Structural, genomic and phenotypic analysis of exopolysaccharide produced by Streptococcus thermophilus ST65

Streptococcus thermophilus is a conventional dairy starter bacterium that is used widely in the dairy industry. In this study, an ideal strain of S. thermophilus, ST65, was isolated from homemade yogurt, which benefited the fermentation process by increasing the fermentation rate, viscosity and overall quality of yogurt. ST65 yielded 305.72 mg/L exopolysaccharide (EPS), and yogurt fermented by ST65 was slimy and smooth. EPS-ST65 was uncharged, had a high molecular weight of 2990 kDa and contained d-glucose and D-galactose at a molar ratio of 1:1. The structure of EPS-ST65 consists of the →3)-α-D-Glcp-(1 → 3)-[β-D-Galp-(1 → 6)]-β-D-Glcp-(1 → 3)-β-D-Galf-(1 → tetrasaccharide repeating unit. A novel eps gene cluster encoding 22 open reading frames, including four glycosyltransferases for EPS assembly of repeating units, was reported. EPS-ST65 showed high ABTS and DPPH free radicals scavenging abilities and scavenged intracellular reactive oxygen species (ROS) level by maintaining high SOD activity. It promoted nitric oxide (NO) and pro-inflammatory cytokines production of RAW264.7 cells, indicating its potential function in anti-oxidation and immunomodulation. Our study provides a theoretical basis for understanding the correlation between the phenotype, structure characteristics of EPS and eps cluster, which ultimately contributes to the application of ST65 and EPS-ST65 in fermented dairy products and functional food.