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Carbon source is an essential factor influencing exopolysaccharide (EPS) production. However, there is still lack of systematic investigation how carbon sources affect EPS characterization in S. thermophilus. In this study, EPSs from S. thermophilusS-3 using lactose, glucose and galactose as the sole carbon source were characterized, and named LacS3EPS, GlcS3EPS and GalS3EPS, respectively. The yield of EPSs was significantly affected by the carbon source. The monosaccharide and linkage type of EPSs were independent of the carbon source. Compared with LacS3EPS, the glucose content of GlcS3EPS increased while galactose content decreased, but the opposite trend occurred for GalS3EPS. The molecular weight of EPSs changed significantly, with the highest for GlcS3EPS (1630 kDa) and the lowest for GalS3EPS (414 kDa). Combined with two wormlike cylinder models, the conformation of EPSs was characterized by random coils with descending rigidity in the order GlcS3EPS > LacS3EPS > GalS3EPS. Additionally, gene transcription and correlation analysis revealed that the variation in EPSs was mainly related to the genes encoding nucleotide sugars and glycosyltransferases. The results suggested that carbon sources could alter molecular characteristics of EPS by influencing gene expression related to EPS biosynthesis.

Lactobacillus salivariusis an important probiotic lactic acid bacterium that plays a major role in innate and acquired immunity. It has gained much attention in metabolic engineering and synthetic biology owing to its probiotic properties. However, few well-characterized constitutive promoters are available to modulate L. salivariusgene expression. Here, a pool of L. salivariusendogenous constitutive promoters was identified using RNA sequencing analysis. The promoters were sorted based on their transcript levels. The application potential of the promoters was evaluated using a fluorescence reporting system. This study provides the first well-characterized endogenous constitutive promoter library from L. salivarius, which has a potential for use as a basic toolbox in various lactic acid bacteria applications.