Breast milk bacteria influence the establishment and development of the infant gut microbiome with continued impact after solid food introduction, as well as decrease the risk for developing allergic diseases, according to a study published in JAMA Pediatrics.

Pia S. Pannaraj, MD, MPH, from the Division of Infectious Diseases at the Children’s Hospital Los Angeles, and colleagues conducted a prospective, longitudinal study to determine the association between the maternal breast milk and areolar skin and infant gut bacterial communities. Bacterial composition was identified with sequencing of the 16S ribosomal RNA gene in breast milk, areolar skin, and infant stool samples of 107 healthy mother-infant pairs. The study was conducted in Los Angeles and St Petersburg, Florida, between January 1, 2010, and February 28, 2015.

Mother-infant pairs were enrolled into the following infant age groups: 0 to 7 days (initial colonization period), 8 to 30 days (first period of full maternal milk supply), 31 to 90 days (before introduction of solids), 91 to 180 days (period of solid food introduction), and 181 to 365 days (after solid food introduction). At the initial study visit, study personnel interviewed mothers to collect clinical data, including basic demographics, pregnancy and delivery history, current or recent medications (including antibiotics), and infant feeding characteristics (age at formula and solid food introduction and frequency of breastfeeding, formula, and solid foods).

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A total of 228 individuals were enrolled, from 121 families, including 107 mother-infant pairs, 2 mothers only, and 12 infants only. Median infant age at the time of specimen collection was 40 days. Eighty (66.1%) of our families were Hispanic/Latino, and 78 (64.5%) infants were born by vaginal delivery. Fifty-two (43.0%) infants were male. Bacterial communities were distinct in milk, areolar skin, and stool, differing in composition and diversity.

The infant gut microbial communities were more closely related to an infant’s mother’s milk and skin compared with that of a random mother (mean difference, 0.012 and 0.014, respectively). Overall, primarily breastfed infants received more bacteria from breast milk and areolar skin compared with those not primarily breastfed (breast milk, 18.5% vs 5.7%; areolar, 5.2% vs 0.001). During the first 30 days of life, infants who breastfed to obtain 75% or more of their daily milk intake received a mean of 27.7% of the bacteria from breast milk and 10.3% from areolar skin.

Early solid food introduction (<4 months) in the infant cohort led to a rapid maturation of the infant stool microbiota. The amount of daily breastfeeding as a proportion of total milk intake continued to influence the infant stool microbiome diversity and membership even after solid foods were introduced.

“Our study confirms a bacterial community in breast milk and tracks that community from mothers into the infant gut,” the authors said. “Breast milk bacteria influence the establishment and development of the infant microbiome with continued impact after solid food introduction. Furthermore, breast milk contributes bacteria associated with a decreased risk for developing allergic diseases.”


  1. Pannaraj PS, Li F, Cerini C, et al. Association between breast milk bacterial communities and establishment and development of the infant gut microbiome. JAMA Pediatr. 8 May 2017. doi:10.1001/jamapediatrics.2017.0378