Human milk microbiota: origins, determinants, and roles in maternal-infant microbial transmission and infant microbiome assembly.

Human milk is a complex and dynamic biological fluid that provides essential nutrients and harbors a diverse, functional microbiota, playing a critical role in infant microbial colonization and early life development. The milk microbiota is derived from multiple maternal and environmental sources, including the maternal gut via the entero-mammary pathway, the mammary and skin microbiota, infant oral microbes through retrograde flow, and environmental exposures. Its composition is influenced by a range of factors, such as maternal metabolic and health status, diet, and antibiotic use, as well as delivery mode, lactation stage, infant characteristics, and geographic context. Human milk contributes to the establishment of oral, airway, and gut microbial communities by transferring key taxa such as Bifidobacterium and Lactobacillus, which are commonly detected in milk; however, direct evidence of specific strains establishing in the infant gut remains limited. Breastfeeding may partially compensate for microbiome deficits in cesarean-delivered, preterm, and antibiotic-exposed infants, supporting protection against infections, allergies, asthma, obesity, and other health outcomes. Translational strategies may help modulate the milk microbiota. These include maternal probiotic or prebiotic supplementation, dietary optimization, and approaches targeting microbiota or bioactive milk components. Such strategies offer feasible and cost-effective means to support healthy infant microbiome development. However, methodological constraints including low-biomass contamination, sequencing biases, and limited strain-level resolution remain significant challenges in accurately characterizing the human milk microbiota. Despite substantial advances, the relative contributions of distinct transmission routes, the persistence of maternal strains, and the efficacy of targeted maternal interventions remain incompletely understood. Addressing these gaps will be essential for refining strategies to promote healthy microbiome maturation and improve lifelong health outcomes.