Baby’s First Microbes: The Microbiome of Human Milk

At the beginning of the twenty-first century, microbiological studies on human milk started to describe the existence of its own microbiota. Hygienically collected milk samples from healthy women contain a relatively low bacterial load consisting mostly of Staphylococcus, Streptococcus, and other Gram-positive bacteria (Corynebacterium, Propionibacterium, Lactobacillus and Bifidobacterium). DNA from strict anaerobic bacteria is also detected in human milk samples. Colostrum and milk bacteria may play a key role in driving the development of the infant gut microbiota, the correct maturation of the infant immune system and the improvement of tolerance mechanisms. A well-balanced human milk microbiota is also relevant for maternal breast health. The origin of human milk bacteria still remains largely unknown. Infant’s oral cavity and maternal skin may contaminate milk. Additionally, selected bacteria of the maternal digestive microbiota may access the mammary glands through oral- and entero-mammary pathways by involving mononuclear cells for their transport. These pathways would provide new opportunities for manipulating maternal-fetal microbiota, reducing the risk of preterm birth or infant diseases.