Recent work has shown that human colostrum and milk, which had been traditionally considered sterile, provides a continuous supply of commensal and potential probiotic bacteria to the infant gut. More than 200 different bacterial species, including staphylococci, lactic acid bacteria and bifidobacteria, have been isolated from human milk samples so far, although the cultivable bacterial diversity found in individual samples is much lower (2 to 8 different species per women). Interestingly, the same bacterial strains have been found in both breast milk and infant feces of different mother-infant pairs, confirming the role of human milk on the bacterial colonization of the infant gut. These commensal bacteria could protect the infant gut and direct, at least partly, the maturation of the immune system, among other functions. Different studies suggest that some bacteria present in the maternal gut could reach the mammary gland during late pregnancy and lactation through a mechanism involving gut dendritic cells and macrophages. Thus, modulation of maternal gut microbiota during pregnancy and lactation could have a direct effect on infant health.