Genomic and phenotypic analyses of exopolysaccharide biosynthesis in Streptococcus thermophilus S-3

Streptococcus thermophilus, one of the most important industrial lactic acid bacteria, is widely used as a starter culture in the dairy industry. Streptococcus thermophilus S-3 isolated from Chinese traditional dairy products has shown great potential for the production of larger amounts of exopolysaccharides (EPS), which significantly affect the organoleptic properties of fermented milk products. To understand the relationship between the genotype and phenotype of S. thermophilus S-3 in terms of EPS biosynthesis, its genome of strain S-3 was sequenced and the genes related to carbohydrate utilization, nucleotide sugars synthesis, and EPS biosynthesis were investigated. The genomic analysis revealed that S. thermophilus S-3 can use sucrose, mannose, glucose, galactose, and lactose. Phenotypic analysis showed that S-3 prefers fermenting lactose to fermenting glucose or galactose. The genetic analysis of nucleotide sugars and EPS biosynthesis revealed that S-3 can synthesize uridine diphosphate (UDP)-glucose, deoxythymidine diphosphate-glucose, deoxythymidine diphosphate-rhamnose, UDP-galactose, UDP-N-acetylgalactosamine, and UDP-N-acetylglucosamine. A high yield of EPS from S-3 cultivated with lactose rather than glucose as the carbon source was correlated with high transcriptional levels of the genes associated with metabolism of these nucleotide sugars and EPS biosynthesis. Our results provide a better understanding of EPS biosynthesis in S. thermophilus and can facilitate enhanced EPS production by lactic acid bacteria fermentation via genetic and metabolic engineering approaches.