New insights into the exopolysaccharide production of Streptococcus thermophilus

The eps gene clusters of four heteropolysaccharide (HePS)-producing Streptococcus thermophilus strains (488, EU20, LY03, and ST111) were cloned and sequenced. For strain 488, a colony type with a truncated eps gene cluster arose in the population, explaining phenotypic instability of HePS production. Whereas the eps gene clusters of strains LY03 and ST111 were similar to those of other S. thermophilus strains, the EU20 eps gene cluster represented a distinct type. Comparing various S. thermophilus strains, 15 distinct types of eps gene clusters and 67 glycosyltransferases (GTFs) could be distinguished. This high number of GTFs more or less reflects the number of monosaccharides present in the different HePS repeating units, because of the high substrate specificity of GTFs, except for the priming GTF. The results indicate considerable variability in the genetic make-up of HePS-producing S. thermophilus strains, which could have an impact on the physical properties of fermented milks and yoghurts.