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Infant Gut Microbiome Correlates With Bacteria in Mother's Breast Milk, Study Finds

NEW YORK – In a new study of the infant gut microbiome, researchers from the University of British Columbia and the University of Manitoba have found that the composition of the gut microbiota is strongly associated with bacterial species found in breast milk.

In their study, which was published on Friday in Cell Host and Microbe, the researchers noted that although it is now generally accepted that breast milk contains live bacteria, it was unclear whether they transferred to the infant gut. Their data showed that certain bacteria, including Streptococcus spp. and Veillonella dispar, occured both in mothers' milk and their infants' stool, and the co-occurrence was reduced when infants received pumped breast milk.

"The relative abundances of commonly shared species are positively correlated between breast milk and stool. Overall, gut microbiota composition is strongly associated with breastfeeding exclusivity and duration but not breast milk feeding mode (nursing versus pumping)," the authors wrote. "Moreover, breast milk bacteria contributed to overall gut microbiota variation to a similar extent as other modifiers of the infant microbiome, such as birth mode."

Pumping breast milk is common and allows many women, especially working mothers, to continue to breastfeed for longer. However, the researchers noted, pumping can affect the bacterial composition of breast milk. Using information from the longitudinal CHILD Cohort Study, they analyzed data on 1,249 mother-infant dyads. The mean duration of any breast milk feeding was 9.4 months, and 61 percent of mothers reported feeding some pumped milk around the time of sample collection, at three to four months into breastfeeding.

At three months, the researchers found that infants no longer fed with breast milk differed in microbiota composition compared with those exclusively or partially fed with breast milk. They also observed a subtler difference between infants fed with breast milk partially versus exclusively.

Infants who were no longer fed with breast milk had higher microbiota diversity and maturity compared with those who were partially or exclusively fed with breast milk. Diversity increased between three months and one year of age among breast milk-fed infants, whereas no further increase was observed among the infants who had ceased breastfeeding before three months.

At one year, breastfeeding duration explained about 1 percent of observed variation in infant gut microbiota composition, and the researchers also observed some differences in gut microbiota diversity and maturity according to breast milk feeding practices.

"Together, these results suggest a dose-dependent impact of breast milk feeding on gut microbiota diversity, and an early or accelerated maturation and diversification in non-breastfed infants," the authors wrote.

The team also assessed the association of breast milk feeding practices with the prevalence and relative abundance of different bacteria in the infant gut and saw differences in the overall relative abundances according to breastfeeding exclusivity at three months.

For example, the investigators found that exclusive breast milk feeding was associated with enriched populations of Haemophilus parainfluenzae, Streptococcus2, and two amplicon sequencing variants (ASVs) classified as Veillonella dispar, whereas taxa enriched among infants who were no longer fed breastmilk included ASVs classified as Blautia, Streptococcus3, and two unclassified Lachnospiraceae. Six of the 12 ASVs also differed in relative abundance between infants exclusively and partially breastfed.

They also noted that while breast milk and the infant gut have distinct microbiota compositions, they do share some bacteria. Only Streptococcus and Veillonella were present in more than 60 percent of both sample types. Overall, Streptococcus and Staphylococcus dominated milk, while Bifidobacterium and Bacteroides dominated the infant gut.

"These results advance the hypothesis that breast milk may act as an incubator that enriches, protects, and transports certain bacteria to a baby's intestinal tract, and this may give us clues about which bacteria could make good probiotics since they appear to withstand the trip to the baby's gut," senior co-author Brett Finlay, professor in the departments of biochemistry and molecular biology, as well as microbiology and immunology at UBC, said in a statement.