Breast milk contains bacteria that may contribute to differences in gut microbial populations between breast-fed and formula-fed infants.

“breast milk is teeming with hundreds of types of bacteria they arrive in the mother's milk from a variety of sources including the mother's skin retrograde flow from the infant's saliva aka backwash and potentially other means exposure to this rich bacterial community via breast milk may contribute to the differences observed in gut microbial populations between breast-fed and formula-fed infants and provides a rationale for the inclusion of probiotics in infant formulas”