A Molecular Basis for the Enrichment of Bifidobacteria in the Infant Gastrointestinal Tract

Breast milk is a complex fluid that has been shaped by evolution to meet all the nutritional requirements of the newborn during the first months of life. In addition, it is a source of an impressive number of bioactive compounds that help in protection and the development of the infant. Among these, human milk oligosaccharides (HMO) are complex substrates that are selectively fermented by beneficial microorganisms found in the infant intestines. These bacteria, mainly belonging to the Bifidobacterium genus, have been shown to utilize these substrates as the sole carbon source, and the genomic sequence of Bifidobacterium infantis ATCC 15697 provided a rationale for this phenotype. In this dissertation, several aspects related to the consumption of HMO were studied at the molecular level. Import of HMO was observed to occur associated to membrane-associated Solute Binding Proteins. Several of these proteins showed specific binding to different molecules found in HMO, and genes encoding them were up-regulated during bacterial growth on HMO. Other molecular determinants critical for bacterial consumption of these substrates are glycosyl hydrolases. The properties of fucosidases, hexosaminidases and galactosidases found in B. infantis were analyzed at the enzymatic and molecular level. Specific genes encoding these enzymes were found to be induced during growth on HMO, and important differences in substrate affinities were determined. Finally, we studied the interactions of a large panel of infant-gut isolates of bifidobacteria with milk glycoproteins containing N-linked glycans. Certain isolates were able to release glycans from glycoproteins, and genes encoding endo-N-acetylglucosaminidases in these microorganisms explained this phenotype. We further characterized the properties of an endoglycosidase from B. infantis, EndoBI-1, shown to have a remarkable glycolytic activity on different types of N-glycans found in host glycoproteins such as milk lactoferrins and immunoglobulins. Together, these findings help defining the molecular adaptations that specific microorganisms found in the infant gut have developed in response to components in breast milk. Furthermore, a detailed description of this mutualistic relationship between these microbes and their host highlights a unique model that could be translated to the application of new functional foods with clearer and reproducible benefits on health.