Maternal stress in the early postpartum period is associated with alterations in human milk microbiome composition

Background: Maternal stress is associated with negative early-life development and (mental)health outcomes. There is recent evidence that maternal stress in the postpartum period impacts the nutrient composition of human milk (HM). However, it is currently not known whether maternal stress is associated with changes in the HM microbiome during the critical early postpartum period. Methods: In this prospective observational cohort study, lactating women were recruited into a high-stress (HS, n = 23) and control (CTL, n = 69) group. The HS group included mothers with infants hospitalized for at least two days. Maternal stress was assessed using validated questionnaires and cortisol concentrations in hair, saliva and HM. HM was collected at days 10 and 24 and its microbiome was analyzed using 16 s rRNA sequencing. HM bacterial composition was compared between study groups and their correlation with maternal stress levels, maternal characteristics and infant outcomes was determined. Results: HM microbiome beta-diversity differed significantly between study groups, with HS mothers displaying decreased abundance of Streptococcus, Gemella, and Veillonella, and increased levels of Staphylococcus, Corynebacterium and Acinetobacter compared to the control group. While the strongest correlation of beta-diversity was with stress, HM microbiome beta-diversity also correlated significantly with maternal education level and infant sex. No correlation between HM microbiome composition and HM cortisol concentrations was found. Conclusions: This study demonstrates stress-associated alterations in the early HM microbiome that could potentially contribute to early gut colonization and subsequent (mental)health outcomes. Future research is needed to elucidate the physiological significance of these changes for infant development and health.