This study characterized the human breast milk (HBM) microbiota of healthy Korean mothers and analyzed both the microbiota and bacterial extracellular vesicles (EVs) to identify key bacteria influencing infant gut colonization. 22 HBM samples were collected, and DNA was extracted from bacteria and bacterial EVs. Alpha-diversity analysis revealed higher richness and evenness in bacterial samples compared to EVs. Firmicutes were the dominant phylum in both, followed by Proteobacteria, Bacteroidetes, and Actinobacteria. *Streptococcus* and *Staphylococcus* predominated in bacterial samples, while *Bacteroides*, *Acinetobacter*, and *Lactobacillaceae* were prevalent in EV samples. Positive correlations were found between several genera, including *Bifidobacterium*, in both sample types. The study suggests the existence of key metabolically active bacteria independent of the major HBM bacterial populations, and that EVs from these bacteria might be involved in vertical gut microbiota transfer.

Su Yeong Kim, Dae Yong Yi