Genetic diversity of Streptococcus agalactiae strains and density of vaginal carriage

We screened 500 pregnant women who had no risk factors for Streptococcus agalactiae vaginal carriage, and isolated 39 S. agalactiae strains (8%). The density of carriage was low in 16 cases (41%), intermediate in 16 cases (41%) and heavy in seven cases (18%). Strains were mostly of serotype III (41%), Ia (26%) and V (18%). Thirty-five strains had at least one of five genetic markers that have been associated with virulent phylogenetic subgroups of strains. Using PCR, nine strains (23%) were identified as belonging to CC17. The 39 vaginal strains that were studied exhibited a substantial genetic diversity; there were 39 PFGE profiles and 13 variants defined on the basis of the five genetic markers studied. The prevalence of the studied genetic characteristics was similar for strains associated with all three classes of density of carriage. These data suggest that genetic features that are markers of S. agalactiae strains able to invade the central nervous system of neonates are not determinants for vaginal adaptation.