Breast milk promotes the seeding of infant gut microbiota

A study, which was the largest in the field and published in JAMA Pediatrics, reveals that breastfeeding favors the implantation of a healthy gut microbiota in babies.


Although babies can thrive perfectly when formula-fed, breast milk is the best nourishment for newborns. As previously explained in this blog, breastfeeding lowers the incidence of infectious diseases, as well as allergies, obesity and diabetes. There are also benefits for women, as longer periods of breastfeeding can help protect the mother against depression and can lower breast cancer risk. For these reasons, among others, the World Health Organisation (WHO) recommends exclusively breastfeeding infants until at least 6 months of age.

Many of the benefits of maternal milk for infant health are linked to the bacteria it contains and also its human milk oligosaccharides (HMOs), which function as a prebiotic helping to establish beneficial microbiota. Nevertheless—and surprisingly—little was known about what happened to the microbes from the mother’s breast milk when they reached the baby’s gut.

Researchers also confirmed that the more babies breastfed, the more their gut microbiota would resemble their mother’s microbiota in the milk.

So, in order to figure it out, a team of scientists conducted a study with 107 pairs of healthy mothers and babies for five years. They examined the microbial content on the women’s breasts—the skin around the nipples—and also in their milk, and analyzed babies’ stool with the aim of identifying which microbes composed the infant gut microbiota.

According to their findings, the bacteria in the infants’ poop were more similar to the microbes from their own mothers than to those of other women enrolled in the study. The authors say this suggests bacteria colonizing the infant gut pass from mother to child through breastfeeding.

The research, published in JAMA Pediatrics, is the largest study to date showing the transfer of microorganisms in the milk to the baby’s gut, claim the authors.

“We were able to show that there are bacteria in milk and that these bacteria could be traced to bacteria in infant stools,” senior study author Dr. Grace Aldrovandi, chief of the division of infectious diseases at Mattel Children’s Hospital at University of California, Los Angeles, told Reuters. “This supports the hypothesis that milk microbes are a mechanism by which breastfeeding provides benefit,” Aldrovandi added.

Bacteria from the mother start colonizing the baby’s gut during birth and continuing through the breastfeeding period. Those inherited microbes are known to have a key role in baby’s health, as they train the immature immune system to recognize bacterial allies and enemies.

“Breast milk is this amazing liquid that, through millions of years of evolution, has evolved to make babies healthy, particularly their immune systems”, states Aldrovandi in a press release.

During the 5-year study, were also able to calculate the relative abundance of each species of bacteria. Thus, they observed that during the first month of life babies who breastfeed to get 75% or more of their daily milk intake received an average of nearly 30% of the bacteria in their guts from breast milk and about 10% from areolar skin. Where the remaining 62% came from, researchers could not identify.

Researchers also confirmed that the more babies breastfed, the more their gut microbiota would resemble their mother’s microbiota in the milk. The study also confirmed that the infants who continued breastfeeding after they began eating solid food continue taking advantage of breast milk supplementation: scientists found a higher population of beneficial bacteria.

“We’ve always assumed that most of these microbes come from the mother,” University of Minnesota gastroenterologist Alexander Khoruts, who was not involved in the research, told Reuters. “They found that breastfeeding is the major source of microbial transfer during the early months of life, and I think the study provides supportive evidence for the current recommendations of exclusive breastfeeding for the first six months and continued breastfeeding to 12 months.”

The study did not address how babies fed only with formula acquire beneficial bacteria in their guts.



Pannaraj P, Li F, Cerini C, et al. Association Between Breast Milk Bacterial Communities and Establishment and Development of the Infant Gut Microbiome. JAMA Pediatrics, 2017; DOI: 10.1001/jamapediatrics.2017.0378

This post was originally published on December 20th, 2017.

Cristina Sáez

Cristina Saez is a freelance science journalist. She works for several media, for instance the Spanish newspaper La Vanguardia, where she coordinates the science section, Big Vang; as well as research centres and scientific societies. She has been awarded for her journalistic work, among others, with the Boehringer Ingelheim Award in Medical Journalism 2015. Follow Cristina on Twitter @saez_cristina