Maternal secretor status and human milk oligosaccharides influence the infant gut resistome.

The infant gut resistome is established early in life and is shaped by perinatal exposures, yet the mechanisms underlying its modulation remain unclear. We combined shotgun metagenomics of fecal samples from 57 one-month-old infants and paired milk samples from 50 mothers in the MAMI cohort to investigate the influence of maternal secretor status on early-life resistome development. Longitudinal follow-up at 6 and 12 months, and also further validation in the independent Lifelines NEXT (LLNEXT) cohort, support our findings. Cesarean section (C-section) was associated with increased antibiotic resistance gene (ARG) diversity, whereas exclusive breastfeeding reduced ARG abundance and diversity. Maternal secretor status further modified resistome composition among exclusively breastfed infants. Human milk oligosaccharide profiling identified specific glycans underlying these associations, with 2'-fucosyllactose and 6'-sialyllactose showing negative correlations with distinct ARG classes. These findings identify human milk composition as a key determinant of early-life resistome assembly and a potential target for modulating antimicrobial resistance.