Whole genome sequencing to characterize capsule locus and predict serogroup of invasive meningococcal isolates

Invasive meningococcal disease is mainly caused by Neisseria meningitidis (Nm) serogroups A, B, C, X, W and Y. Serogroup is typically determined by slide agglutination serogrouping (SASG) and real-time PCR (rt-PCR). We describe a whole-genome sequencing (WGS)-based method to characterize the capsule polysaccharide synthesis ( cps ) locus, classify Nm serogroups, and identify mechanisms for nongroupability using 453 isolates from a global strain collection. We identified novel genomic organizations within functional cps loci, consisting of insertion-sequence (IS) elements in unique positions that did not disrupt the coding sequence. Genetic mutations (partial gene deletion, missing genes, IS insertion, internal stop, and phase variable off) that led to nongroupability were identified. WGS and SASG were in 91–100% agreement for all serogroups, while WGS and rt-PCR showed 99–100% agreement. Among isolates determined nongroupable by WGS (31 of 453), all three methods agreed 100% for those without a capsule polymerase gene. However, 61% (WGS vs. SASG) and 36% (WGS vs. rt-PCR) agreements were observed for isolates particularly with phase variations or internal stops in cps loci, which warrant further characterization by additional tests. Our WGS-based serogrouping method provides comprehensive characterization of the Nm capsule, which is critical for meningococcal surveillance and outbreak investigations.