Polymorphism of eps loci in Streptococcus thermophilus: sequence replacement by putative horizontal transfer in S. thermophilus IP6757

The phylogenetic relationships between 26 Streptococcus thermophilus strains were investigated by hybridization of 15 DNA probes. They indicate that IP6757 and NST2280 strains are genetically very closely related. They are included in a group of eight closely related strains which contain identical eps loci, apart from IP6757 which possesses a different one. The IP6757 and NST2280 eps loci contain identical epsA–epsD region whereas the remainder of the loci are completely different in size and sequence. The IP6757 genes are conserved in only one of the 26 S. thermophilus strains tested, LMG18311. The IP6757 eps locus could result from sequence replacement and the new sequence may have been acquired by putative horizontal transfer. Hybridization data and sequence comparison of IP6757 and NST2280 eps loci with those of other S. thermophilus strains show that eps loci are highly variable and could quickly evolve by sequence replacement.