Recent understanding of human milk oligosaccharides in establishing infant gut microbiome and roles in immune system

[Display omitted] •A mechanism level review on determinants of HMOs on infant health.•HMOs are excellent source for protecting infant from microbial pathogens.•Fucosylated and sialylated status of HMOs are important for its biological functions.•Ability of Bifidobacteria to utilize HMOs is irrespective of their population size. Human milk oligosaccharides (HMOs) are complex sugars with distinctive structural diversity present in breast milk. HMOs have various functional roles to play in infant development starting from establishing the gut microbiome and immune system to take it up to the mature phase. It has been a major energy source for human gut microbes that confer positive benefits on infant health by directly interacting through intestinal cells and generating short-chain fatty acids. It has recently become evident that each species of Bifidobacterium and other genera which are resident of the infant gut employ distinct molecular mechanisms to capture and digest diverse structural HMOs to avoid competition among themselves and successfully maintain gut homeostasis. HMOs also directly modulate gut immune responses and can decoy receptors of pathogenic bacteria and viruses, inhibiting their binding on intestinal cells, thus preventing the emergence of a disease. This review provides a critical understanding of how different gut bacteria capture and utilize selective sugars from the HMO pool and how different structural HMOs protect infants from infectious diseases.