Capsular switching in group B Streptococcus CC17 hypervirulent clone: a future challenge for polysaccharide vaccine development.

Background: The capsular polysaccharide (CPS) is an important virulence factor and a vaccine target of the major neonatal pathogen group B Streptococcus (GBS). Population studies revealed no strong correlation between CPS type and multilocus sequence typing (MLST) cluster, with the remarkable exception of the worldwide spread of hypervirulent GBS CC17, which were all until recently CPS type III.

Methods: A total of 965 GBS strains from invasive infection isolated in France were CPS typed and the presence of the CC17-specific surface protein encoding gene hvgA gene was investigated. Three hvgA-positive GBS strains screened were surprisingly CPS type IV and thus further characterized by MLST typing, pulsed-field gel electrophoresis (PFGE), and whole genome sequencing.

Results: MLST and PFGE demonstrated a capsular switching from CPS type III to IV within the highly homogeneous GBS CC17. Sequence analysis revealed that this capsular switch was due to the exchange of a 35.5-kb DNA fragment containing the entire cps operon.

Conclusions: This work shows that GBS CC17 hypervirulent strains have switched one of their main vaccine targets. Thus, continued surveillance of GBS population remains of the utmost importance during clinical trials of conjugate GBS vaccines.