Mobile genetic elements provide evidence for a bovine origin of clonal complex 17 of Streptococcus agalactiae.

We sought an explanation for epidemiological changes in Streptococcus agalactiae infections by investigating the link between ecological niches of the bacterium by determining the prevalence of 11 mobile genetic elements. The prevalence of nine of these elements differed significantly according to the human or bovine origin of the isolate. Correlating this distribution with the phylogeny obtained by multilocus sequence analysis, we observed that human isolates harboring GBSi1, a clear marker of the bovine niche, clustered in clonal complex 17. Our results are thus consistent with the emergence of this virulent human clone from a bovine ancestor.

Acquisition of insertion sequences and the GBSi1 intron by Streptococcus agalactiae isolates correlates with the evolution of the species.

The prevalence of insertion sequences IS1548, IS861, IS1381, and ISSa4 and of the group II intron GBSi1 within Streptococcus agalactiae human isolates strongly correlates with the genetic lineages obtained by multilocus sequence typing. Our results yielded an evolutionary scheme for the acquisition of these genetic elements linked to the ecosystems from which the isolates were obtained.