New insights into the evolution of emerging serotype IV Streptococcus agalactiae. Streptococcus agalactiae

Streptococcus agalactiae (Group B Streptococcus, GBS) causes life-threatening infections in newborns, adults with chronic medical conditions and the elderly. GBS serotype IV strains are emerging both among carriers and as cause of invasive disease and recent studies revealed two prevalent Sequence Types (STs), ST-452 and ST-459 assigned to Clonal Complexes CC23 and CC1, respectively.Whole genome sequencing of 70 type IV GBS clinically relevant isolates and subsequent phylogenetic analysis of an extended dataset elucidated the localization of type IV isolates in a SNP-based GBS phylogenetic tree and suggested that ST-452 could have originated through genetic recombination. SNPs density analysis of the core genome confirmed that this lineage emerged from a single large horizontal gene transfer event between CC23 and the hypervirulent CC17. Indeed, ST-452 genomes are composed by two parts that are nearly identical to corresponding regions in ST-24 (CC23) and ST-291 (CC17). Chromosome mapping of the major GBS virulence factors showed that ST-452 strains have an intermediate yet unique profile among strains from CC23 and CC17.We described an unreported large recombination event, involving the cps IV operon and resulting in the expansion of serotype IV to CC23. This work sheds further light on the evolution of GBS providing new insights on the recent emergence of serotype IV.