Emergence and Global Dissemination of Host-Specific Streptococcus agalactiae Clones

ABSTRACT To examine the global diversity of Streptococcus agalactiae (group B streptococci [GBS]) and to elucidate the evolu-tionary processes that determine its population genetics structure and the reported changes in host tropism and infection epide-miology, we examined a collection of 238 bovine and human isolates from nine countries on five continents. Phylogenetic analy-sis based on the sequences of 15 housekeeping genes combined with patterns of virulence-associated traits identified a genetically heterogeneous core population fromwhich virulent lineages occasionally emerge as a result of recombination affect-ing major segments of the genome. Such lineages, like clonal complex 17 (CC17) and two distinct clusters of CC23, are exclu-sively adapted to either humans or cattle and successfully spread globally. The recent emergence and expansion of the human-associated and highly virulent sequence type 17 (ST17) could conceivably account, in part, for the increased prevalence of neonatal GBS infections after 1960. The composite structure of the S. agalactiae genome invalidates phylogenetic inferences ex-clusively based onmultilocus sequence typing (MLST) data and thereby the previously reported conclusion that the human-associated CC17 emerged from the bovine-associated CC67. IMPORTANCE Group B streptococci (GBS) (Streptococcus agalactiae) have long been recognized as important causes of mastitis in cattle. After 1960, GBS also became the most prevalent cause of invasive and often fatal infections in newborns. At the same time, GBS are carried by a substantial proportion of healthy individuals. The aims of this study were to elucidate the genetic mecha-nisms that lead to diversification of the GBS population and to examine the relationship between virulence and host preferenc