Long before a child learns to speak, a quieter conversation is already underway between mother and infant, carried in each feeding through the bacteria that travel with milk. Researchers at the University of Chicago, working from 507 samples across 195 mother-infant pairs, found that breast milk holds its own living community, largely made of Bifidobacteria, and that some of these microbes settle into the infant's gut. Their findings suggest this early seeding helps shape how the body learns to digest, defend itself, and tell friend from threat. The larger question, whether these early exchang
Breast Milk Carries Its Own Microbes, Which May Help Seed Infant Gut Health and Immune Development
Long before a child learns to speak, a quieter conversation is already underway between mother and infant, carried in each feeding through the bacteria that travel with milk. Researchers at the University of Chicago, working from 507 samples across 195 mother-infant pairs, found that breast milk holds its own living community, largely made of Bifidobacteria, and that some of these microbes settle into the infant's gut. Their findings suggest this early seeding helps shape how the body learns to digest, defend itself, and tell friend from threat. The larger question, whether these early exchanges influence health decades later, remains open.
Researchers at the University of Chicago set out to look for bacteria in breast milk and in infant stool, and they kept finding the same microbes in both. Their study, published in Nature Communications, examined 507 samples of milk and stool from 195 mother-infant pairs enrolled in the Mothers and Infants LinKed for Healthy Growth study.
The central finding is that breast milk has its own distinct bacterial community, dominated by Bifidobacteria, especially B. longum, B. breve and B. bifidum. Earlier studies had mostly reported other bacteria, such as Staphylococcus and Streptococcus, but the newer metagenomic approach revealed a richer and more complex milk microbiome. In 12 cases, identical strains appeared in both a mother's milk and her infant's gut, which the team read as strong evidence of transmission during breastfeeding.
The bacteria matter because of what they do. Bifidobacteria produce lactate and acetate, which lower the pH of the infant intestine and make it less friendly to pathogens. Their signals also help prompt immune cells to form regulatory T cells, which act as instructors that teach the body when to respond and when to remain tolerant. Maternal antibodies and human milk compounds add further influence, and breastfed infants often carry far higher shares of Bifidobacteria than formula-fed infants.
Because early gut communities play a role in allergy risk, scientists see this window of infancy as a possible opportunity for prevention. The team plans multi-omic follow-up work on milk metabolites and environmental contaminants, aiming to follow children over longer periods. Whether milk and early-life factors can predict later health is the question still to be answered.
Researchers at the University of Chicago went looking for bacteria in breast milk and in infant stool, and they found the same microbes in both. Their work, published in Nature Communications, analyzed 507 samples of milk and stool from 195 mother-infant pairs enrolled in the Mothers and Infants LinKed for Healthy Growth study.
The central finding is that breast milk carries its own distinct community of bacteria, and that community is dominated by Bifidobacteria. The three species that stood out were Bifidobacterium longum, B. breve and B. bifidum. More than half of the milk samples contained B. longum, a species found in over 98 percent of infant gut microbiomes.
The result departs from what earlier studies suggested. Pamela Ferretti, one of the researchers, said previous work used less sophisticated sequencing methods and mostly turned up other bacteria, such as Staphylococcus and Streptococcus. The new study used a metagenomic approach, which revealed a richer and more complex community in the milk.
The most direct evidence of transfer came from 12 cases in which identical bacterial strains appeared in both a mother's milk and her infant's gut. The researchers read this as strong evidence of vertical transmission during breastfeeding. Some of the shared strains were beneficial commensals that help digest the sugars in human milk and support gut development. Others were pathobionts, including E. coli and Klebsiella pneumoniae, which can live harmlessly in healthy people but may cause infection under certain conditions. All of the mothers and infants in the study were healthy, which the researchers took to mean that microbial diversity in milk does not by itself signal disease.
Why the bacteria matter is where immunology enters. Dr. Kirsi Jarvinen-Seppo, an immunologist who studies gut microbiome signaling, explained that Bifidobacteria do more than occupy the gut. They produce large amounts of lactate and acetate, which lower the pH of the infant intestine and make it less hospitable to pathogens. Gut bacterial metabolites also support the production of certain immunoglobulins, and bacterial signals prompt immune cells to generate regulatory T cells, whose levels are shaped by the makeup of the gut microbiota. Jarvinen-Seppo described these cells as instructors that help determine whether the body reacts to environmental signals or stays tolerant, a link between early microbial exposure and immune balance later in life.
Maternal antibodies add another layer. Research on antibodies in breast milk has shown they promote long-term intestinal homeostasis by regulating both the gut microbiota and host gene expression. Breastfed infants can have up to 90 percent Bifidobacteria in their gut flora, a share significantly higher than in formula-fed infants. Breastfeeding shapes the gut directly, through the microbes in milk, and indirectly, through human milk oligosaccharides, secretory IgA and antimicrobial compounds that affect bacterial growth and metabolism.
Scientists increasingly describe early life as a window of opportunity in which changes to the gut microbiota can have lasting effects. Because the composition and metabolism of a newborn's gut bacteria play a role in allergic disease risk, the capacity of breast milk to influence that early community is seen as a possible tool for allergy prevention.
The study also expands the available data, nearly doubling the number of publicly available metagenomic breast milk samples. The team plans multi-omic follow-up work covering metabolites such as human milk oligosaccharides, along with environmental factors like PFAS and antimicrobial resistance that can pass through milk. Ferretti said the ultimate aim is to follow longer health trajectories and test whether factors in milk and early life predict later health outcomes. That question remains open, and it is the one to watch.
Quer a matéria completa?
Leia o original em Natural News ↗
Science & Space