Breast Milk Microbiome Affect Mom and Baby

A Mother’s (microbial) milk

The overwhelming belief prior to microbiome research was that a mother’s breast milk was sterile. Researchers believed that milk could only be cultured when a woman had mastitis, an infection of the breast tissue. Yet, one researcher who began studying breast milk microbiome in the 1990s found that the opposite is true: breast milk actually serves as a potential source of microbes to infants. The scientific community believed that he studied samples contaminated from the mothers’ skin or babies’ mouths; however, the bacterial strains found in the breast milk did not exist in either mouth or skin communities. His team later confirmed that the bacteria found in the breast milk were found in the infant gut microbiome.

Later in 2011, another team of researchers characterized the human breast milk microbiome from 16 women and found extremely diverse microbial communities. While samples varied from woman to woman, the most abundant bacteria were found to be StreptococcusStaphylococcusSerratia, and Corynebacteria. The variation in samples reveals that the mother passes on her microbiome to her offspring, essentially training the immune system and expanding the infant’s exposure.

Researchers now question the affects these various microbial sources have on infants. One primate study published in Nature Communications revealed that macaques on a high fat diet throughout pregnancy and lactation birthed babies with altered gut microbiomes. Although researchers are not sure what exactly causes these changes, they think breastfeeding is a key and critical component not only to the introduction and influence on the infant gut microbiome but also how it changes its metabolism.

Breast Milk Microbiome: researching a mother’s microbial role

Although we know that the mother’s microbes affect newborns, the microbial communities found within the placenta, vagina, and breast milk are neglected when it comes to fetal and infant development. However, the medical field has recognized that the mother’s microbiome plays an important role for women’s health. One rodent study in 2012 found that changes in the gut microbiome during pregnancy may be linked to weight gain and reductions in insulin sensitivity.

One scientist hypothesized that breast milk microbiome is critical to the health of the mother. Mastitis, an infection that causes severe pain, is typically treated with antibiotics. His research observed significant sample-to-sample dysbiosis of the breast-milk microbiome. A single pathogenic strain of bacteria dominated each sample and the genus Lactobacillus disappeared from the samples altogether.

Lactation researchers at the Washington State University recently confirmed these results in an additional study, which collected breast milk microbiome samples from another group of women. The majority of samples from women who complained of pain or discomfort in the breast or who had recently taken antibiotics were dominated by a single strain and once again lacked Lactobacillus.

With this study, researchers questioned whether the bacteria found in the dysbiotic milk originated in the gut. Women took supplements containing the depleted bacteria and then found those specific strains in their breast milk. In addition, the women reported that their mastitis cleared after three weeks of taking the supplements. Subsequent trials studied whether probiotics were more effective at treating mastitis during breastfeeding compared with antibiotics.

Researchers have also found that even when not lactating, the breast microbiome exists. Researchers suspect that the bacteria in the breast might even play a role in disease, such as cancer. However, to study this link between the breast microbiome and health requires normalizing the microbiome, which is not an easy task. However, we do know that a healthy microbiome, whether a baby or mother’s harbor diverse bacteria. Researchers are just on the brink of discovering how diverse microbial communities affect our general health.


Aagaard, K., Ma, J., Antony, K.M. et al. The Placenta Harbors a Unique Microbiome. Science Translational Medicine. 6(237): 237-65. 2014. DOI: 10.1126/scitranslmed.3008599

Bode L, McGuire M, Rodriguez JM, Geddes DT, Hassioutou F, Hartmann PE, McGuire MK. It’s alive: Microbes and cells in human milk and their potential benefits to mother and infant.Adv Nutr 5:571-3. 2014. doi: 10.3945/​an.114.006643

Hunt, K.M., Foster, J.A., Forney, L.J. et al. Characterization of the Diversity and Temporal Stability of Bacterial Communities in Human Milk. PLoS ONE 6(6): e21313. 2011. doi: 10.1371/journal.pone.0021313

Ma, J., Prince, A.L., Bader, D. et al. High-fat maternal diet during pregnancy persistently alters the offspring microbiome in a primate model. Nature Communications. 5:3889. 2014. DOI: 10.1038/ncomms4889

Koren, et al. Host remodeling of the gut microbiome and metabolic changes in pregnancy. Cell. 150(3): 470-80, 2012. DOI:

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