LAUSR

BACKGROUND Streptococcus thermophilus is a major starter used in the dairy industry and it could improve the flavor of fermented products. It is necessary to improve biomass of S. thermophilus for its application and industrialization. The utilization of carbon sources directly affects the biomass of S. thermophilus. Therefore, the carbohydrate metabolism of S. thermophilus should be studied. RESULTS In this study, metabolic parameters and gene expression of S. thermophilus S-3 with different carbon sources were investigated. The physicochemical results showed that S. thermophilus S-3 had high lactose utilization. Transcriptome analysis found that approximately 104 genes were annotated onto 15 carbohydrate metabolic pathways, of which 15 unigenes were involved in the phosphotransferase system (PTS) and 75 were involved in the ABC transporter system. In addition, 171 differentially expressed genes (DEGs) related to carbohydrate metabolism were identified. Expression of the galactose metabolism genes lacSZ and galKTEM increased significantly from the lag phase to the mid-exponential growth phase due to the global regulator protein CcpA. The high expression of galK in the mid- to late- phases indicated that the metabolite galactose is re-transported for intracellular utilization. CcpA regulation may also induce high expressions of glycolytic pathway regulated-genes related to lactose utilization, including ldh, fba, eno, pfkA, bglA, pgi, pgm and pyk, producing optimal glycolytic flux and S. thermophilus S-3 growth. CONCLUSIONS This study provides new insights into the carbon metabolism regulation and provide theoretical support for high-density fermentation of S. thermophilus S-3. This article is protected by copyright. All rights reserved.