Typical skin and oral bacterial species present in human milk are not the result of contamination during the sampling process.

Abstract

Human milk has a low microbial biomass with a microbiome dominated by typical skin and oral taxa, raising concerns about contamination during sample collection. However, to date, no study has directly compared samples collected with and without aseptic technique, leaving questions related to potential contamination within the field. To address this, we compared the microbiota of hand-expressed milk samples collected from 23 mothers before and after cleansing of the hands and breast. Metataxonomic analysis showed that taxonomic profiles were largely unaffected by cleansing, with only Rothia mucilaginosa significantly more abundant in non-aseptically collected samples (P = 0.007). Although aseptically and non-aseptically collected samples were taxonomically similar, there was a higher level of bacterial richness (P = 0.003) and evenness (Shannon diversity, P = 0.0002) in non-aseptically collected samples, suggesting that multiple low-abundance taxa are introduced via skin contamination. These findings support the use of aseptic collection methods to minimize external contamination and accurately assess milk microbial diversity. Importantly, they also suggest that common skin and oral taxa detected in human milk are likely true members of the mammary microbiome.