Human milk was traditionally considered to be sterile, but recent research has shown that it provides a continuous supply of bacteria to the infant gut. Both colostrum and breast milk contain commensal bacteria and the bacteria present in the gut microbiota of breastfed infants has been shown to reflect that found in their mother’s breast milk.
Intestinal bacteria are transferred to the breast tissue via the dendritic cells during pregnancy in preparation for breast milk production. This ‘mammary microbiota’ development has been shown to start during the last trimester of pregnancy, reaching the highest levels at the end of this period. It then remains at relatively constant levels throughout breastfeeding, declines quickly once weaning starts and disappears completely when there is no milk in the mammary gland.
Breast milk also contains oligosaccharides that play a key role in driving the diversity of the infant gut microbiota by selectively stimulating the growth and activity of beneficial bacteria.
The composition of breast milk reflects the health and diet of the mother and supplementation of the mother’s diet with probiotics and prebiotics during both pregnancy and lactation may help to optimise this bacterial colonisation of the baby’s GI tract. Providing supplementation directly to the baby may also help to support this and is particularly useful if the infant is formula-fed.