Birthmode and environment-dependent microbiota transmission dynamics are complemented by breastfeeding during the first year.

The composition and maturation of the early-life microbiota are modulated by a number of perinatal factors, whose interplay in relation to microbial vertical transmission remains inadequately elucidated. Using recent strain-tracking methodologies, we analyzed mother-to-infant microbiota transmission in two different birth environments: hospital-born (vaginal/cesarean) and home-born (vaginal) infants and their mothers. While delivery mode primarily explains initial compositional differences, place of birth impacts transmission timing-being early in homebirths and delayed in cesarean deliveries. Transmission patterns vary greatly across species and birth groups, yet certain species, like Bifidobacterium longum, are consistently vertically transmitted regardless of delivery setting. Strain-level analysis of B. longum highlights relevant and consistent subspecies replacement patterns mainly explained by breastfeeding practices, which drive changes in human milk oligosaccharide (HMO) degrading capabilities. Our findings highlight how delivery setting, breastfeeding duration, and other lifestyle preferences collectively shape vertical transmission, impacting infant gut colonization during early life.