Abstract: To explore the changes in microbial community diversity and to analyze the function of the microbiota in breast milk during lactation, this study recruited 12 full-term lactating women. A total of 70 breast milk samples were collected from the 1st to 24th week after birth. The composition and dynamic changes of the breast milk microbiota were analyzed by metagenomic sequencing. Meanwhile, a comprehensive analysis of the function of the microbial community, antibiotic resistance genes, and bacteriophages was conducted. The results demonstrated that the diversity of the microbiota increased significantly in the 20th and 24th weeks, but did not significantly change from the 2nd to the 18th week. The relative abundances of Firmicutes and Proteobacteria significantly changed during the first six months. Genus-level analysis showed that the relative abundances of Staphylococcus and Streptococcus gradually decreased during the lactation period, while the relative abundance of Acinetobacter increased. The dominant species gradually shifted from Staphylococcus epidermidis and Streptococcus salivarius to Acinetobacter ursingii. During different lactation stages, 18 differential metabolic pathways were identified. In the early lactation stage, the major function of the microbiota was the catabolism of various components in breast milk, while in the later stage, it shifted towards amino acid synthesis. The diversity of antibiotic resistance genes increased significantly as the lactation period progressed, but their relative abundance did not significantly change. Additionally, bacteriophages showed a significant positive correlation with the relative abundances of 10 genera, including Acinetobacter and Agrobacterium. In summary, this study provides data support for future research on maternal-infant microecology and offers a reference for the development of infant products tailored to different stages of growth.

Key words: breast milk microbiota, lactation stage, metagenome, metabolic pathway