Breast milk provides essential nutrients, delivers antibodies, and promotes healthy infant development. New research now supports the idea that breast milk helps establish the infant gut microbiota by transmitting beneficial bacterial strains, metabolic functions, and some antimicrobial resistance genes.

The findings, published in Nature Communications, may inform strategies to improve early-life gut health as well as infant nutrition and disease prevention.

Scientists have known that breast milk contains some bacteria that can influence the infant gut microbiota, but the detailed composition of the breast milk microbiota and which specific bacterial strains are shared with the infant gut remain unclear.

So, Pamela Ferretti at the University of Chicago in Illinois and her colleagues analyzed breast milk and infant stool samples from 195 mothers and their babies, most of whom were exclusively breastfed during their first six months.

Shared microbes

The researchers found that although the types of bacteria in milk and baby stool are different, both are dominated by Bifidobacterium longum. Breast milk included other Bifidobacterium species, skin bacteria, and mouth bacteria, while babies’ guts were dominated by Bifidobacterium species and some gut and oral bacteria.

From one to six months of age, B. longum became more common in the guts of babies, especially those who were exclusively breastfed, while other bacteria such as E. coli decreased. The presence of B. longum was linked to a more stable gut microbiota over time, the researchers found.

About 10% of the bacteria in a one-month-old baby’s stool were also found in their mother’s milk. Besides B. longum, other bacteria such as B. bifidum, E. coli, and some oral microbes were most frequently passed from milk to infant gut. Some of these strains persisted in the baby’s gut for several months.

Antimicrobial resistance

Further analyses revealed that both breast milk and infants’ gut bacteria harbor genes that allow them to produce essential nutrients such as amino acids. Both milk and infant guts also contain genes that provide resistance to antibiotics. 

Some antimicrobial resistance genes were shared between a mother’s milk and her baby’s gut, and babies whose guts were dominated by Bifidobacterium had fewer resistance genes, the researchers found.

“Our results indicate that maternal breast milk plays a role in infant gut microbiome and resistome establishment, development, and temporal stability,” the authors say. The work, they add, als offers a detailed picture of the bacteria in breast milk and how they relate to the bacteria in a baby’s gut, providing better information for future research.