Adaptive mechanism of L6 in soymilk environment based on metabolism of nutrients and related gene-expression profiles.

L6 isolated from naturally fermented tofu-whey was characterized as potential probiotics. To give insight into the adaptive mechanism of L6 in soymilk, the gene-expression profiles of this strain and changes of chemical components in fermented soymilk were investigated. The viable counts of L6 in soymilk reached 10 CFU/mL in the stationary phase (10 hr). The main sugars reduced gradually while the acidity value significantly increased from 45.33° to 95.88° during fermentation. About 50 genes involved in sugar metabolization and lactic acid production were highly induced during soymilk fermentation. The concentration of total amino acid increased to 668.38 mg/L in the logarithmic phase, and 45 differentially expressed genes (DEGs) in terms of nitrogen metabolism and biosynthesis of amino acid were detected. Other genes related to lipid metabolism, inorganic ion transport, and stress response were also highly induced. Besides, the concentration of isoflavone aglycones with high bioactivity increased from 14.51 mg/L to 36.09 mg/L during the fermentation, and the expression of 6-phospho--glucosidase gene was also synchronously induced. This study revealed the adaptive mechanism of L6 in the soymilk-based ecosystem, which gives the theoretical guidance for the application of this strain in other soybean-derived products.