Lactation time influences the composition of and at species level in human breast milk.

Human breast milk is a source of microorganisms for infants that play an important role in building infant gut health and immunity. The bacterial composition in human breast milk is influenced by lactation time. This study aimed to investigate the influence of lactation time on bacteria in breast milk at the genus level and the species levels of and on days 2-4, 8, 14, and 30. Eighteen individuals were recruited and 60 milk samples were collected. The 16S rRNA gene, and the bifidobacterial and lactobacilli genes were used for amplicon sequencing. The results revealed that the alpha diversities of colostrum and transition 1 (day 8) milk were lower than that of transition 2 (day 14) and mature milk. PCoA analysis showed that bacterial composition in colostrum and transition 1 milk differed from transition 2 and mature milk. A lower relative abundance of was found in colostrum and transition 1 milk compared with mature milk and lower abundances of , and were found in transition 1 compared with mature milk. , and were the predominant species across all four lactation stages, while was lower in transition 1, and and were higher in transition 1 milk. This study indicated that the bacterial composition in colostrum was more similar to that of transition 1 milk, whereas the bacterial community in transition 2 milk was similar to that of mature milk which suggests that bacterial composition in human breast milk shows stage-specific signatures even within a short period at both genus level and and species levels, providing insights into probiotic supplementation for the nursing mother.