Mammalian milk microbiomes: sources of diversity, potential functions, and future research directions.

Milk is an ancient, fundamental mammalian adaptation providing nutrition and biochemical communication to offspring. Microbiomes have been detected in milks of all species studied to date. In this review, we discuss (a) routes by which microbes may enter milk, (b) evidence for proposed milk microbiome adaptive functions, (c) variation in milk microbiomes across mammals, and (d) future research directions, including suggestions for how to address outstanding questions on the viability and functionality of milk microbiomes. Milk microbes may be sourced from maternal gastrointestinal tract, oral, skin, and mammary gland microbiomes and from neonatal oral and skin microbiomes. Given the variety of microbial sources, stochastic processes strongly influence milk microbiome assembly, but milk microbiomes appear to be influenced by maternal evolutionary history, diet, environment, and milk nutrients. Milk microbes have been proposed to colonize the neonatal intestinal tract and produce gene and metabolic products that influence physiology, metabolism, and immune system development. Limited epidemiological data indicate that early-life exposure to milk microbes can result in positive, long-term health outcomes. Milk microbiomes can be modified by dietary changes including providing the mother with probiotics and prebiotics. Milk replacers (i.e., infant formula) may benefit from supplementation with probiotics and prebiotics, but data are lacking on probiotics usefulness and supplementation should be evidence-based. Overall, milk microbiome literature outside of human and model systems is scarce. We highlight the need for mechanistic studies in model species paired with comparative studies across mammals to further our understanding of mammalian milk microbiome evolution. Broader study of milk microbiomes has the potential to inform animal care with relevance to ex-situ endangered species.