Transfer of Bacteria From Mothers to Infants Through Breast Milk: A Systematic Review

There is a critical early window during infancy for establishing the intestinal microbiota. Increasing evidence shows that breast milk (BM) harbors its own distinct microbiota. However, the extent of its contribution to the infant intestinal microbiota remains unclear. A systematic search was done to identify original studies investigating the transfer of bacteria from mothers to the infant's intestine through BM in the first 2 years of life. We identified 56 studies investigating a total of 2509 children, 3481 BM and 4635 stool samples. Many studies reported a higher bacterial diversity of the BM microbiota compared with the infant stool microbiota. Most studies found a higher overlap of bacteria between BM and infant stool in mother-infant pairs compared with unrelated mother-infant pairs. It was estimated that BM bacteria contribute anywhere from 1% to 68% of the infant intestinal bacteria. This large variation is explained by a high degree of methodological heterogeneity between studies, both for microbiota analysis and estimation of overlapping bacteria, including different taxonomic levels analyzed. Several studies reported that the overlap between bacteria in BM and infant stool was higher during the first week of life compared with later time points. Genera that were most frequently reported to overlap include Bifidobacterium, Streptococcus, Staphylococcus, Lactobacillus, Veillonella, Escherichia/Shigella and Bacteroides. In total, identical strains were isolated from BM and infant stool for 25 bacterial species. The first weeks of life are critical for shaping the infant intestinal microbiota, but the role of BM as a source of microbial transmission requires further investigation. The relationship between BM and infant stool microbiota remains poorly understood, hindered by methodological variability and the risk of cross-contamination during BM collection. Future research should focus on clarifying the origins of the BM microbiota and quantifying its contribution to the infant intestinal microbiota.