Human milk glycobiome and its impact on the infant gastrointestinal microbiota

Abstract

Human milk contains an unexpected abundance and diversity of complex oligosaccharides apparently indigestible by the developing infant and instead targeted to its cognate gastrointestinal microbiota. Recent advances in mass spectrometry-based tools have provided a view of the oligosaccharide structures produced in milk across stages of lactation and among human mothers. One postulated function for these oligosaccharides is to enrich a specific "healthy" microbiota containing bifidobacteria, a genus commonly observed in the feces of breast-fed infants. Isolated culture studies indeed show selective growth of infant-borne bifidobacteria on milk oligosaccharides or core components therein. Parallel glycoprofiling documented that numerous Bifidobacterium longum subsp. infantis strains preferentially consume small mass oligosaccharides that are abundant early in the lactation cycle. Genome sequencing of numerous B. longum subsp. infantis strains shows a bias toward genes required to use mammalian-derived carbohydrates by comparison with adult-borne bifidobacteria. This intriguing strategy of mammalian lactation to selectively nourish genetically compatible bacteria in infants with a complex array of free oligosaccharides serves as a model of how to influence the human supraorganismal system, which includes the gastrointestinal microbiota.

Fig. 1.

Human milk composition. (Left) Macronutrient composition of pooled human milk, with lactose being the most abundant component at 70 g/L, followed by lipids at 40 g/L. The third most abundant component is HMO at an estimated 5–15 g/L, followed by protein at 8 g/L. (Right) Pull-out pie chart showing composition of the most abundant HMOs. Masses of individual HMO structures are shown, along with their relative abundances, which were calculated from peak intensities presented in Ninonuevo et al. (21).

Fig. 2.

Characteristic HMO structures. The structures of several characteristic HMO structures found across human milk samples, along with their isomers, are shown. Two isomers with mass 709.3: (A) LNT, lacto-N-tetraose and (B) LNnT, lacto-N-neotetraose; (C) mass 1511.6: MSMFLNnH, monofucosylmonosialyllacto-N-hexaose; four isomers with mass 855.3: (D) LNFP I, lacto-N-fucopentaose, (E) LNFP II, (F) LNFP III, and (G) LNFP V; and five isomers with mass 1220.4: (H) MFLNH I, monofucosyllacto-N-hexaose, (I) MFLNH III, (J) MFpLNH IV, monofucosyl-paralacto-N-hexaose, (K) IFLNH I, isomer 1 fucosyl-paralacto-N-hexaose, and (L) IFLNH III, isomer 3 fucosyl-paralacto-N-hexaose. Glc, D-glucose; Gal, D-galactose; GlcNAc, N-acetylglucosamine; Fuc, L-fucose; Neu5Ac, N-acetyl-neuraminic acid.

Fig. 3.

HMO-related gene cluster 1 from B. longum subsp. infantis ATCC15697. HMO gene cluster 1, shown here, contains all of the necessary glycosidases (sialidase, fucosidase, galactosidase, and hexosaminidase) and carbohydrate transporters necessary for importing and metabolizing HMOs.

Fig. 4.

Strain-specific strategies for HMO import and catabolism. The most common infant-borne bifidobacteria, B. bifidum, B. longum subsp. infantis, and B. breve, possess different modes for consumption of HMO. B. longum subsp. infantis imports lower molecular-weight HMO via specific soluble binding proteins and transporters, followed by intracellular catabolism by a complement of glycosidases before entry of the monosaccharides into central metabolic pathways. B. bifidum exports fucosidases and lacto-N-biosidase for extracellular hydrolysis to remove lacto-N-biose (LNB) from the HMO structure, internalizes the free LNB, and catabolizes it intracellularly. B. breve consumes the various monomer constituents of HMO, imports them as monosaccharides, followed by intracellular catabolism. Glc, D-glucose; Gal, D-galactose; GlcNAc, N-acetylglucosamine; Fuc, L-fucose.