Population structure of Streptococcus agalactiae reveals an association between specific evolutionary lineages and putative virulence factors but not disease.

To evaluate the genetic diversity and relationships in a collection of 85 Danish strains of Streptococcus agalactiae (group B streptococcus) we have performed restriction fragment length polymorphism analysis on EcoRI- and MspI-digested whole-cell DNA using as probes rRNA, DNA fragments representing the genes encoding hyaluronidase, C5a-peptidase, alpha-antigen, and beta-antigen as well as two randomly selected genomic DNA fragments for which the coding potential is unknown. In addition, we have assayed for expression of hyaluronidase activity and beta-antigen. Combined analyses of our data and those previously obtained by multilocus enzyme electrophoresis and serotyping revealed a population separating into six major lineages that correlate with individual serotypes. The significant linkage disequilibrium of alleles indicates that the S. agalactiae population examined is predominantly clonal. Notably, strains expressing the serotype III capsule divide into two distant evolutionary lineages, of which one lacks expression of hyaluronidase activity. Six North American isolates of serotype III clustered together with multiple Danish serotype III strains, showing that the combinations of characters on which the phylogenetic tree was based are conserved worldwide. Occurrence of beta-antigen correlated with a specific version of the alpha-antigen gene and was exclusively associated with a single major phylogenetic lineage. Comparisons with the clinical history of the strains revealed no evidence of differences in pathogenic potential among the six major genetic divisions.