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Published On November 19, 2015

CLINICAL RESEARCH

The Secret in Mother’s Milk

Does the body have a hidden highway between a mother’s digestive tract and the milk she produces?

Mastitis is a painful infection of breast tissue that tends to strike breast-feeding mothers. In 2007, scientists at Complutense University of Madrid tried a novel treatment. One test group of new mothers received antibiotics, and a second group received probiotics—bacteria meant to help the body, in this case lactobacilli bacteria that had originally been isolated from healthy breast milk. And instead of applying the probiotics directly to the nipple, the scientists gave them orally. 

Administering probiotics in this way was an unorthodox approach. The probiotics were anaerobes, meaning that it isn’t easy for them to survive in the presence of oxygen. The only obvious route for an imbibed microbe to get to the breast was to travel from the end of the digestive tract across a vast expanse of aerated skin. That would have been difficult for these microbes to do.

At the end of a month, there were two important results. The women taking probiotics improved much more than those taking antibiotics, suggesting a new avenue of treatment for mastitis. And perhaps more important, the lactobacilli taken orally showed up in breast milk. Somehow these microbes had traveled from gut to breast.

The study of breast milk has undergone something of a renaissance in the past decade. Scientists now know that protein and fat content aside, breast milk contains antibodies that protect the infant from infection—immune-signaling proteins that activate or soothe the infant’s immune response, and anti-microbial substances. As researchers point out, breast milk is the only food “designed” for human consumption.

The latest research also shows that this perfect food may cater to a child’s microbial community. It contains hundreds of sugars that the infant can’t use but which help nourish important microbes, such as bifidobacteria and staphylococcus. The study at Complutense also raised the possibility that mother’s milk conveys an assortment of microbes from the mother’s gut to the infant’s mouth.    

How would such a bacterial route function? We usually imagine the immune system as designed to attack rather than aid bacteria. But research on the human microbiome, the community of microbes that inhabit our bodies, mostly in our large intestine, has revealed a much more complex relationship. In some respects, the gut immune system seems to farm our intestinal microbes. And Juan M. Rodríguez, the principal investigator at Complutense, thinks that the same immune cells that defend a mother from infection might also serve as bacterial “ushers”—reaching into the gut, grasping microbes, and then shuttling them through the body, like VIPs through a nightclub, to the mammary glands. Rodríguez calls this hypothetical gut-to-breast path of microbial transmission the “enteromammary pathway.”

The route has not yet been proven to exist, but additional studies on the bacteria in breast milk seem to support the idea. In 2013 researchers at The Swiss Federal Institute of Technology in Zurich isolated the same anaerobic species in the mother’s gut, in her breast milk, and in the child’s gut. The authors suggested that the transmission of intestinal bacteria through the breast might be “a novel way of mother-neonate communication.”

Others remain skeptical. David Mills and Steve Frese, who have studied human milk at the University of California, Davis, believe that proponents of the enteromammary route underestimate the hardiness of anaerobic bacteria, which can persist in an aerated environment long enough to move externally from fecal matter to breast. Mother and infant exist in a shared bacterial “aura,” they say, that makes it easy for them to swap bacteria.

Even if the route does exist, however, Mills and Frese question its importance, given that infants are deluged with bacteria from many other sources. Before ever encountering breast milk, infants get a mouthful of microbes during birth from their mother’s vaginal and digestive environments. This first inoculation is massive in both volume and diversity, particularly compared with what they see as the meager microbial counts in breast milk. Their thesis is that it’s not the milk-borne microbes, but the selective pressure of different elements in breast milk that help and hinder bacterial growth which shapes the bacterial community.

Eight years ago, scientists at the Nestlé Research Center in Lausanne, Switzerland, captured what might be the best real-time evidence yet of white blood cells “ushering” bacteria along the enteromammary route. They matched microbial DNA signatures across mothers’ feces, breast milk, infant feces and, critically, white blood cells taken from the mothers. That the same microbes appeared in all four places was evidence, they argued, suggesting that microbes move in white blood cells from gut to breast and then to infant. The Nestlé scientists reproduced the study in animals, yielding an additional nuance: In mice, bacterial DNA began showing up in white blood cells only in late pregnancy, as if preparing for lactation.

Mills calls the study “wonderful work,” but cautions against drawing too many conclusions. The study didn’t measure living microbes, only microbial DNA, which may mean that the researchers were seeing bacterial DNA that has diffused around the body during late pregnancy for other reasons.

Why would clarity on the enteromammary question be helpful? Proponents argue that a mother’s microbes may improve her infant’s health. Exposure to them might prevent later diseases of immune hypersensitivity, like hay fever. A robust community of maternal bacteria may also keep more harmful bacteria from gaining a foothold; in settings with high infectious loads, some evidence suggests that children fed with heat-treated, and thus relatively sterile, breast milk get more infections than those fed with “raw” breast milk. A mother’s bacteria might even “fit” an infant’s genotype. A 2011 study by scientists at the Yakult Honsha European Research Center for Microbiology in Belgium found that mother-infant pairs shared specific strains. Microbial heirlooms—bacteria perhaps passed down the maternal line, possibly for generations—might have beneficial effects.

If it exists, the enteromammary route may present a new intervention point for doctors. Once researchers know which microbes prevent disease and how the microbiome matures over time, giving lactating mothers the right probiotics might later protect infants against developing miserable and costly diseases, including asthma, inflammatory bowel disease and obesity.