USES OF BIFIDOBACTERIUM LONGUM TRANSITIONAL MICROORGANISM Field of the Invention The present invention is related to probiotics and prebiotics, in particular a Bifidobacterium longum transitional microorganism or a prebiotic that promotes the growth and/or survival of a Bifidobacterium longum transitional microorganism for use in preventing and/or reducing the risk of an infection in an infant or young child. of the Invention Interactions between the immune system and the microbiome play a crucial role in human health. These interactions start in the prenatal period and are critical for the maturation of the immune system in new-borns and infants. Several factors influence the composition of the infant’s microbiota and subsequently the development of the immune system. They include maternal infection, antibiotic treatment, environmental exposure, mode of delivery, breastfeeding, and food introduction. It is known that the modulation of the gut microbiota during infancy can prospectively have a great influence on future health status. For example the gut flora can have influence on the development of a strong immune system, normal growth and even on the development of obesity later in life. The gut microbiota and its evolution during the development of the infant is, however, a fine balance between the presence and prevalence (amount) of many populations of gut bacteria. Some gut bacteria are classified as "generally positive" while other ones are "generally negative" (or pathogenic) as to their effect on the overall health of the infant. The weaning period has been described as a non-redundant window for immune imprinting (Cahenzli et al., Cell Host Microbe, 2013, 14(5), 559-70; Olszak et al., Science, 2012, 336(6080): 489-93; Nabhani et al., Immunity, 2019, 50(5), 1276-1288). Healthy immune imprinting promotes appropriate immune responses against environmental challenges, including infections. There remains a need to develop new strategies for preventing and/or reducing the risk of an infection in an infant or young child. Summary of the Invention The present inventors have determined that Bifidobacterium longum subsp microorganisms (B. longum transitional) of a clade that is present in the gut microbiome of the transitional feeding period of mammals, particularly humans, may have beneficial effects on preventing and/or reducing the risk of developing infection. For example, the inventors have shown that the B. longum transitional microorganisms are capable of modulating levels of protective cytokines (e.g. IL-6) and/or short-chain fatty acids (SCFAs); and modulating gut barrier permeability, for example following an insult or exacerbation to gut barrier permeability. Thus, in a first aspect the present invention provides a Bifidobacterium longum transitional microorganism for use in preventing and/or reducing the risk of an infection in an infant or young child. The invention further provides a prebiotic for use in preventing and/or reducing the risk of an infection in an infant or young child by promoting the growth and/or survival of a Bifidobacterium longum transitional microorganism in the gut of the infant or young child, wherein the prebiotic is: i. a glycan substrate, suitably selected from the group recited in any of Tables 1 to 3; and/or ii. a human milk oligosaccharide (HMO), suitably selected from the group consisting of 2’-O-fucosyllactose (2’-FL), 3-O-fucosyllactose (3-FL), lactodifucotetraose/difucosyllactose (di-FL), 3’-O-sialyllactose (3’-SL), 6’-O- sialyllactose (6’- SL), lacto-N-tetraose (LNT) and lacto-N-neotetraose (LNnT) and any combination thereof. The invention also provides a combination of a Bifidobacterium longum transitional microorganism and a prebiotic for use in preventing and/or reducing the risk of an infection in an infant or young child; wherein the prebiotic is: i. a glycan substrate, suitably selected from the group recited in any of Tables 1 to 3; and/or ii. a human milk oligosaccharide (HMO), suitably selected from the group consisting of 2’-O-fucosyllactose (2’-FL), 3-O-fucosyllactose (3-FL), lactodifucotetraose/difucosyllactose (di-FL), 3’-O-sialyllactose (3’-SL), 6’-O- sialyllactose (6’-SL), lacto-N-tetraose (LNT) and lacto-N-neotetraose (LNnT) and any combination thereof. The invention further provides a prebiotic for use in preventing and/or reducing the risk of an infection in an infant or young child by promoting the growth of a Bifidobacterium longum transitional microorganism in the gut of the infant or young child. The invention also provides a combination of a Bifidobacterium longum transitional microorganism and a prebiotic for use in preventing and/or reducing the risk of an infection in an infant or young child. Brief Description of the Drawings Figure 1 - Average Nucleotide Identity (ANI) UPGMA based phylogenetic tree of strains belonging to the B. longum species. The scale represents the percentage of identity at each branch point. Figure 2 - Short chain fatty acids (SCFAs) production (i.e acetate, butyrate and propionate) over 48h of batch fermentation with 3-fucosylactose (3FL). A. Heatmap shows the z score of the Nuclear magnetic resonance (NMR) peak intensity of 3FL, TCA cycle, SCFAs intermediates and SCFAs. Three conditions were tested, i.e., fermentation with no supplementation, supplementation with B longum transitional strain (NCC5004), or supplementation with B longum infantis (NCC3089). Each condition was performed in triplicate using one baby fecal inoculum. Samples were collected at the beginning (T0) at 24 h (T24) and at the end of the fermentation (T48). B. Abundance of B longum transitional strain (NCC5004) and B longum infantis (NCC3089) at T0, T24 and T48 of batch fermentation with 3FL as measured by strain specific qPCR. Figure 3 - Short chain fatty acids (SCFAs) production (i.e. acetate, butyrate and propionate) over 48h of batch fermentation with pea fiber rich in arabinan. A. Heatmap shows the z score of the Nuclear magnetic resonance (NMR) peak intensity of TCA cycle, SCFAs intermediates and SCFAs. Three conditions were tested, i.e., fermentation with no supplementation, supplementation with B longum transitional strain (NCC5002), or supplementation with B longum infantis (NCC3089). Each condition was performed in triplicate using one baby fecal inoculum. Samples were collected at the beginning (T0) at 24 h (T24) and at the end of the fermentation (T48). B. Abundance of B longum transitional strain (NCC5002) and B longum infantis (NCC3089) at T0, T24 and T48 of batch fermentation with pea fiber as measured by strain specific qPCR. Figure 4 – Interleukin 6 (IL-6) production by monocytes following training with different probiotics and stimulated thereafter with LPS. Bars represent median IL-6 response with dotted line demonstrating the IL-6 level by untrained monocytes Figure 5 - Transepithelial electrical resistance (TEER) of Caco-2 monolayers incubated with transitional B. longum NCC5002 (black line), B lactis NCC2818 (grey line) or vehicle (dotted line) for 24h followed by a challenge with pro-inflammatory cytokines. Figure 6 - Measurement of the flux of micromolecules across Caco-2 cell monolayers after incubation with transitional B longum NCC5002 (black line), B lactis NCC2818 (grey line) or vehicle (dotted line) followed by a challenge with pro-inflammatory cytokines. Figure 7 - Representation of glycoside hydrolases (GH) and polysaccharide lyases (PL) in the genomes of the B. longum clade. Heatmap shows presence (light) and absence (dark) of GH and PL genes, and the size of the circles represent the number of these genes per genome of a particular strain. Figure 8 - Growth of B. longum transitional strain NCC5001 was promoted in a complex gut microbiota community by pectin (sugar beet) and arabinogalactan (larch wood). P **** < 0.0001, *** <0.001, ** <0.01, * <0.05, one-way ANOVA with uncorrected Fisher's LSD. Figure 9 - Growth of B. longum transitional strain NCC5002 was promoted in a complex gut microbiota community by arabinogalactan (larch wood) and starch (potato). P **** < 0.0001, *** <0.001, ** <0.01, * <0.05, one-way ANOVA with uncorrected Fisher's LSD. Figure 10 – Representative CAZyme sequences Figure 11 - Schematic representation of the organization of the genes implicated in the degradation and the metabolization of fucosylated human milk oligosaccharides in the B. longum transitional strains, compared to B. longum subsp. infantis ATCC 15697 and B. kashiwanohense DSM 21854. Values represent percentage (%) of identity between the different genes. Figure 12 - Growth of B. longum transitional strains and B. longum subsp. infantis LMG 11588 on glucose, 2’-FL or 3-FL as sole carbon source (0.5% final). Significant differences between 2’-FL and 3-FL growth for each strain were calculated using one-way ANOVA, followed by a Sidak’s multiple comparison test (ns=non-significant, * p-value <0.05, ** p-value <0.01). Figure 13 - Growth ratios of 3-FL over 2’-FL of B. longum transitional strains and B. longum subsp. infantis LMG 11588. Figure 14 - Schematics of the experimental set up for a preclinical model for efficacy testing of B. longum transitional strain in infection model. At post natal day (PND) 5, C57BL/6 WT pups received different combinations of nutritional ingredients (HMOs + probiotic mixture) via oral gavage while being nursed by mothers fed on low fiber diet . Broad antibiotics was supplied through the drinking water from PND16 to 26. Following weaning on PND21, a selective fiber mix (adapted to B. longum transitional strain) was introduced in the diet of these mice coupled with oral gavage of the same nutritional ingredients (reduced dose of HMOs + probiotic mixture). Infection with pneumonia virus of mice was performed at PND35. Control groups were nursed by mothers fed either with a low fiber diet only (susceptible group) or with a high fiber diet only (protected group) before weaning and kept on the same diet after weaning. Figure 15 – Kinetics of weight change following airway viral infection with pneumonia virus of mice from 0 to 10 dpi. Each dot represents the mean with error bars indicating standard error of the mean with N=8 / experimental group. Statistical difference between different groups was calculated by a 2way ANOVA. *,£ p value <0.05, **, ££ p value <0.005, $$$, $$$$ p value < 0.0001. Detailed Description of the invention All percentages are by weight unless otherwise stated. The terms “about” or “approximatively” as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, are meant to encompass variations of and from the specific value, such as the variation of 1/-10% or less, 1/-5% or less, 1/-1% or less, and +/0.1% or less of and from the specific value, insofar such variations are appropriate to perform in the disclosed invention. It is to be understood that the value to which the modifier “about” or “approximately” refers is itself also specifically, and preferably, disclosed. The terms “subject”, “individual” and “patient” are used interchangeably to refer to a vertebrate, preferably a mammal, more preferably a human. Mammals include but are not limited to murines, simians, humans, farm animals, sport animals and pets. The term “infant” means a human subject under the age of 12 months or an age equivalent non-human animal. The terms “young child” or “toddler” as used herein may mean a human subject aged between 12 months and 5 years of age. Suitably, a “young child” may refer to an age equivalent non- human animal. The expressions “complementary feeding period”, “complementary period”, “transitional period”, “transitional feeding period” and “weaning period” can be interchangeably used and refer to the period during which the milk, either breast milk or formula, is substituted by other foods in the diet of an infant or a young child. The infant or the young child is typically moved or transitioned gradually from exclusive milk-feeding, either breast feeding or formula feeding, to mixed diet comprising milk and/or solid foods. The transitional period depends on the infant or young child but typically falls between about 4 months and about 18 months of age, such as between about 6 and about 18 months of age, but can in some instances extend up to about 24 months or more. For humans, the weaning period typically starts between 4 and 6 months of age and is considered completed once the infant and/or the young child is no longer fed with breast milk or infant formula, typically at about 24 months of age. In some embodiments, the weaning period is between 4 and 24 months. The expressions “composition” or “nutritional composition” refer to any kind of composition or formulation that provides a nutritional benefit to an individual and that may be safely consumed by a human or an animal. A nutritional composition may be in solid (e.g. powder), semi-solid or liquid form and may comprise one or more macronutrients, micronutrients, food additives, water, etc. For instance, the nutritional composition may comprise the following macronutrients: a source of proteins, a source of lipids, a source of carbohydrates and any combination thereof. Furthermore, the nutritional composition may comprise the following micronutrients: vitamins, minerals, fiber, phytochemicals, antioxidants, prebiotics, probiotics, bioactives, metabolites (e.g. butyrate, Docosahexaenoic acid (DHA), Eicosapentaenoic acid (EPA), Gamma-Linolenic acid (GLA)) and any combination thereof. The composition may also contain food additives such as stabilizers (when provided in liquid or solid form) or emulsifiers (when provided in liquid form). The amount of the various ingredients (e.g. the oligosaccharides) can be expressed in g/100 g of composition on a dry weight basis when it is in a solid form, e.g. a powder, or as a concentration in g/L of the composition when it refers to a liquid form (this latter also encompasses liquid composition that may be obtained from a powder after reconstitution in a liquid such as milk, water, e.g. a reconstituted infant formula or follow-on/follow-up formula or infant cereal product or any other formulation designed for infant or young child nutrition). Generally, a nutritional composition can be formulated to be taken enterally, orally, parenterally, or intravenously, and it usually includes one of more nutrients selected from: a lipid or fat source, a protein source, and a carbohydrate source. Preferably, a nutritional composition is for oral use. In a particular embodiment, the nutritional composition is a “synthetic nutritional composition”. The expression “synthetic nutritional composition” means a mixture obtained by chemical and/or biological means. The expression "infant formula" as used herein refers to a foodstuff intended for particular nutritional use by infants during the first months of life and satisfying by itself the nutritional requirements of this category of person (Article 2(c) of the European Commission Directive 91/321/EEC 2006/141/EC of 22 December 2006 on infant formulae and follow-on formulae). It also refers to a nutritional composition intended for infants and as defined in Codex Alimentarius (Codex STAN 72-1981) and Infant Specialities (incl. Food for Special Medical Purpose). The expression "infant formula" encompasses both “starter infant formula” and “follow-up formula” or “follow-on formula”. A “follow-up formula” or “follow-on formula” is given from the 6th month onwards. It constitutes the principal liquid element in the progressively diversified diet of this category of person. The expression “baby food” means a foodstuff intended for particular nutritional use by infants or young children during the first years of life. The expression “infant cereal composition” means a foodstuff intended for particular nutritional use by infants or young children during the first years of life. The expression “growing-up milk” (or GUM) refers to a milk-based drink generally with added vitamins and minerals, that is intended for young children or children. The terms “fortifier” refers to liquid or solid nutritional compositions suitable for fortifying or mixing with human milk, infant formula, growing-up milk or human breast milk fortified with other nutrients. Accordingly, the fortifier can be administered after dissolution in human breast milk, in infant formula, in growing-up milk or in human breast milk fortified with other nutrients or otherwise it can be administered as a stand-alone composition. When administered as a stand-alone composition, the milk fortifier can be also identified as being a “supplement”. The term “metabolize” is used herein to mean that a substrate can by broken down, adsorbed and/or utilized by a microorganism. For example, the substrate may promote and/or contribute to the growth and/or survival of the microorganism. Suitably, the term “capable of metabolizing the glycan substrate” may mean that the B. longum transitional strain encodes at least one CAZyme which is capable of utilizing the glycan substrate. For example, the CAZyme may be capable of catalyzing the hydrolysis of a glycosidic bond within the glycan substrate. Suitably, the B. longum transitional strain may encode at least one, at least two, at least three, at least four or at least five CAZymes that are capable of utilizing the glycan substrate. Suitably, the term “capable of metabolizing the glycan substrate” may mean that the glycan substrate (or a fiber or ingredient comprising the glycan substrate) is capable of promoting growth and/or survival of the B. longum transitional strain (e.g. when added to an anaerobic culture of the B. longum transitional strain). Growth and/or survival of the B. longum transitional strain may be determined by measuring the abundance of 16S rDNA – for example using PCR methods. An illustrative assay for measuring growth of a B. longum transitional strain in the presence of glycan substrates (e.g. in the form of fiber) is provided in the present examples. Suitably, the glycan substrate is capable of being metabolized by the B longum transitional microorganism. Suitably, the glycan substrate may be capable of promoting growth and/or survival of the B. longum transitional strain. Glycan substrates capable of promoting growth and/or survival of the B. longum transitional strain may be determined by e.g. anaerobic culture of the B. longum transitional strain with the glycan substrate to be tested. Growth and/or survival of the B. longum transitional strain may be determined by measuring bacteria cell number, cell density (e.g. measured by optical density) and/or the abundance of 16S rDNA – for example using PCR methods. An illustrative assay for measuring growth of a B. longum transitional strain in the presence of glycan substrates is provided in the Examples. A glycan substrate capable of promoting growth and/or survival of the B. longum transitional strain may increase the number of B. longum transitional bacteria in an anaerobic culture by at least 20%, at least 30%, at least 40%, at least 50%, at least 75% or at least 100% compared to the number of B. longum transitional bacteria in a control anaerobic culture which does not comprise the HMO. Suitably, a glycan substrate capable of promoting growth and/or survival of the B. longum transitional strain may increase the number of B. longum transitional bacteria in an anaerobic culture by a statistically significant amount (e.g. p-value <0.05 as determined by one-way ANOVA) compared to the number of B. longum transitional bacteria in a control anaerobic culture which does not comprise the glycan substrate. A “glycan substrate” refers to a glycan that can be metabolized by a microorganism. A glycan substrate may be, for example, a glycoconjugate, oligo- or polysaccharide. Glycoconjugate glycans may comprise N-linked glycans or O-linked glycans within glycoproteins and proteoglycans, or glycolipids. For example, an O-linked glycan may comprise a protein or peptide where the oxygen atom of a serine or threonine residue is linked to a monosaccharide, oligo- or polysaccharide as in the case with glycosaminoglycans (GAGs). Further examples of “glycan substrates” are cellulose, which is a glycan composed of β-1,4-linked D-glucose, and chitin, which is a glycan composed of β-1,4-linked N-acetyl-D-glucosamine. Glycans may be homo- or heteropolymers of monosaccharide residues and can be linear or branched. “Glycan substrate” as used herein encompasses, for example, oligosaccharides and polysaccharides. The “oligosaccharide” may refer to a carbohydrate that has greater than 2 but relatively few monosaccharide units (typically 3, 4, 5, 6, and up to 10). Exemplary oligosaccharides include, but are not limited to, fructo-oligosaccharides, galacto-oligosaccharides (raffinose, stachyose, verbascose), maltooligosaccharides, gentio-oligosaccharides, cellooligosaccharides, milk oligosaccharides (e.g., those present in secretions from mammary glands), isomalto- oligosaccharides, lactosucrose, mannooligosaccharides, melibiose-derived oligosaccharides, pectic oligosaccharides, xylo-oligosaccharides. The term “polysaccharide” may refer to a carbohydrate that has more than ten monosaccharide units. Exemplary polysaccharides include, but are not limited to, starch, arabinogalactan, laminarin, chrysolaminarin, xylan, arabinoxylan, mannan, fucoidan and galactomannan. It is to be understood that there is not a precise cut-off or distinction between the terms oligosaccharide and polysaccharide, nor is such a distinction necessary to practice the invention. The term “glycosaminoglycan” (GAG) or mucopolysaccharide refers to long linear polysaccharides consisting of repeating disaccharide units (i.e. two-sugar units). The repeating two-sugar unit consists of a uronic sugar and an amino sugar, with the exception of keratan, where in the place of the uronic sugar it has galactose. GAGs are classified into four groups based on core disaccharide structures. “Mucins”, as used herein, may refer to a family of high molecular weight, heavily glycosylated proteins (glycoconjugates). Mucins' key characteristic is their ability to form gels; therefore they are a key component in most gel-like secretions, serving functions from lubrication to cell signaling to forming mechanical and chemical barriers. The term “HMO” or “HMOs” refers to human milk oligosaccharide(s). These carbohydrates are highly resistant to enzymatic hydrolysis, indicating they may display essential functions not directly related to their caloric value. It has been especially illustrated they play a vital role in the early development of infants and young children, such as the maturation of the immune system. Many different kinds of HMOs are found in the human milk. Each individual oligosaccharide is based on a combination of glucose, galactose, sialic acid (N- acetylneuraminic acid), fucose and/or N-acetylglucosamine with many and varied linkages between them, thus accounting for the enormous number of different oligosaccharides in human milk – over 130 such structures have been identified so far. Almost all of them have a lactose moiety at their reducing end while sialic acid and/or fucose (when present) occupy the terminal position at the non-reducing ends. Depending on the presence of fucose and sialic acid in the oligosaccharide structure, the HMOs can be divided as non-fucosylated (neutral) or fucosylated (neutral) and sialylated (acidic) and non-sialylated molecules, respectively. The expression “fucosylated oligosaccharide” refers to an oligosaccharide having a fucose residue. It has a neutral nature. Some examples are 2’-fucosyllactose (2-FL), 3-fucosyllactose (3-FL), difucosyllactose (DiFL), lacto-N-fucopentaose (e.g. lacto-N-fucopentaose I, lacto-N- fucopentaose II, lacto-N-fucopentaose III, lacto-N-fucopentaose V), lacto-N-fucohexaose, lacto-N-difucohexaose I, fucosyllacto-N-hexaose, fucosyllacto-N-neohexaose, difucosyllacto- N-hexaose I, difucosyllacto-N-neohexaose II and any combination thereof. Fucosylated oligosaccharides represents the largest fraction of human milk with 2’-FL constituting up to 30% of the total HMOs. Fucosylated oligosaccharides are thought to reduce the risk of infections and inflammations and to boost growth and metabolic activity of specific commensal microbes reducing inflammatory response. The expression “N-acetylated oligosaccharide(s)” encompasses both “N-acetyl-lactosamine” and “oligosaccharide(s) containing N-acetyl-lactosamine”. They are neutral oligosaccharides having an N-acetyl-lactosamine residue. Suitable examples are LNT (lacto-N-tetraose), para- lacto-N-neohexaose (para-LNnH), LNnT (lacto-N-neotetraose), DSLNT (disialyllacto-N- tetraose), and any combinations thereof. Other examples are lacto-N-hexaose, lacto-N- neohexaose, para- lacto-N-hexaose, para-lacto-N-neohexaose, lacto-N-octaose, lacto-N- neooctaose, iso- lacto-N-octaose, para- lacto-N-octaose and lacto-N-decaose. The expressions “at least one fucosylated oligosaccharide” and “at least one N-acetylated oligosaccharide” should be understood as “at least one type of fucosylated oligosaccharide” and “at least one type of N-acetylated oligosaccharide”. The term “sialylated oligosaccharide” refers to an oligosaccharide having a charged sialic acid residue. It has an acidic nature. Some examples are 3’-sialyllactose (3-SL), 6’-sialyllactose (6- SL), sialyllacto-N-tetraose (Lst – e.g. Lst-a, Lst-b or Lst-c). Suitably, the term “capable of metabolizing the HMO” may mean that the B. longum transitional strain encodes at least one CAZyme which is capable of utilizing the HMO. For example, the CAZyme may be capable of catalyzing the hydrolysis of a glycosidic bond within the HMO. Suitably, the B. longum transitional strain may encode at least one, at least two, at least three, at least four or at least five CAZymes that are capable of utilizing the HMO. Suitably, the term “capable of metabolizing the HMO” may mean that the HMO is capable of promoting growth and/or survival of the B. longum transitional strain (e.g. when added to an anaerobic culture of the B. longum transitional strain). Growth and/or survival of the B. longum transitional strain may be determined by measuring the abundance of 16S rDNA – for example using PCR methods. The term fibers is used herein to refer to carbohydrates that are indigestible by a human or animal. Such fibers are also discussed in relation to carbohydrates herein. Suitably, the fiber can be fermented by one or more B. longum transitional microorganisms provided in the present use or composition and/or within one or more regions in the gastrointestinal tract within an organism, such as a human or non-human animal. As used herein, the expressions “fiber” or “fibers” or “dietary fiber” or “dietary fibers” within the context of the present invention indicate the indigestible portion, in small intestine, of food derived from plants which comprises two main components: soluble fiber, which dissolves in water and insoluble fiber. Mixtures of fibers are comprised within the scope of the terms above mentioned. Soluble fiber is readily fermented in the colon into gases and physiologically active byproducts and can be prebiotic and viscous. Insoluble fiber does not dissolve in water, is metabolically inert and provides bulking, or it can be prebiotic and metabolically ferment in the large intestine. Chemically, dietary fiber consists of carbohydrate polymers with three or more monomeric units which are not hydrolyzed by endogenous enzymes in the small intestine such as arabinoxylans, cellulose, and many other plant components such as resistant starch, resistant dextrins, inulin, lignin, chitins, pectins, arabinans, arabinogalactans, galactans, xylans, beta-glucans, and oligosaccharides. Non-limiting examples of dietary fibers are: prebiotic fibers such as Fructo- oligosaccharides (FOS), inulin, galacto-oligosaccharides (GOS), fruit fiber, vegetable fiber, cereal fiber, resistant starch such as high amylose corn starch. As used herein, “added fiber” or “added dietary fiber” indicates an ingredient mainly or totally constituted by fiber which is added to the complementary nutritional composition and whose content in fiber contributes to the total fiber content of the composition. The total fiber content of the complementary nutritional composition is provided by the sum of amount of fiber naturally present in ingredients used in the recipe (for example from whole grain cereal flour) plus amount of added fiber. The term “prebiotic” means non-digestible carbohydrates that beneficially affect the host by selectively stimulating the growth and/or the activity of healthy bacteria such as bifidobacteria in the colon of humans (Gibson GR, Roberfroid MB. Dietary modulation of the human colonic microbiota: introducing the concept of prebiotics. J Nutr.1995;125:1401-12). The term “probiotic” means microbial cell preparation or components of microbial cells with a beneficial effect on the health or well-being of the host (Salminen S, Ouwehand A. Benno Y. et al. “Probiotics: how should they be defined” Trends Food Sci. Technol.1999:10107-10). The microbial cells according to the present invention are generally bacteria. The term “cfu” should be understood as colony forming unit. The “gut microbiota” is the composition of microorganisms (including bacteria, archaea and fungi) that live in the digestive tract. The term “gut microbiome” may encompass both the “gut microbiota” and their “theater of activity”, which may include their structural elements (nucleic acid, proteins, lipids, polysaccharides), metabolites (signaling molecules, toxins, organic and inorganic molecules) and molecules produced by coexisting hosts and structured by the surrounding environmental conditions (Berg, G., et al., 2020. Microbiome, 8(1), pp.1-22). Bifidobacterium longum transitional microorganism Bifidobacterium longum subsp microorganisms of a clade that is present in the gut microbiome of the transitional feeding period of mammals, particularly humans, have previously been identified. B. longum microorganisms belonging to this clade are referred to herein as Bifidobacterium longum transitional (B. longum transitional). B. longum transitional strains NCC 5000, NCC 5001, NCC 5002, NCC 5003 and NCC 5004 were deposited with the Collection nationale de cultures de micro-organisms (CNCM), Institute Pasteur by SOCIÉTÉ DES PRODUITS NESTLÉ S.A according to Budapest Treaty on 11th of May 2021 receiving the deposit numbers CNCM I-5683, CNCM I-5684, CNCM I-5685, CNCM I-5686 and CNCM I-5687, respectively. In US provisional patent application 63/216127, it was shown that the B. longum transitional microorganisms are greater in relative abundance during the transitional feeding period (e.g. weaning period) than either B. longum subsp. infantis (B. infantis) or B. longum subsp longum. Indeed, the relative abundance of B. longum subsp. infantis decreases at the beginning of the transitional feeding period until the end of the transitional feeding period while B. longum subsp. longum begins to increase in abundance. Vatanen et al. demonstrated that this distinct Bifidobacterium longum clade expanded with introduction of solid foods and harbored enzymes for utilizing both breast milk and solid food substrates (Vatanen et al.; 2022, Cell 185, 1–18; published online 1 November 2022; https://doi.org/10.1016/j.cell.2022.10.011). Suitably, the B. longum transitional microorganism may encode one or more CAZymes selected from the groups recited in Table 1. Suitably, the B. longum transitional microorganism may encode one or two CAZymes selected from the group recited in Table 1. Suitably, the B. longum transitional microorganism encodes at least one CAZyme selected from the group recited in Table 1 and one or more of the CAZymes selected from the groups recited in Table 2 and 3. For example, the B. longum transitional microorganism may encode at least 2, at least 5, at least 10, at least 20 or at least 30 of the CAZymes selected from the groups recited in Table 2 and 3. Suitably, B. longum transitional microorganism encodes (i) at least one CAZyme selected from the group recited in Table 1 and (ii) each of the CAZymes recited in Table 3 or each of the CAZymes recited in Table 3 apart from GH5_44. Suitably, B. longum transitional microorganism encodes (i) at least one CAZyme selected from the group recited in Table 1 and (ii) each of the CAZymes recited in Table 3 or each of the CAZymes recited in Table 3 apart from GH25. Suitably, the B. longum transitional microorganism does not encode one or more of the CAZymes recited in Table 4. Suitably, the B. longum transitional microorganism does not encode any of the CAZymes recited in Table 4. The B. longum transitional microorganism of the present invention advantageously harbors genes coding for CAZymes allowing cleavage of sialic residues from glycans such as sialilated oligosachharides, glycoproteins and glycolipids. This allows effective utilization of the sialylated oligosaccharides that are present in the breast milk at weaning and hence can participate to an appropriate development of the gut microbiome of an infant and/or a young child. It may also help in preventing presence of enteropathogens. In some embodiments, a B. longum transitional microorganism comprises a sialidase or neuraminidase family 33 (GH33, sialidase or neuraminidase) gene having at least 60% identity with BLON_2348 gene present in Bifidobacterium longum subsp. infantis ATCC 15697. In some embodiments, a B. longum transitional microorganism comprises sialidase or neuraminidase family 33 (GH33, sialidase or neuraminidase) gene having about 60%, about 61%, about 62%, about 63%, about 64%, about 65%, about 66%, about 67%, about 68%, about 69%, about 70%, about 71%, about 72%, about 73%, about 74%, about 75%, about 76%, about 77%, about 78%, about 79%, about 80%, about 81%, about 82%, about 83%, about 84%, about 85%, about 86%, about 87%, about 88%, about 89%, about 90%, about 91%, about 92%, about 93%, about 94%, about 95%, about 96%, about 97%, about 98%, about 99%, about 100% identity with BLON_2348 gene present in B. longum subsp. infantis ATCC 15697. In some embodiments, a B. longum transitional microorganism comprises sialidase or neuraminidase family 33 (GH33, sialidase or neuraminidase) gene having at least 60%, at least 61%, at least 62%, at least 63%, at least 64%, at least 65%, at least 66%, at least 67%, at least 68%, at least 69%, at least 70%, at least 71%, at least 72%, at least 73%, at least 74%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% identity with BLON_2348 gene present in B. longum subsp. infantis ATCC 15697. In some embodiments, the B. longum transitional microorganism used according to the present invention comprises a glycosyl hydrolase family 95 (GH95, α-L-galactosidase; α-L- fucosidase; a-1,2-L-fucosidase) gene having at least 60% of identity with BLON_2335 gene present in B. longum subsp. infantis ATCC 15697 and/or a glycosyl hydrolase family 29 (GH29, α-L-fucosidase; α-1,3/1,4-L-fucosidase;α-1,2-L-fucosidase) having at least 60 % identity with BLON_2336 gene present in B. longum subsp. infantis ATCC 15697. In some embodiments, the B. longum transitional microorganism used according to the present invention comprises a sialidase or neuraminidase family 33 (GH33, sialidase or neuraminidase) gene having at least 60% identity with BLON_2348 gene present in B. longum subsp. infantis ATCC 15697. In some embodiments, the B. longum transitional microorganism preferentially utilizes 3- fucosyllactose (3-FL) over 2’ -fucosyllactose (2’-FL). Suitably, B. longum transitional microorganism may preferentially utilize 3-FL over 2’-FL in a ratio between 0.1:5, preferably in a ratio between 0.1:4, more preferably in a ratio between 0.2:2. In some embodiments, the B. longum transitional microorganism used in the present invention utilizes 3-FL more efficiently than 2’-FL, as demonstrated by a better growth, for example as shown in the present Examples. In some embodiments, a B. longum transitional microorganism has an Average Nucleotide Identity (ANI) of at least 96% with at least one B. longum strain selected in the group consisting of CNCM I-5683, CNCM I-5684, CNCM I-5685, CNCM I-5686, CNCM I-5687, and CMCC- P0001 (ATCC BAA-2753), and any combination thereof. In some embodiments, a Bifidobacterium longum transitional microorganism has an ANI of about 96%, 96.1%, 96.2%, 96.3%, 96.4%, 96.5%, 96.6%, 96.7%, 96.8%, 96.9%, 97%, 97.1%, 97.2%, 97.3%, 97.4%, 97.5%, 97.6%, 97.7%, 97.8%, 97.9%, 98%, 98.1%, 98.2%, 98.3%, 98.4%, 98.5%, 98.6%, 98.6 %, 98.7 %, 98.8 %, 98.9 %, 99 %, 99.1 %, 99.2 %, 99.3 %, 99.4 %, 99.5 %, 99.6 %, 99.7 %, 99.8 %, 99.9 %, or 100 % with at least one Bifidobacterium longum strain selected in the group consisting of CNCM I-5683, CNCM I-5684, CNCM I-5685, CNCM I-5686, CNCM I-5687 and CMCC-P0001 (ATCC BAA-2753), and any combination thereof. In some embodiments, a Bifidobacterium longum transitional microorganism has an ANI of at least 96%, of at least 96.1%, of at least 96.2%, of at least 96.3%, of at least 96.4%, of at least 96.5%, of at least 96.6%, of at least 96.7%, of at least 96.8%, of at least 96.9%, of at least 97%, of at least 97.1%, of at least 97.2%, of at least 97.3%, of at least 97.4%, of at least 97.5%, of at least 97.6%, of at least 97.7%, of at least 97.8%, of at least 97.9%, of at least 98%, of at least 98.1%, of at least 98.2%, of at least 98.3%, of at least 98.4%, of at least 98.5%, of at least 98.6%, of at least 98.6 %, of at least 98.7 %, of at least 98.8 %, of at least 98.9 %, of at least 99 %, of at least 99.1 %, of at least 99.2 %, of at least 99.3 %, of at least 99.4 %, of at least 99.5 %, of at least 99.6 %, of at least 99.7 %, of at least 99.8 %, of at least 99.9 % with at least one Bifidobacterium longum strain selected in the group consisting of CNCM I-5683, CNCM I-5684, CNCM I-5685, CNCM I-5686, CNCM I-5687 and CMCC-P0001 (ATCC BAA-2753), and any combination thereof. In some embodiments, a B. longum transitional microorganism has an Average Nucleotide Identity (ANI) of at least 98% with at least one B. longum strain selected in the group consisting of CNCM I-5683, CNCM I-5684, CNCM I-5685, CNCM I-5686 and CNCM I-5687, and any combination thereof. Suitably, the B. longum transitional microorganism encodes one or more CAZymes selected from the group recited in Table 1 and has an ANI of at least 98% with at least one B. longum strain selected in the group consisting of CNCM I-5683, CNCM I-5684, CNCM I-5685, CNCM I-5686 and CNCM I-5687, and any combination thereof. Suitably, the B. longum transitional microorganism encodes one or more GH31 CAZymes and has an ANI of at least 98% with at least one B. longum strain selected in the group consisting of CNCM I- 5683, CNCM I-5684, CNCM I-5685, CNCM I-5686 and CNCM I-5687, and any combination thereof. In some embodiments, a B. longum transitional microorganism has an ANI of about 98% to 100% with at least one B. longum strain selected in the group consisting of CNCM I- 5683, CNCM I-5684, CNCM I-5685, CNCM I-5686 and CNCM I-5687, and any combination thereof. In some embodiments, a B. longum transitional microorganism has an ANI of at least 98%, 98.1%, 98.2%, 98.3%, 98.4%, 98.5%, 98.6%, 98.6 %, 98.7 %, 98.8 %, 98.9 %, 99 %, 99.1 %, 99.2 %, 99.3 %, 99.4 %, 99.5 %, 99.6 %, 99.7 %, 99.8 %, 99.9 %, or 100 % with at least one B. longum strain selected in the group consisting of CNCM I-5683, CNCM I-5684, CNCM I-5685, CNCM I-5686 and CNCM I-5687, and any combination thereof. In some embodiments, a B. longum transitional microorganism has an ANI of at least 98.6%, of at least 98.6 %, of at least 98.7 %, of at least 98.8 %, of at least 98.9 %, of at least 99 %, of at least 99.1 %, of at least 99.2 %, of at least 99.3 %, of at least 99.4 %, of at least 99.5 %, of at least 99.6 %, of at least 99.7 %, of at least 99.8 %, of at least 99.9 % or of at least 100% with at least one B. longum strain selected in the group consisting of CNCM I-5683, CNCM I-5684, CNCM I-5685, CNCM I-5686 and CNCM I-5687, and any combination thereof. Methods for sequencing microbial genomes are well known in the art (see e.g. Segerman; Front. Cell. Infect. Microbiol.; 2020; 10; Article 527102 & Donkor; Genes; 2013; 4(4); 556-572). By way of example, metagenomics methods may be used. Suitable metagenomics methods may be performed using shotgun sequencing data, for example. Suitable metogenomics methods are known in the art and include MetaPhlAn 3.0, for example (see Beghini et al.; eLife 2021;10: e65088; https://huttenhower.sph.harvard.edu/metaphlan). The “Average Nucleotide Identity (ANI)” is a term of art that refers to a distance-based approach to delineate species based on pair-wise comparisons of their genome sequences and is an in silico alternative to the traditional DNA-DNA hybridization (DDH) techniques that have been used for phylogenetic definition of a species (Goris et al., 2007, “DNA-DNA hybridization values and their relationship to whole-genome sequence similarities”, Int. J. Syst. Evol. Microbiol.57: 81-91). Based on DDH, strains with greater than 70% relatedness would be considered to belong to the same species (see e.g., Wayne et al., 1987, Report of the Ad- Hoc-Committee on Reconciliation of Approaches to Bacterial Systematics. Int J Syst Bacteriol 37: 463-464). ANI is similar to the aforementioned 70% DDH cutoff value and can be used for species delineation. ANI has been evaluated in multiple labs and has become the gold standard for species delineation (see e.g., Kim et al., 2014, “Towards a taxonomic coherence between average nucleotide identity and 16S rRNA gene sequence similarity for species demarcation of prokaryotes”, Int. J. Syst. Evol. Micr.64: 346-351; Richter et al., 2009, “Shifting the genomic gold standard for the prokaryotic species definition”, P Natl Acad Sci USA 106: 19126-19131; and Chan et al., 2012, “Defining bacterial species in the genomic era: insights from the genus Acinetobacter”, Bmc. Microbiol.12)). The ANI of the shared genes between two strains is known to be a robust means to compare genetic relatedness among strains, and that ANI values of about 95% correspond to the 70% DNA-DNA hybridization standard for defining a species. See, e.g., Konstantinidis and Tiedje, Proc Natl Acad Sci USA, 102(7):2567-72 (2005); and Goris et al., Int Syst Evol Microbiol.57(Pt 1):81-91 (2007). The ANI between two bacterial genomes is calculated from pair-wise comparisons of all sequences shared between any two strains and can be determined, for example, using any of a number of publicly available ANI tools, including but not limited to OrthoANI with usearch (Yoon et al. Antonie van Leeuwenhoek 110:1281-1286 (2017)); ANI Calculator, JSpecies (Richter and Rossello-Mora, Proc Natl Acad Sci USA 106:19126-19131 (2009)); and JSpeciesWS (Richter et al., Bioinformatics 32:929-931 (2016)). Other methods for determining the ANI of two genomes are known in the art. See, e.g., Konstantinidis, K. T. and Tiedje, J. M., Proc. Natl. Acad. Sci. U.S.A., 102: 2567-2572 (2005); and Varghese et al., Nucleic Acids Research, 43(14):6761-6771 (2015). In a particular embodiment, the ANI between two bacterial genomes can be determined, for example, by averaging the nucleotide identity of orthologous genes identified as bidirectional best hits (BBHs). Protein-coding genes of a first genome (Genome A) and second genome (Genome B) are compared at the nucleotide level using a similarity search tool, for example, NSimScan (Novichkov et al., Bioinformatics 32(15): 2380-23811 (2016)). The results are then filtered to retain only the BBHs that display at least 70% sequence identity over at least 70% of the length of the shorter sequence in each BBH pair. The ANI of Genome A to Genome B is defined as the sum of the percent identity times the alignment length for all BBHs, divided by the sum of the lengths of the BBH genes. These and ANI determination techniques are known in the art. Suitably, a B. longum transitional microorganism selected from the group consisting of CNCM I-5683, CNCM I-5684, CNCM I-5685, CNCM I-5686 and CNCM I-5687, represents the reference genome to which a microbial genome is compared. Suitably, a Bifidobacterium longum microorganism selected from the group consisting of CNCM I-5683, CNCM I-5684, CNCM I-5685, CNCM I-5686, CNCM I-5687 and CMCC-P0001 (ATCC BAA-2753), represents the reference genome to which a microbial genome is compared. Genome sequences for B. longum transitional strains NCC 5000 (CNCM I-5683), NCC 5001 (CNCM I-5684), NCC 5002 (CNCM I-5685), NCC 5003 (CNCM I-5686) and NCC 5004 (CNCM I-5687) are available via Joint Genome Project (JGI) Study number: Gs0156595 (https://genome.jgi.doe.gov/portal/). Analysis project numbers and taxon numbers for each genome are as follows: In some embodiments, the B. longum transitional microorganism for use in the present invention is isolated from a human. In some other embodiments, the B. longum transitional microorganism is not of the subspecies B. longum subsp. longum or B. longum subsp. infantis. Suitably, the B. longum transitional microorganism is provided as a probiotic. Suitably, the B. longum transitional microorganism is provided in a composition. Preventing and/or reducing the risk of an infection ‘Infection’, as used herein, may refer to a disease or disorder caused by an infectious agent or pathogen (including symptoms thereof). “Preventing”, as used herein, may refer to administering the B. longum transitional microorganism and/or prebiotic and/or composition of the invention to a subject who has not yet contracted an infection and/or who is not showing any symptoms of the infection to prevent or impair the cause of the disease or to reduce or prevent development of at least one symptom associated with the disease. The subject may have a predisposition for, or be thought to be at risk of developing, the disease. “Reducing the risk of an infection”, may refer to administering the B. longum transitional microorganism and/or prebiotic and/or composition of the invention to a subject who has not yet contracted an infection and/or who is not showing any symptoms of the infection to reduce the likelihood of the infant or young child developing a disease caused by infectious agent or pathogen. The administration may prevent or impair the cause of the disease or reduce or prevent development of at least one symptom associated with the disease. The subject may have a predisposition for, or be thought to be at risk of developing, the disease. The present use to prevent and/or reduce the risk of an infection may be referred to as a prophylactic use to delay or prevent the onset of the symptoms of the infection and/or reduce the number or severity of symptoms of the infection. Suitably, administering the B. longum transitional microorganism and/or prebiotic and/or composition of the invention to a subject may reduce the magnitude and/or amount of symptoms of an infection caused by the infectious agent or pathogen. Suitably, the present B. longum transitional microorganism, prebiotic and/or composition may be administered to an infant or young child. Suitably, the present B. longum transitional microorganism, prebiotic and/or composition may prevent and/or reduce the risk of an infection in an infant or young child in an infant or young child. Suitably, the present B. longum transitional microorganism, prebiotic and/or composition may be administered to an infant or young child and prevent and/or reduce the risk of an infection in an infant or young child in an infant or young child. The infection may be a viral, bacterial or fungal infection. Suitably, the infection may be a viral airway or respiratory infection. For example, the infection may be selected from an influenza virus, respiratory syncytial virus, rhinovirus, parainfluenza viruses, metapneumovirus, coronavirus, adenovirus, and bocavirus infection. Suitably the infection may be an influenza virus, respiratory syncytial virus or rhinovirus infection. Influenza virus is the infectious agent that causes influenza (flu). Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms begin from one to four days after exposure to the virus (typically two days) and last for about 2–8 days. Diarrhea and vomiting can occur, particularly in children. There are four types of influenza virus, termed influenza viruses A, B, C, and D. Aquatic birds are the primary source of Influenza A virus (IAV), which is also widespread in various mammals, including humans and pigs. Influenza B virus (IBV) and Influenza C virus (ICV) primarily infect humans, and Influenza D virus (IDV) is found in cattle and pigs. IAV and IBV circulate in humans and cause seasonal epidemics, and ICV causes a mild infection, primarily in children. IDV can infect humans but is not known to cause illness. In humans, influenza viruses are primarily transmitted through respiratory droplets produced from coughing and sneezing. Transmission through aerosols and intermediate objects and surfaces contaminated by the virus also occur. Respiratory syncytial virus (RSV) a negative-sense, single-stranded RNA virus. It is the single most common cause of respiratory hospitalization in infants, with infection rates typically higher during the cold winter months, causing bronchiolitis. RSV is spread through contaminated air droplets and can cause outbreaks both in the community and in hospital settings. Following initial infection via the eyes or nose, the virus will infect the epithelial cells of the upper and lower airway, causing inflammation, cell damage, and airway obstruction. Rhinovirus is the most common viral infectious agent in humans and is the predominant cause of the common cold. The three species of rhinovirus (A, B, and C) include around 160 recognized types of human rhinovirus that differ according to their surface proteins (serotypes). They are lytic in nature and are among the smallest viruses, with diameters of about 30 nanometers. Symptoms of rhinovirus infection may include sore throat, runny nose, nasal congestion, sneezing and cough; sometimes accompanied by muscle aches, fatigue, malaise, headache, muscle weakness, or loss of appetite. Suitably, the B. longum transitional microorganism and/or prebiotic is not for use to reduce or prevent the presence of enteropathogens. Suitably, the B. longum transitional microorganism and/or prebiotic is not for use to reduce or prevent the presence of enteropathogens in the gut of an infant and/or a young child. Suitably, the B. longum transitional microorganism is not for use to reduce or prevent the presence of enteropathogens. Suitably, the B. longum transitional microorganism prebiotic is not for use to reduce or prevent the presence of enteropathogens in the gut of an infant and/or a young child. Suitably, the B. longum transitional microorganism and/or prebiotic may increase the levels of IL-6 in the infant or young child. IL-6 is secreted by macrophages in response to pathogen-associated molecular patterns (PAMPs). As such, IL-6 is an important component of fever and of the acute phase response. In addition, IL-6 is responsible for stimulating acute phase protein synthesis, as well as the production of neutrophils in the bone marrow. It supports the growth of B cells and is antagonistic to regulatory T cells. IL-6 has been shown to have an important role in preventing and/or controlling a number of infections including, for example, vaccinia virus and Listeria monocytogenes (Kopf et al; 1994; Nature; 368; 339-342); herpes simplex virus (LeBlanc et al; 1999; J Virol; 73(10)); influenza virus (Pyle et al.; 2017; PLoS Pathogens; 13(9), Dienz et al.; 2012; Mucosal Immunol; 5(3); 258-266, Gou et al; 2019; Front Immunol; 10:3102); enteric bacterial pathogens (Dann et al.; 2008; J Immunol; 180(10); 6816-6826); Escherichia coli (Dalrymple et al.; 1996; Infect Immun; 64(8): 3231-3235); Pulmonary Aspergillosis (Cenci et al.; 2001; J Infect Dis; 184(5); 610-617) and Candida albicans (van Enckevort et al.; 1999; Med Mycol; 37(6): 419-426). Suitably, the B. longum transitional microorganism and/or prebiotic may increase the levels of short-chain fatty acids (SFCA) in the infant or young child. Suitably, the SCFA may be selected from acetate (Ethanoate, C1:0), butyrate (Butanonate, C4:0) and/or propionate (Propanoate, C3:0). SCFAs are produced when dietary fiber is fermented in the colon. SCFAs have diverse physiological roles in body functions; they can affect the production of lipids, energy and vitamins; affect appetite and cardiometabolic health; and have roles in lowering blood pressure in experimental models. SCFAs have been shown to have an important role in preventing and/or controlling a number of infections and immune responses (Kim et al.; Cell Host & Microbe; 2016; 20(2); 202-214). For example, SCFAs have been shown to have a protective affect against RSV (Antunes et al.; Nat Comm; 2019; 10; 3273); influenza virus (Trompette; Immunity; 2018; 48(5); 992-1005 and Moriyama and Ichinobe; PNAS; 2018; 16(8); 3118-3125); viral bronchiolitis (Lynch et al.; J Exp Med; 2018; 215(2); 537-557) and general microbe infection (Schulthess et al.; Immunity; 2019; 50(2); 432-445). Notably, SCFA produced in the gut impacts systemic levels and local SFCA levels in other local organs, for example, the lungs. The cytokine and SCFA effects mediated by the present probiotic and/or prebiotic may be systemic. As such, the cytokine effects (e.g. increase in the levels of IL-6 and/or SCFAs) may systemically prevent or reduce the risk of an infection as described herein. The cytokine effects may occur locally in the gut, the lungs and/or the skin of the infant or young child. Suitably, the SCFA effects may systemically prevent or reduce the risk of an infection as described herein. The SCFA effects may occur locally in the gut, the lungs and/or the skin of the infant or young child. Suitably, the cytokine or SCFA effects may occur in the gut of the infant or young child. Suitably, the cytokine or SCFA effects may occur in the lungs of the infant or young child. Accordingly, the cytokine or SCFA effects may prevent or reduce the risk of an infection in a particular organ or system. Suitably, the B. longum transitional microorganism and/or prebiotic may modulate the permeability of the gut epithelial barrier of the infant or young child. Suitably, the B. longum transitional microorganism and/or prebiotic may decrease the permeability of the gut epithelial barrier. Increased permeability of the gut epithelial barrier may be associated with an increase crossing of e.g. pathogens across the intestinal epithelium. Accordingly, decreased permeability of the gut epithelial barrier may be associated with a decreased crossing of e.g. pathogens across the intestinal epithelium. The B. longum transitional microorganism and/or prebiotic may reduce and/or prevent an exacerbation of symptoms of an infection. For example, the B. longum transitional microorganism and/or prebiotic may reduce and/or prevent an exacerbation of symptoms caused by inflammation. The inflammation may be – for example – a pro-inflammatory response to an existing infection. The existing infection may be the present infection or a separate infection caused by a different infectious agent or pathogen. For example, the present examples show that a B. longum transitional microorganism reduced the level of increased permeability in a model of gut epithelial barrier function following a pro-inflammatory insult. Without wishing to be bound be theory, it is considered that the reduced gut epithelial barrier permeability following an inflammatory insult may reduce the number/levels of pathogens that pass through the gut epithelial barrier during an inflammatory episode and thus prevent and/or reduce the risk of an infection; and/or prevent and/or reduce the risk of an exacerbation of symptoms of an existing infection. Prebiotic The invention further provides a prebiotic for use in preventing and/or reducing the risk of an infection in an infant or young child by promoting the growth of a Bifidobacterium longum transitional microorganism in the gut of the infant or young child, wherein the prebiotic is: i. a glycan substrate, suitably selected from the group recited in any of Tables 1 to 3; and/or ii. a human milk oligosaccharide (HMO), suitably selected from the group consisting of 2’-O-fucosyllactose (2’-FL), 3’-O-fucosyllactose (3-FL), lactodifucotetraose/difucosyllactose (di-FL), 3’-O-sialyllactose (3’-SL), 6’-O- sialyllactose (6’-SL), lacto-N-tetraose (LNT) and lacto-N-neotetraose (LNnT) and any combination thereof. In another aspect, the invention provides a prebiotic for use in preventing and/or reducing the risk of an infection in an infant or young child by promoting the growth of a Bifidobacterium longum transitional microorganism in the gut of the infant or young child. Glycan Substrate / Carbohydrate-Active Enzymes (CAZymes) The B. longum transitional microorganisms encode a profile of Carbohydrate-Active Enzymes (CAZymes). Without wishing to be bound by theory, it is considered that targeting these CAZymes by, for example, providing suitable glycan substrates in the form of a prebiotic, may promote the growth and/or survival of the Bifidobacterium longum transitional microorganisms in the gut microbiota of an infant or young child. Suitably, promoting the growth and/or survival of the B longum transitional microorganism may refer to increasing the number and/or concentration of the B longum transitional microorganism in the gut microbiota. In particular, the CAZymes encoded by each of the Bifidobacterium longum transitional strains NCC 5000, NCC 5001, NCC 5002, NCC 5003 and NCC 5004, which were deposited with the Institute Pasteur according to Budapest Treaty on 11 ^th of May 2021 receiving the deposit numbers CNCM I-5683, CNCM I-5684, CNCM I.5685, CNCM I-5686 and CNCM I-5687, respectively; have been determined. Carbohydrate-active enzymes (CAZymes) are responsible for the synthesis and breakdown of glycoconjugates, oligo- and polysaccharides. They typically correspond to 1-5% of the genes in the living organism. Glycoconjugates, oligo- and polysaccharides play essential roles in many biological functions, for example as structure and energy reserve components and in many intra- and intercellular events. The Carbohydrate Active Enzyme (CAZy) classification is a sequence-based family classification system that correlates with the structure and molecular mechanism of CAZymes (www.cazy.org). CAZymes include glycoside hydrolyases (GH), glycosyltransferases (GT), polysaccharide lyases (PL), carbohydrate esterases (CE), and carbohydrate-binding module families (CBM) Suitably, the CAZyme may be a glycoside hydrolyase (GH). GHs catalyze the hydrolysis of glycosidic bonds between two or more carbohydrates or between a carbohydrate and a non- carbohydrate moiety. In most cases, the hydrolysis of the glycosidic bond is catalyzed by two amino acid residues of the enzyme: a general acid (proton donor) and a nucleophile/base. Depending on the spatial position of these catalytic residues, hydrolysis occurs via overall retention or overall inversion of the anomeric configuration. A GH classification system is provided by the CAZy classification. Herein, GHs are divided into families based on molecular function (e.g., GH1, GH2, GH3, GH4, etc.). These families are then further divided into subfamilies based on subgroups found within a family that share a more recent ancestor and, typically more uniform in molecular function (e.g., GH13_1, GH13_2, GH13_3, GH13_4, etc.). Table 1 provides details of CAZymes that are unique to the Bifidobacterium longum transitional strains (i.e., not encoded by Bifidobacterium longum suis/suillum, Bifidobacterium longum longum or Bifidobacterium longum infantis strains). Table 1 also provides a summary of the glycan substrate metabolized by each CAZyme and illustrative dietary fiber sources/ingredients. Table 1 Table 2 provides details of CAZymes that were present in at least one Bifidobacterium longum transitional strain but not present in at least one of the groups selected from the Bifidobacterium longum subsp. suis/suillum, Bifidobacterium longum subsp. longum or Bifidobacterium longum subsp. infantis strains presented in Figure 7. Table 2 also provides a summary of the glycan substrate metabolized by each CAZyme and illustrative dietary fiber sources/ingredients. Table 2 Table 3 provides details of CAZymes that were present in all Bifidobacterium longum strains analysed (i.e., Bifidobacterium longum transitional, Bifidobacterium longum subsp. suis/suillum, Bifidobacterium longum subsp. longum and Bifidobacterium longum subsp. infantis). Table 3 also provides a summary of the glycan substrate metabolized by each CAZyme and illustrative dietary fiber sources/ingredients. Table 3 Table 4 provides details of the CAZymes that are not encoded by Bifidobacterium longum transitional strains but are encoded by one or more of Bifidobacterium longum subsp. suis/suillum, Bifidobacterium longum subsp. longum and Bifidobacterium longum subsp. infantis. Table 4 Representative sequences for the CAZymes listed in Tables 1-4 are shown in Figure 7. Suitably, the CAZyme referred to in any of Tables 1-4 may comprise or consist of the corresponding sequence shown in Figure 7. Suitably, the CAZyme may comprise or consist of a variant of the corresponding sequence shown in Figure 7, which variant retains at least one of the functions of the corresponding CAZyme as recited in Table 1-4. Suitably, the variant may provide each of the functional activities of the corresponding CAZyme as recited in Table 1-4. Suitably, the variant may comprise or consist of an amino acid sequence which has at least 70% sequence identity to the sequence listed in Figure 7, and retains at least one of the functional activities, preferably each of the functional activities, of the corresponding CAZyme as recited in Table 1-4. Suitably, the variant may comprise or consist of an amino acid sequence, which has at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity to the corresponding sequence listed in Figure 7. The variant retains at least one of the functional activities, preferably each of the functional activities, of the corresponding CAZyme as recited in Table 1-4. Suitably, the prebiotic for use in the present invention may comprise a glycan substrate selected from the groups recited in any of Tables 1 to 3. Suitably, the prebiotic for use in the present invention may comprise a combination of glycan substrates selected from the groups recited in any of Tables 1 to 3. The combination of glycan substrates may comprise at least 2, at least 4, at least 10, at least 20, at least 30, at least 40 or at least 50 of the glycan substrates selected from the groups recited in Tables 1 to 3. The combination may comprise each of the glycan substrates recited in Tables 1 to 3. Suitably, the prebiotic may comprise one or more glycan substrates selected from the group recited in Table 1 or Table 2. The prebiotic may comprise at least 2, at least 4, at least 10, at least 20, or at least 30 of the glycan substrates recited in Tables 1 and 2. The prebiotic may comprise each of the glycan substrates recited in Tables 1 and 2. Suitably, the glycan substrate may comprise or consist of pectin, arabinogalactan and/or starch. Suitably, the glycan substrate may comprise or consist of pectin. Suitably, the glycan substrate may comprise or consist of arabinogalactan. Suitably, the glycan substrate may comprise or consist of starch. Suitably, the glycan substrate is provided in the form of a dietary fiber. For example, the dietary fiber may be a prebiotic fiber. Suitably, the glycan substrate may be comprised in an ingredient, for example a dietary ingredient. The ingredient containing one or several glycan substrates may be selected from the group consisting of purified polysaccharide or purified oligosaccharide, a dietary fiber ingredient, a semi-purified food ingredient, a raw food ingredient, a food additive, a HMO, a semi-purified or purified peptido-glycan. The semi-purified food ingredient may be a fruit, vegetable or cereal extract. The raw food ingredient may be a fruit, vegetable, cereal, algae or microalgae. The food additive may be a guar gum or gum arabic. Suitably, the peptide-glycan may be a GAG. Suitably, the glycan substrate may be comprised in a purified fiber. Illustrative ingredients and/or purified fibers comprising suitable glycan substrates are provided in Tables 1 to 3. In particular, dietary fibers and/or ingredients that comprise a given glycan substrate are identified in the same row as the glycan substrate. The pectin may be comprised in fruit or vegetable pectin. Accordingly, suitable ingredients comprising pectin include, but are not limited to, fruits (e.g., apple, pear), vegetables, legumes (peas), and roots (e.g., sugar beet). Suitable purified fibers comprising arabinogalactan include peach pectin. Suitably, the pectin extracted from sugar beet contains arabinan, galactans and arabinogalactans and may be provided as an ingredient. The arabinogalactan may be comprised in fruit or vegetable pectin. Illustrative suitable ingredients comprising arabinogalactan include, but are not limited to, fruits, vegetables, whole grain cereals and sea weed dietary fiber. Suitable purified fibers comprising arabinogalactan include peach pectin, larch wood arabinogalactan, and Arabic gum. Suitably, the arabinogalactan may be provided in larch wood arabinogalactan. The starch may be comprised in resistant-starch from cereals (whole grains), legumes, vegetables (e.g., corn) and roots (e.g., potato). Illustrative suitable ingredients comprising starch include, but are not limited to, corn. Suitable purified fibers comprising starch include high amylose starch and resistant dextrin. Suitably, the starch may be provided in a potato, corn or other ingredient. Suitably, the starch may be comprised in a potato ingredient. Human milk oligosaccharide (HMO) Suitably, the prebiotic comprises an HMO. Suitably, the HMO is capable of being metabolized by the B longum transitional microorganism. Suitably, the HMO may be capable of promoting growth and/or survival of the B. longum transitional strain. HMOs capable of promoting growth and/or survival of the B. longum transitional strain may be determined by e.g. anaerobic culture of the B. longum transitional strain with the HMO to be tested. Growth and/or survival of the B. longum transitional strain may be determined by measuring bacteria cell number, cell density (e.g. measured by optical density) and/or the abundance of 16S rDNA – for example using PCR methods. An illustrative assay for measuring growth of a B. longum transitional strain in the presence of HMOs is provided in present Example 6. An HMO capable of promoting growth and/or survival of the B. longum transitional strain may increase the number of B. longum transitional bacteria in an anaerobic culture by at least 20%, at least 30%, at least 40%, at least 50%, at least 75% or at least 100% compared to the number of B. longum transitional bacteria in a control anaerobic culture which does not comprise the HMO. Suitably, HMO capable of promoting growth and/or survival of the B. longum transitional strain may increase the number of B. longum transitional bacteria in an anaerobic culture by a statistically signifiicant amount (e.g. p-value <0.05 as determined by one-way ANOVA) compared to the number of B. longum transitional bacteria in a control anaerobic culture which does not comprise the HMO. The HMO may be a fucosylated oligosaccharide (i.e. an oligosaccharide having a fucose residue; e.g.2’ fucosyllactose (2-FL), 3-fucosyllactose (3-FL), difucosyllactose (DiFL), lacto- N-fucopentaose (e.g. lacto-N-fucopentaose I, lacto-N-fucopentaose II, lacto-N-fucopentaose III, lacto-N-fucopentaose V), lacto-N-fucohexaose, lacto-N-difucohexaose I, fucosyllacto-N- hexaose, fucosyllacto-N-neohexaose, difucosyllacto-N-hexaose I, difucosyllacto-N- neohexaose II and any combination thereof), an N-acetylated oligosaccharide (e.g. LNT (lacto-N-tetraose), para-lacto-N-neohexaose (para-LNnH), LNnT (lacto-N-neotetraose), DSLNT (disialyllacto-N-tetraose), lacto-N-hexaose, lacto-N-neohexaose, para- lacto-N- hexaose, para-lacto-N-neohexaose, lacto-N-octaose, lacto-N- neooctaose, iso- lacto-N- octaose, para- lacto-N-octaose and lacto-N-decaose and any combinations thereof) and/or a sialylated oligosaccharide (e.g. 3’-sialyllactose (3-SL), 6’-sialyllactose (6-SL), or Lst (sialyllacto-N-tetraose), Lst-a, Lst-b or Lst-c)). The prebiotic may comprise at least one prebiotic oligosaccharide selected from the group consisting of: 2’-O-fucosyllactose (2’FL), 3’-O-fucosyllactose (3FL), lactodifucotetraose/difucosyllactose (DFL), 3’-O-sialyllactose (3-SL), 6’-O- sialyllactose (6- SL), lacto-N-tetraose (LNT) and lacto-N-neotetraose (LNnT); and any combination thereof. The prebiotic may comprise 34 wt% to 85 wt% of 2’-FL, 10 wt% to 40 wt% of LNT, 4 wt% to 14 wt% of DFL and 9 wt% to 31 wt% of 3-SL and 6-SL combined. In some embodiments, the prebiotic comprises - 26 wt% to 65 wt% of 2’-FL, preferably 32 wt% to 54 wt%; - 10 wt% to 40 wt% of LNT, preferably 11 wt% to 20 wt%; - 4 wt% to 14 wt% of DFL, preferably 4 wt% to 8 wt%; - 9 wt% to 31 wt% of 3’-SL and 6’-SL combined, preferably 8 wt% to 22 wt%; and - 12 wt% to 38 wt % of 3-FL, preferably 17 wt% to 31 wt%. The prebiotic may comprise between 0.001 g/L to 12 g/L of 2’-FL, preferably between 0.002 g/L to 10 g/L of 2’-FL, more preferably between 0.005 g/L to 5 g/L of 2’-FL. The prebiotic may comprise between 0.001 g/L to 5 g/L of DFL, preferably between 0.002 g/L to 4 g/L of DFL, more preferably between 4 g/L to 3 g/L of DFL. The prebiotic may comprise between 0.01 g/L to 6 g/L of LNT, preferably between 0.025 g/L to 5 g/L of LNT, more preferably between 0.05 g/L to 1 g/L of LNT. The prebiotic may comprise between 0.001 g/L to 2 g/L of 6’-SL, preferably between 0.002 g/L to 1.5 g/L of 6’-SL, more preferably between 0.005 g/L to 1 g/L of 6’-SL. The prebiotic may comprise between 0.01 g/L to 2 g/L of 3’-SL, preferably between 0.025 g/L to 1.5 g/L of 3’-SL, more preferably between 0.05 g/L to 1 g/L of 3’-SL. The prebiotic may comprise between 0.01 g/L to 7 g/L of 3-FL, preferably between 0.025 g/L to 6 g/L of 3-FL, more preferably between 0.05 g/L to 5 g/L of 3-FL. Suitably, the 3’-O-fucosyllactose (3’FL) and lacto-N-tetraose (LNT) comprised in the prebiotic promote the growth of a Bifidobacterium longum transitional microorganism that preferentially utilizes 3- fucosyllactose (3-FL) over 2’ -fucosyllactose (2’-FL). Combination of a B longum transitional microorganism and a prebiotic The invention further provides a combination of a B longum transitional microorganism and a prebiotic for use according to the present invention. The B longum transitional microorganism and prebiotic may be administered separately, simultaneously or sequentially. Suitably, the B longum transitional microorganism and prebiotic may be administered in a combined composition. Suitably, a combination of a B longum transitional microorganism and a prebiotic may be referred to as a “synbiotic”. In aspects of the invention in which a combination of a B longum transitional microorganism and a prebiotic (e.g. a glycan substrate) are used, each may be selected such that the B longum transitional microorganism is capable of metabolising the glycan substrate provided in the combination. Such a selection may be made, for example, by selecting a B longum transitional microorganism that encodes a CAZyme from the same row of Tables 1-3 as the glycan substrate (or selecting an ingredient comprising said glycan substrate). The combinations of the invention are not limited to requiring that the B longum transitional microorganism is capable of metabolizing the glycan substrate provided in the combination. As such, any combinations of B longum transitional microorganism(s) and glycan substrates disclosed herein are encompassed by the invention. Suitably, the composition comprises one or more glycan substrates as described herein. Suitably, the composition comprises B longum transitional preferentially utilizing 3- fucosyllactose (3-FL) over 2’ -fucosyllactose (2’-FL) mixed with 3’-O-fucosyllactose (3-FL) and lacto-N-tetraose (LNT). The composition may comprise between 10 ³ to 10 ¹² cfu of probiotic strain, more preferably between 10 ⁷ and 10 ¹² cfu such as between 10 ⁸ and 10 ¹⁰ cfu of probiotic strain per g of composition on a dry weight basis mixed with 3-Fl in an amount between 0.01 g/L to 7 g/L of 3-FL, preferably between 0.025 g/L to 6 g/L of 3-FL, more preferably between 0.05 g/L to 5 g/L of 3-FL and with LNT in an amount between 0.01 g/L to 6 g/L of LNT, preferably between 0.025 g/L to 5 g/L of LNT, more preferably between 0.05 g/L to 1 g/L of LNT. Compositions The B longum transitional microorganism, prebiotic or synbiotic for use in the present invention may be provided in the form of a composition. The composition may suitably be administered to an individual, for example an infant or a young child, in any suitable form such as a nutritional composition in a dosage unit (for example a tablet, a capsule, a sachet of powder, etc). The composition may be in powder, semi-liquid or liquid form. The composition may be added to a nutritional composition, an infant formula, a food composition, a supplement for infant or young child, a baby food, a follow-up formula, a growing-up milk, an infant cereal or a fortifier. In some embodiments, the composition of the present invention is an infant formula, a baby food, an infant cereal, a growing-up milk, a supplement or fortifier that may be intended for infants or young child. By way of example, the composition may comprise further components which may be beneficial in preventing and/or reducing the risk of an infection. In addition, or alternatively, the composition may comprise further components may be beneficial during the weaning period. The B longum transitional microorganism can be included in the composition in an amount from about 10 ³ to 10 ¹² cfu of probiotic strain, more preferably between 10 ⁷ and 10 ¹² cfu such as between 10 ⁸ and 10 ¹⁰ cfu of probiotic strain per g of composition on a dry weight basis. In one embodiment, the B longum transitional microorganism is viable. In another embodiment the B longum transitional microorganism is non-replicating or inactivated. There may be both viable and inactivated Bifidobacterium longum transitional microorganisms in some other embodiments. Suitably, the composition comprises one or more glycan substrates as described herein. In some embodiments, the composition comprises at least one prebiotic oligosaccharide selected in the group consisting of 2’-O-fucosyllactose (2FL), 3’-O-fucosyllactose (3FL), lactodifucotetraose/difucosyllactose (DFL), 3’-O-sialyllactose (3’-SL), 6’-O- sialyllactose (6’- SL) and lacto-N-tetraose (LNT) and any combination thereof. In some embodiments, the composition comprises - 26 wt% to 65 wt% of 2’-FL, preferably 32 wt% to 54 wt%; - 10 wt% to 40 wt% of LNT, preferably 11 wt% to 20 wt%; - 4 wt% to 14 wt% of DFL, preferably 4 wt% to 8 wt%; - 9 wt% to 31 wt% of 3’-SL and 6’-SL combined, preferably 8 wt% to 22 wt%; and - 12 wt% to 38 wt % of 3-FL, preferably 17 wt% to 31 wt%. The composition may comprise between 0.001 g/L to 12 g/L of 2’-FL, preferably between 0.002 g/L to 10 g/L of 2’-FL, more preferably between 0.005 g/L to 5 g/L of 2’-FL. The composition may comprise between 0.001 g/L to 5 g/L of DFL, preferably between 0.002 g/L to 4 g/L of DFL, more preferably between 4 g/L to 3 g/L of DFL. The composition may comprise between 0.01 g/L to 6 g/L of LNT, preferably between 0.025 g/L to 5 g/L of LNT, more preferably between 0.05 g/L to 1 g/L of LNT. The composition may comprise between 0.001 g/L to 2 g/L of 6’-SL, preferably between 0.002 g/L to 1.5 g/L of 6’-SL, more preferably between 0.005 g/L to 1 g/L of 6’-SL. The composition may comprise between 0.01 g/L to 2 g/L of 3’-SL, preferably between 0.025 g/L to 1.5 g/L of 3’-SL, more preferably between 0.05 g/L to 1 g/L of 3’-SL. The composition may comprise between 0.01 g/L to 7 g/L of 3-FL, preferably between 0.025 g/L to 6 g/L of 3-FL, more preferably between 0.05 g/L to 5 g/L of 3-FL. In some embodiments, the composition comprises Bifidobacterium longum transitional microorganism preferentially utilizing 3- fucosyllactose (3-FL) over 2’ -fucosyllactose (2’-FL) mixed with 3’-O-fucosyllactose (3-FL) and lacto-N-tetraose (LNT). The composition may comprise between 10 ³ to 10 ¹² cfu of probiotic strain, more preferably between 10 ⁷ and 10 ¹² cfu such as between 10 ⁸ and 10 ¹⁰ cfu of probiotic strain per g of composition on a dry weight basis mixed with 3-Fl in an amount between 0.01 g/L to 7 g/L of 3-FL, preferably between 0.025 g/L to 6 g/L of 3-FL, more preferably between 0.05 g/L to 5 g/L of 3-FL and with LNT in an amount between 0.01 g/L to 6 g/L of LNT, preferably between 0.025 g/L to 5 g/L of LNT, more preferably between 0.05 g/L to 1 g/L of LNT. Suitably, the 3’-O-fucosyllactose (3’FL) and lacto-N-tetraose (LNT) comprised in the composition promote the growth of a Bifidobacterium longum transitional microorganism that preferentially utilizes 3- fucosyllactose (3-FL) over 2’ -fucosyllactose (2’-FL). Methods In another aspect, the present invention provides a method for preventing and/or reducing the risk of an infection in an infant or young child; wherein the method comprises administered an effective amount of a Bifidobacterium longum transitional microorganism, a prebiotic or a combination of a Bifidobacterium longum transitional microorganism and a prebiotic to a subject in need thereof. In a further aspect, the present invention relates to the use of Bifidobacterium longum transitional microorganism, a prebiotic or a combination of a Bifidobacterium longum transitional microorganism and a prebiotic for the preparation of a medicament for preventing and/or reducing the risk of an infection in an infant or young child. The Bifidobacterium longum transitional microorganism may be a Bifidobacterium longum transitional microorganism as described herein. The prebiotic may be a prebiotic as described herein. The combination of a Bifidobacterium longum transitional microorganism and a prebiotic may be provided in any form as described herein. For example, the combination may be provided in a composition as described herein. Embodiments The present invention provides the embodiments according to the following numbered clauses: 1. A Bifidobacterium Longum transitional microorganism for use in preventing and/or reducing the risk of an infection in an infant or young child. 2. A prebiotic for use in preventing and/or reducing the risk of an infection in an infant or young child; wherein the prebiotic is a glycan substrate or a human milk oligosaccharide (HMO). 3. The Bifidobacterium Longum transitional microorganism for use according to clause 1 wherein the Bifidobacterium Longum transitional microorganism is used in combination with a prebiotic selected from a glycan substrate or a human milk oligosaccharide (HMO). 4. A prebiotic for use according to clause 2, wherein the prebiotic is used in combination with a Bifidobacterium Longum transitional microorganism. 5. A combination of a Bifidobacterium Longum transitional microorganism and a prebiotic for use in preventing and/or reducing the risk on an infection in an infant or young child; wherein the prebiotic is selected from glycan substrate or a human milk oligosaccharide (HMO). 6. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any of clauses 2-5 wherein the prebiotic is a glycan substrate selected from the group recited in any of Tables 1 to 3. 7. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any of clauses 2-5, wherein the prebiotic is a HMO selected from the group consisting of 2’-FL, 3-FL, di-FL, 3’-SL, 6’-SL, LNT and LNnT, and any combination thereof. 8. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any one of clauses 2-5, wherein the HMO is 3-FL. 9. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any one of the preceding clauses, wherein the Bifidobacterium longum transitional microorganism is capable of metabolizing the HMO(s) and/or the glycan substrate(s). 10. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any one of the preceding clauses wherein the Bifidobacterium longum transitional microorganism preferentially utilizes 3’- fucosyllactose (3’-FL) over 2’- fucosyllactose (2’-FL). 11. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any of the preceding clauses, wherein the Bifidobacterium longum transitional microorganism is capable of metabolizing a glycan substrate selected from the group recited in any of Tables 1 to 3. 12. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any of the preceding clauses wherein the Bifidobacterium longum transitional microorganism encodes one or more CAZymes selected from the group recited in Table 1, preferably wherein the Bifidobacterium longum transitional microorganism further encodes one or more CAZymes selected from Table 2 and 3. 13. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any of the preceding clauses, wherein the Bifidobacterium longum transitional microorganism has an Average Nucleotide Identity (ANI) of at least 98% with at least one Bifidobacterium longum strain selected in the group consisting of CNCM I-5683, CNCM I-5684, CNCM I-5685, CNCM I-5686, CNCM I-5687, CMCC-P0001 (ATCC BAA-2753), and any combination thereof. 14. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any of the preceding clauses, wherein the Bifidobacterium longum transitional microorganism has an Average Nucleotide Identity (ANI) of at least 98% with at least one Bifidobacterium longum strain selected in the group consisting of CNCM I-5683, CNCM I-5684, CNCM I-5685, CNCM I-5686, CNCM I-5687, and any combination thereof. 16. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any one of the preceding clauses, wherein the Bifidobacterium longum transitional microorganism and/or prebiotic increases the levels of IL-6 in the infant or young child. 17. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any one of the preceding clauses, wherein the Bifidobacterium longum transitional microorganism and/or prebiotic increases the levels of short-chain fatty acids (SCFA) in the infant or young child. 18. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to clause 16 wherein the SCFA is selected from acetate, butyrate and/or propionate. 19. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any of the preceding clauses, wherein the Bifidobacterium longum transitional microorganism and/or prebiotic modulates the permeability of the gut epithelial barrier; preferably wherein the Bifidobacterium longum transitional microorganism and/or prebiotic decreases the permeability of the gut epithelial barrier. 20. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any of the preceding clauses, wherein the infection is a viral, bacterial or fungal infection. 21. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any one of the preceding clauses, wherein the infection is an airway infection. 22. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to clause 21, wherein the infection is a viral airway infection; suitably selected from influenza virus, respiratory syncytial virus, parainfluenza viruses, metapneumovirus, rhinovirus, coronaviruses, adenoviruses, and bocaviruses. 23. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to clause 22, wherein the viral airway infection is influenza virus, respiratory syncytial virus or rhinovirus. Those skilled in the art will understand that they can freely combine all features of the present invention disclosed herein. In particular, features described for the product of the present invention may be combined with the method of the present invention and vice versa. Further, features described for different embodiments of the present invention may be combined. Where known equivalents exist to specific features, such equivalents are incorporated as if specifically referred to in this specification. Further advantages and features of the present invention are apparent from the figures and non-limiting examples. Examples Example 1: Transitional B. longum increase short-chain fatty acid production 3-fucosylactose (3FL), short-chain fatty acid (SCFA), tricarboxylic acid (TCA) intermediates and SCFA intermediates were measured by 1H-NMR technique. Results are shown in Figures 2 and 3. The heatmaps highlight the dynamic of consumption and production of key metabolites in SCFAs pathways by showing the Z score of each metabolite abundance at T0, T24 and T48. Total SCFAs corresponds to the sum of the peak integrals of acetate, butyrate, and propionate. Significant difference in metabolite Z score between Bifidobacterium longum transitional or Bifidobacterium longum spp infantis and no supplementation is calculated with ANOVA and highlighted with a star symbol (* p-value<0.05, ** p-value<0.01, *** p- value<0.001). Significant difference in metabolite Z score between Bifidobacterium longum transitional and Bifidobacterium longum spp infantis is calculated with ANOVA and highlighted with a round symbol (°p-value<0.05, ° ° p-value<0.01, ° ° ° p-value<0.001). The box plots indicate the strain abundance (i.e. the strain specificity gene copy measured by qPCR) of Bifidobacterium longum transitional or Bifidobacterium longum spp infantis over 48h or fermentation. Figure 2 and Figure 3 are proofs of concept that Bifidobacterium longum transitional is well implanted in the microbial community, is metabolically active on 3FL or Pea fiber and produces more SCFAs than Bifidobacterium longum subsp. Infantis. Figure 2 shows SCFAs production (i.e acetate, butyrate and propionate) over 48h of batch fermentation with 3-fucosylactose (3FL). Figure 3 shows SCFAs production (i.e acetate, butyrate and propionate) over 48h of batch fermentation with pea fiber (rich in arabinan). Example 2: Transitional B. longum increases the anti-infection cytokine, IL-6 Monocytes were isolated from the buffy coat of healthy donors. One hundred thousand monocytes were seeded in each well of a 96-well plate and incubated with 1e6 CFU of B. longum transitional for 24 hours for immune training. Cells were washed by centrifugation and allowed to rest for 6 days. Monocytes were stimulated with LPS for 24hrs. IL-6 was thereafter measured in the cell culture supernatants to assess immune training (see Figure 4). Bars indicate the median IL-6 production from 3 donors with dotted line indicating the IL-6 level by untrained monocytes. Method Immunoprofiling with PBMC cells Peripheral blood mononuclear cells (PBMC) were isolated from buffy coats obtained from healthy adults by density gradient. PBMC were then seeded at 1.5x10 ⁶ cells /ml in a 48-well bottom plate in complete Isocove’s modified Dulbecco’s medium (cIMDM) containing 10% fetal bovine serum, 1% glutamine, 1% penicillin/streptomycin and 0.1% gentamycin. PBMC were stimulated for 36 hours in the presence of different bacterial strains including all transitional B longum isolates at 10 ⁷ CFU/ml and probiotic strains. Cell culture supernatants were collected to assess cytokine expression for IL-10 and IL-12p40 by ELISA. Standard curve for each cytokine was used to calculate absolute amount (picogram/ml) from optical density readouts. Example 3: Transitional B. longum increases gut epithelial barrier resistance In vitro experiments using a human colorectal adenocarcinoma cell line (Caco-2) have shown that the transitional B.longum strains were able to increase the transepithelial electrical resistance (TEER) when incubated with the epithelial cells. Caco-2 cells were seeded on Transwell and grown for 3 weeks. Caco-2 monolayers were pre- incubated with transitional B. longum NCC5002 (black line) at 4.10 ⁶ CFU/well, B lactis NCC2818 (grey line) at 4.10 ⁶ CFU/well or vehicle (dotted line) in the presence of 10ng/mL IFNγ for 24 hours (0-24). After that period, cells were challenged with 50ng/mL TNFα proinflammatory cytokine for another 24 hours (24-48) followed by a recovery phase of 24 hours (48-72). Transepithelial electrical resistance was measured at 0, 24, 48 and 72 hours. Data is represented as mean ± SD. For each time point, statistical difference was assessed using two-way ANOVA with Dunnett test for multiple comparison and represented by asterisks or hash marks for NCC5002 and NCC2818, respectively. * ^/# = p<0.05; ** ^/## =P<0.01 compared to control group (Figure 5). Caco-2 cells were seeded on Transwell and grown for 3 weeks. Caco-2 monolayers were pre- incubated with transitional B. longum NCC5002 (black line) at 4.10 ⁶ CFU/well, B lactis NCC2818 (grey line) at 4.10 ⁶ CFU/well or vehicle (dotted line) in the presence of 10ng/mL IFNγ for 24 hours (0-24). After that period, cells were challenged with 50ng/mL TNFα proinflammatory cytokine for another 24 hours (24-48) followed by a recovery phase of 24 hours (48-72). At the 72-hour timepoint, permeability of the caco-2 monolayers was assessed by measuring the flux of fluorescein sulfonic acid (478 Daltons) across the epithelium for 180 minutes. Data is represented as mean ± SD. For each time point, statistical difference was assessed using two-way ANOVA with Dunnett test for multiple comparison and represented by asterisks. *= p<0.05 compared to control group (Figure 6). Method CACO-2 cells culture and transepithelial electrical resistance measurement Caco-2 cells (HTB-37; American Type Culture Collection) were seeded in 24-well semi- permeable inserts. Caco-2 monolayers were cultured for 14 days, with three medium changes/week, until a functional cell monolayer with a transepithelial electrical resistance (TEER) was obtained. Cells were maintained in Dulbecco’s Modified Eagle Medium (DMEM) containing glucose and glutamine and supplemented with HEPES and 20% (v/v) heat- inactivated fetal bovine serum. Before addition of bacteria to the apical compartment, the TEER of the Caco-2 monolayers was measured (= 0h time point). The TEER of an empty insert was subtracted from all readings to account for the residual electrical resistance of an insert. Then, probiotic strains (directly taken from a glycerol stock) were diluted in Caco-2 complete medium and apically added to the Caco-2-bearing inserts at 2x10E6 colony-forming unit. Cells were also exposed to Caco-2 complete medium (CM) in both chambers as control and to 0.75% glycerol in the apical compartment as vehicle control. Cells were treated for 24h and TEER was measured at several time points (2h, 4h, 6h and 24h). After subtracting the TEER of the empty insert, all timepoint values were normalized to its own 0h value (to account for the differences in initial TEER of the different inserts) and are presented as percentage of initial value. Example 4: Analysis of Carbohydrate Active Enzyme (CAZyme) genes of Bifidobacterium longum transitional microorganism Genomes of Bifidobacterium longum subspecies listed in Figure 7 were annotated to CAZymes combining dbCAN2 (Zhang et al., Nucleic Acids Res.46(W1):W95-W101 (2018)) tools and databases HMMdb (v9) and Diamond (v2.0.8). Query sequences with > 0.50 coverage and e-value < 1e-15 were annotated with HMMER according to the dbCAN CAZyme domain HMM database. Diamond was also used to annotate query sequences with hits in the CAZy database (Drula et al., Nucleic Acids Res. 50(D1):D571-D577 (2022)) (http://www.cazy.org/) with > 0.90 identity, and e value < 1e-102. HMMER annotation was prioritized and used in instances of mismatched CAZyme annotations of query sequences between HMMER and DIAMOND tools. Only CAZyme families and subfamilies encoding Glycoside Hydrolases (GHs) and Polysaccharide Lyases (PLs) were used for comparative analyses of B. longum subspecies (see Figure 7). Example 5: Utilization of glycan substrates Pulverized or homogenized stool samples were mixed 10-fold by adding PBS/glycerol (1/10) (w/v) before centrifugation at 2000g for 2 minutes. The slurry and pellet were then stored at - 80°C. Frozen fecal samples were thawed from storage at -80°C before centrifugation at 2000g for 2 minutes. The resulting supernatant was inoculated with media based on that disclosed in Daguet et al. (Journal of Functional Foods; 2016; 20; 369-379). This media was supplemented with specific fibers to be tested at 5 g/L and a Bifidobacteria supplement of 5E07 CFU/ml. The culture was set up at 37°C, N2 gas flow to ensure anaerobic conditions and gentle stirring. Aliquots were taken and analyzed at the time points indicated. B. longum transitional strain NCC5001 growth is promoted by pectin (sugar beet) and arabinogalactan (larch wood) (Figure 8). B. longum transitional strain NCC5002 growth is promoted by arabinogalactan (larch wood) and starch (potato) (Figure 9). Example 6: Characterization of B.longum transitional microorganism B. longum transitional strains were isolated from the feces of breast-fed infants using Eugon Tomato Agar (ETA). Obtained isolates were sequenced using PacBio to obtain a fully closed assembled genome for each of the strain. Each strain was deposited in the internal Nestlé Culture Collection (NCC, Lausanne, Switzerland) and at the Collection Nationale de Microorganisms (CNCM) at the Pasteur Institute (Paris, France) together with their genome sequence data. The genome of the strains was compared by Average Nucleotide Identity (ANI) using OrthoAni (https://www.ezbiocloud.net/tools/orthoani) to other publicly available genomes representing the overall diversity of the B. longum species (Table 5), and to the Metagenomic Assembled Genomes (MAG) obtained from metagenomic sequences issued from infant feces of the same cohort. Table 5 – list of genomes used for ANI analysis and their publicly available references. (T) stands for typestrain. _ _ The analysis demonstrates that the newly described strains group together with the MAGs obtained from the same cohort, defining a well delineated clade belonging to the B. longum species. Two previously isolated strains BSM11-5 and 3_mod are found to be grouped within this newly described clade. The clade is genetically different from B. longum subspecies longum (96.40 % ANI) subspecies. The clade is related, while still clearly distinct, to B. longum subspecies. suis/suillum (98.207%), and to the group of strains (JDM301, CMCC_P0001 and BXY01) previously suggested to be a new B. longum subspecies (O’Callaghan et al.2015), sharing an identity of 98.260 % to this group of strains. Figure 1 shows ANI UPGMA based phylogenetic tree. The scale represents the percentage (%) of identity at each branch point. A selection of the above mentioned genomes, representing the diversity of the B. longum subspecies, were annotated for Carbohydrate-Active enZYmes(CAZY) using the dbCAN annotation pipeline (http://bcb.unl.edu/dbCAN/). Results showed that B. longum subsp. longum, B. longum subsp. Suis and B. longum subsp.suillum strains contained a GH20 (lacto- N-biosidase) enzyme, implicated in the degradation and metabolization of Lacto-N-tetraose (LNT). Similarly to B. longum subsp.infantis strains, B.longum transitional strains also possessed a similar enzyme, and in addition harbored GH29 (fucosidase) encoding genes which are implicated in the degradation and metabolization of fucosylated human milk oligo- saccharides, such as 2’FL, 3’FL or diFL. Additionally, three of the strains (CNCM I-5684, BSM1-15& 3_mod) also harbor a GH 33 (sialidase) encoding gene implicated in the degradation and metabolization of sialilated HMO such as 3’SL or 6’SL (Table 6). Table 6 – Number of genes encoding for GH20 (lacto-N-biosidase), GH29 (α-fucosidase), GH95 ((α -fucosidase/( α -galactosidase) and GH33 (sialidase) glucohydrosylhydrase family enzymes in each of the represented genomes. CAZYmes_pred Genome N° of N° of N° of N° of reference predicted GH20 predicted GH29 predicted predicted encoding genes encoding genes GH95 GH33 encoding encoding All newly obtained genomes were compared and aligned with the genome of two strains (B. longum subsp. Infantis ATCC15697 and B. kashiwanohense DSM 21854) belonging to species for which the genes responsible for fucosylated HMOs utilization were elucidated (James et al.2019). Results As shown in Figure 11, all newly described strains contained genes responsible for the utilization of fucosylated HMOs. While NCC 5001 organization reflects the one of B. longum subsp. Infantis ATCC 15697, all other strains (NCC 5000, NCC 5002, NCC 5003, NCC 5004) harbor a gene organization closer to that of B. kashiwanohense DSM 21854. Overall, the similarity to the well described fucosidases of B. longum subsp. Infantis ATCC 15697 is above 77% (for BLON_2334) and 88% (BLON_2335) in all newly described strains. Example 7: Utilization of fucosylated HMOs All strains retrieved from the Nestlé Culture Collection (Table 7) were reactivated from a freeze-dried stock, using two successive culturing steps (16h, 37°C, anaerobiosis) in MRS supplemented with 0.05 % cysteine (MRSc). Reactivated cultures were then centrifuged, washed and resuspended in 1 volume of PBS. Washed cells were used to inoculate MRS based medium without a carbon source (MRSc-C) (10 g l-1 of bacto proteose peptone n°3, 5 g l-1 bacto yeast extract, 1 g l-1 Tween 80, 2 g l-1 di-ammonium hydrogen citrate, 5 g l-1 sodium acetate, 0.1 g l-1 magnesium sulphate, 0.05 g l-1 manganese sulfate, 2 g l-1 di-sodium phosphate, 0.5 g l-1 cysteine) in which glucose, 2’FL or 3’FL were added as unique carbon source at a concentration of 0.5%. Growth was then performed in a 96 well microplate, with a volume of 200 µl per well. Incubation was performed in anaerobiosis for 48h, and optical density was measured in a spectrophotometer at 600 nm. As shown in Figure 9, all B.longum transitional strains grew on fucosylated HMOS. Results All B.longum transitional strains grew better on 3’FL than 2’FL and reached a higher cell density on this carbohydrate. This behavior indicates a preference for 3’FL over 2’FL (ratios from 1.8 to 2.8- see Figure 12), which is not observed in B. longum subsp. infantis LMG 11588. Table 7 – list of the strains (and corresponding numbers) used for the individual fucosylated HMO growth studies Example 8: Preclinical model for efficacy testing of B. l. iuvenis in infection model An in-vivo preclinical model of infection was developed, as shown in Figure 14. At post natal day (PND) 5, C57BL/6 WT pups received different combinations of nutritional ingredients (HMOs + probiotic mixture) via oral gavage while being nursed by mothers fed on low fiber diet . Broad antibiotics were supplied through the drinking water from PND16 to 26. Following weaning on PND21, a selective fiber mix (adapted to B. longum transitional strain) was introduced in the diet of these mice coupled with oral gavage of the same nutritional ingredients (reduced dose of HMOs + probiotic mixture). Infection with pneumonia virus of mice was performed at PND35. Control groups were nursed by mothers fed either with a low fiber diet only (susceptible group) or with a high fiber diet only (protected group) before weaning and kept on the same diet after weaning. As shown in Figure 15, the nutritional composition containing the B. longum transitional strain mix confers better protection against airway viral infection post weaning, as shown by the increased weight (%PND35) on the days post infection when compared to the low fibre diet, or the low fibre diet mix without the B. longum transitional strain. All publications mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described methods and system of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in molecular biology or related fields are intended to be within the scope of the following claims.

CLAIMS 1. A Bifidobacterium Longum transitional microorganism for use in preventing and/or d i h ik f i f i i i f hild ant or haride aim 1 h i h Bifidb i L ii l i i i d i bi i ih Table 1, preferably wherein the Bifidobacterium longum transitional microorganism further encodes one or more CAZymes selected from Table 2 and 3. 10. The Bifidobacterium longum transitional microorganism, prebiotic or combination for use according to any of the preceding claims, wherein the Bifidobacterium longum transitional microorganism has an Average Nucleotide Identity (ANI) of at least 98% with at least one

ABSTRACT Uses of Bifidobacterium longum transitional microorganism