Breast milk has long been known as a rich source of nutrients and antibodies essential for newborn’s health and protection against infection. Formula-fed babies are at higher risk of necrotizing enterocolitis, lower respiratory infection, asthma, obesity, and type 2 diabetes. A recent hypothesis suggests a new component of breast milk that contributes to its beneficial properties – microbes.
Several papers describe culturing bacteria from breast milk, including species of Lactobacillus, Enterococcus, and Staphylococcus and a recent metagenomics study suggests a much more diverse community. It appears that the presence of these microbes is not due to reverse colonization from the newborn, but rather is endogenous to the mother. There is also evidence to suggest that these bacteria become residents of the infant gut, making them a source of the microflora in addition to primary sources, the vaginal microflora and the environment.
How the bacteria actually arrive in the breast milk is still a mystery – however the existence of a gut-mammary axis during pregnancy is clearly described for IgA secreting B cells. The chemokine CCL28 and the adhesion molecules VCAM-1 and alpha4 integrins direct the traverse of B cells from the gut to the mammary gland. A similar mechanism is believed to be involved in the migration of dendritic cells that sample maternal gut flora, assume a quiescent state, and establish residency in breast milk. During breast-feeding, they would then deliver the bacteria to the intestine in an anti-inflammatory environment allowing them to establish themselves as commensals.
This idea is still yet to be supported experimentally, but regardless, the finding that breast milk is not sterile is another eye opener into the diversity of the microbial world – and another microbiome to study!