The human milk microbiota plays a role in health of the infant



Human milk is a complex biological fluid, which satisfies the nutritional requirements of rapidly growing infants. In a recent study, it has been suggested that human milk microbiota may play a major role in the health of the infants.1 It educates the infant immune system and confers a certain degree of protection against pathogens.2 These effects reflect the synergistic action of many bioactive molecules which are present in colostrum and milk such as: immunocompetent cells, immunoglobulins, fatty acids, polyamines, oligosaccharides, lysozyme, lactoferrin and other glycoproteins and antimicrobial peptides.3 These molecules inactivate pathogens individually, additively, and synergistically.4 Many studies have recently suggested that colostrum and breast milk are continuous sources of probiotic bacteria to the infant gut.5-12

Many bacterial species and bacterial DNA sequencies have been isolated from human milk. There is predominance of staphylococci, streptococci, lactic acid bacteria (LAB), propionibacteria and closely related Gram-positive bacteria in human milk.5,6,13,14 Human milk has also been shown to be a source of live bifidobacteria in the infant’s gut.12    These microbes have now been found to be secreted by the mammary gland and are not considered contamination,  but nature’s way of colonizing the babies gut with the proper bacteria.1

The mammary microbiota (see above) are transient in nature.  Its development starts during the last trimester of pregnancy, reaches the highest complexity at the end of last trimester, remains constant throughout lactation, then declines sharply at weaning and rapidly disappears when there is no milk in the mammary gland.

It has been suggested that exposure of the breastfed infant to such a number of bacterial species may exert beneficial effects against diarrheal and respiratory diseases and may reduce the risk of other diseases, such as diabetes or obesity.15,16

Bacteria in human milk may play several roles in the infant gut. They can contribute to the reduction, incidence and severity of infections in the breastfed infant by different mechanisms, such as production of antimicrobial compounds,7,10,11,17  Such bacteria may improve the intestinal barrier function by increasing mucine production and reduce intestinal permeability.17 Recently, the administration of a human milk Lactobacillus strain in infants during 6 months led to 46%, 27%, and 30% reductions in the incidence of gastrointestinal infections, upper respiratory tract infections, and total number of infections, respectively.18

Bacteria in milk may also participate in the promoting maturation of the infant immune system, as some strains may modulate both natural and acquired immune responses in humans.19,20,21 These bacteria enhance macrophage production of cytokines in the absence of an inflammatory stimulus and behave as potent activators of NK cells, moderate activators of CD4+ and CD8+ T cells and regulatory T cells which play important role in immune responses. Human milk bacteria also have a remarkable potential to play metabolic roles in the infant. Some lactobacilli and bifidobacteria may help to create a specific “healthy” microbiota in the infant gut.22,23 These microorganisms might also contribute to infant digestion through the breakdown of sugars and proteins.

Lactobacilli strains are metabolically active in the infant gut and increase the production of functional metabolites such as butyrate, which is the main energy source for intestinal cells and also modulates intestinal function. As a result, they improve the intestinal habitat of infants by increasing fecal moisture, stool frequency and volume.24,25


Thus, human milk is a source of bacteria to the infant gut, where they may play a variety of anti-infectious, immunomodulatory, and metabolic roles.


Sonia Shoukat  M.D.

Thomas W. Hale Ph.D.

InfantRisk Center



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25) Asakuma S, Hatakeyama E, Urashima T, Yoshida E, Katayama T, Yamamoto K, et al. Physiology of consumption of human milk oligosaccharides by infant gut-associated bifidobacteria. Journal of Biological Chemistry 2011;286:34583–92.