The human breast milk microbiome: a landscape review of its composition, origins, and impact on infant health.

Breast milk has long been recognized as the optimal source of nutrition for newborns, yet emerging research has revealed an additional dimension of its complexity: a diverse and dynamic microbial community. The breast milk microbiome constitutes one of the earliest and most consequential sources of microbial colonization in the infant gut, with direct relevance to neonatal immune development and long-term health. The core milk microbiome, dominated by Staphylococcus, Streptococcus, Lactobacillus, and Bifidobacterium, arrives principally via the entero-mammary pathway, maternal skin, and retrograde infant oral transfer, and is substantially modulated by maternal factors including diet, mode of delivery, antibiotic use, BMI, and lactation stage. Beyond live microorganisms, human milk oligosaccharides serve as critical prebiotics that selectively promote beneficial microbial taxa in the infant gut, while also exhibiting direct antimicrobial and immunomodulatory properties. Extracellular vesicles in breast milk carry bioactive payload, including proteins, microRNAs, and microbial components, that influence infant gut epithelial development and immune signaling, representing an underexplored frontier in milk biology. This review synthesizes current knowledge on the composition, origins, and functional significance of the breast milk microbiome. Based on a wide-ranging literature survey, the functional roles of the breast milk microbiome in colonization resistance, immune education, and metabolic programming in the developing infant are evaluated. Disruptions to this microbial community have been associated with conditions such as infant colic, atopic disease, and obesity. By mapping the available literature, we identify emerging research trends in the breast milk microbiome research and their translational implications for infant health optimization.