Breast Milk’s New Protective Benefit: A Mice Study

Breast milk has a new protective benefit: it shapes the gut microbiota of infants and protects them from certain bacterial infections, according to a mice study.


Breastfeeding is widely recognized as the best way to provide nutrition and immunity to infants. However, the mechanisms of how breast milk protects against infections are not fully understood. A new study published in the journal Cell has uncovered a novel role for breast milk’s complement proteins, which are part of the immune system, in shaping the gut microbiota of infant mice and protecting them from certain disease-causing bacteria.

Complement proteins in breast milk

Complement proteins are molecules that can work with antibodies to attack bacteria and other pathogens. They are found in blood and other body fluids, including breast milk. However, their role in breast milk has been largely overlooked until now.

The researchers, led by Professor Fengyi Wan from the Johns Hopkins Bloomberg School of Public Health, found that mouse breast milk contains complement proteins that can directly eliminate some types of gut-dwelling bacteria. This reshapes the gut microbiota of infant mice, making them less susceptible to Citrobacter rodentium infection, a bacterium that causes diarrhea in mice and is similar to some types of E. coli that can infect humans.

Mice study

The researchers used genetically modified mice that lacked a key complement protein called C3 in their breast milk. They compared the gut microbiota and infection susceptibility of mouse pups that nursed from these mice or from normal mice.

They found that pups that nursed from C3-deficient mice had different gut microbe populations than pups that nursed from normal mice. They also had higher levels of Citrobacter rodentium in their feces and intestines, indicating a higher infection rate. Moreover, they had more severe inflammation and tissue damage in their colons, suggesting a worse disease outcome.

The researchers also confirmed that human breast milk contains complement proteins that can target specific bacteria in vitro. They suggest that human breast milk may have a similar protective effect on infants’ gut microbiota and infection resistance.


The study reveals a new protective benefit of breast milk: its complement proteins shape the gut microbiota of infant mice and protect them from certain bacterial infections. This expands our understanding of breast milk’s immune functions and may have implications for infant health and nutrition.

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