Comparison of molecular serotyping approaches of Streptococcus agalactiae from genomic sequences

Group B streptococcus (GBS) capsular polysaccharide (CPS) is one of the major virulence factors underlying invasive GBS disease and a component of forthcoming vaccines. Serotype classification of GBS is based on the capsule polysaccharide of which ten variants are known to exist (Ia, Ib, II-IX). Current methods for GBS serotype assignment rely on latex agglutination or PCR while more recently a whole genome sequencing (WGS) method was reported. In this study, three distinct algorithms for serotype assignment from WGS data were evaluated using a panel of 854 clinical isolates. The first approach utilised the entire CPS locus coupled with a mapping methodology. The second approach continues from the first and utilized a SNP-based methodology across the conserved cpsD-G region to differentiate serotypes Ia-VII and IX. Finally the third approach used the variable cpsG –K region coupled with a mapping methodology. All three approaches were evaluated for typeability (percentage of isolates assigned a serotype) and concordance to the latex agglutination methodology.Following comparisons, the third approach using the variable cpsG-K region demonstrated the best performance with 99.9% typeability and 87.1% concordance. Overall, of the 107 discordant isolates, only 14 were found to be truly discordant on re-testing whereas typing of the remainder was resolved following retesting of latex agglutination and WGS. These results demonstrated that molecular capsular typing using WGS is a viable alternative to the traditional serotyping method.