Maternal microbiota transmission through breastfeeding shapes infant tolerance to skin commensals and protects against dermatitis

Summary: Abstract Body: The establishment of tolerance to skin commensals early in life is thought to play a crucial role in the development of dermatitis, but direct evidence remains limited. In this study, we analyzed the skin and fecal microbiome of 102 mother-infant pairs from a high-risk birth cohort for atopic dermatitis (AD), alongside comprehensive blood tests and questionnaires administered during pregnancy, at birth, and at 1, 2, and 6 months of age. From the 769 factors examined, we identified that infants whose mothers routinely wiped their nipples clean before breastfeeding exhibited a higher incidence of AD between 2 and 6 months of age. Comparative microbiome analysis revealed that the maternal nipple microbiota closely resembled the infant’s facial microbiota (nose, cheeks, and eyebrows) but differed significantly from microbiota on other infant body sites (e.g., extremities). Furthermore, the nipple microbiota was the only maternal site closely associated with the infant’s fecal microbiota, suggesting that mother-to-child bacterial transmission occurs during breastfeeding. We investigated whether maternal nipple-cleaning habits influenced bacterial transmission and found that these habits affected the facial and fecal microbiota of 1- and 2-month-old infants but not those of 6-month-olds. Infants whose mothers wiped their nipples had reduced abundance of certain skin commensals, including Staphylococcus epidermidis, on both facial skin and in feces. This reduction was associated with a higher risk of subsequent dermatitis development. These findings suggest that bacterial inoculation through maternal breastfeeding during the neonatal period may promote tolerance to skin commensals and suppress later skin inflammation in infants, emphasizing the importance of early microbial exposure in preventing AD. Hiroshi Kawasaki^{1, 3}, Kanae Masuda², Yoshimasa Aoto², Jun Isayama², Yoshihiro Ito¹, Makoto Irahara⁴, Tatsuki Fukuie⁴, Hideaki Morita⁴, Yukihiro Ohya⁴, Masayuki Amagai¹ 1. Keio University Schiool of Medicine, Tokyo, Japan. 2. JKiC, JSR Corporation, Tokyo, Japan. 3. RIKEN IMS, Yokohama, Japan. 4. Allergy Center, NCCHD, Tokyo, Japan. Clinical Research: Epidemiology and Observational Research