Strain Resolution Analyses of Early Mother to Child Transmission of Bacteroides

Bacteroides represents one of the most important early colonizers of the human infant gut. Although it is known that Bacteroides is transmitted from mother to child, we still lack basic knowledge on how, when, and which Bacteroides strains are transmitted. Therefore, the aim of this study was to address the mechanisms of mother to child transmission of Bacteroides. The samples analyzed were collected during early infancy, from the PreventADALL study, which included infant skin swabs taken immediatly after birth, fecal samples collected within the first 5 days after birth, at 3 months of age, and from their mothers at 18 weeks of gestation. We first screened 464 meconium samples for Bacteroides presence, with subsequent selection of 144 mother child pairs for longitudinal analysis based on Bacteroides presence, longitudinal sample availability, and delivery mode. From the subset, 96 infants were born vaginally, and 48 by cesarean section. Bacteroides were mainly detected in samples from vaginally delivered infants. The difference in abundance and presence of Bacteroides in samples from cesarean-section and vaginally born infants could not be explained by the abundance of Bacteroides in their mother's feces. We identified the species Bacteroides vulgatus, dorei, and caccae to potentially be transmitted from mother to child, while this was not reflected on strain-level for B. dorei. We observed a potentially globally distributed B. caccae strain, and a local strain of B. vulgatus, based on their prevalence in available metagenomes from the infant and adult population worldwide. Based on our results, we porpose that Bacteroides exposure during birth determines their early presence in the gut community.