Surveillance of serotypes and antimicrobial susceptibility profile in group B streptococcus (GBS) in Belgium

BACKGROUND Today GBS vaccines for prevention of severe neonatal disease through transplacental delivery of antibodies directly from immunized mothers are in advanced stage of development. For the introduction of any GBS vaccine there are urgent needs for pre and post vaccine enhanced surveillance studies of strains isolated from both neonatal diseases and vagino-rectal colonization of pregnant women. In Belgium, surveillance of invasive isolates is regularly done by the NRC. We report in this study a surveillance of colonizing isolates of GBS.

METHODS In 2012, 344 GBS isolates were obtained from a Belgian surveillance for vagino-rectal colonization among pregnant women (max. 5 isolates/lab). Capsular types were determined by agglutination (Strep-B-latex, SSI, Denmark) and MICs by using a microdilution method (Sensititre) and Etest® (EUCAST interpretive criteria). Furthermore, for the erythromycin (E) resistant (R) isolates, the inducible (iMLS), constitutive (cMLS) and M phenotypes were assessed by a double-disk diffusion test.
RESULTS Serotype III was the more common (27.6%) followed by V, II, Ia, Ib, IV, IX, VII and VI (18.1%, 16.4%, 13.4%, 7%, 4.7%, 2.5%, 0.8%, 0.5%) and 8.9% were non typable. All isolates were susceptible to penicillin ; 29% were R to E with a higher rate among serotypes IV and V (p<0.05). Among these E-R isolates, 93% exhibited the MLS phenotype (R to E and CC): 66% were cMLS with E MIC50>256 mg/L and 27% iMLS with E MIC50/MIC90 2/>8 mg/L. The M phenotype (R to E and S to C) was expressed by 7% of E-R isolates with E MIC50/MIC90 2/4 mg/L.

CONCLUSION Compared with Belgian data relating to neonatal invasive strains (NRC reports) 1) Serotype V and II are more frequent and III less frequent among colonizing isolates 2) Prevalence of E-R is similar in percentage and phenotypes with the MLS R phenotype as major mechanism. Extended surveillance of both invasive and colonizing isolates is needed currently to prepare the follow-up in the future vaccine era.