Examining the Distribution and Impact of Single-Nucleotide Polymorphisms in the Capsular Locus of Streptococcus pneumoniae Serotype 19A

Abstract

Streptococcus pneumoniae serotype 19A prevalence has increased after the implementation of the PCV7 and PCV10 vaccines. In this study, we have provided, with high accuracy, the genetic diversity of the 19A serotype in a cohort of Dutch invasive pneumococcal disease patients and asymptomatic carriers obtained in the period from 2004 to 2016. The whole genomes of the 338 pneumococcal isolates in this cohort were sequenced and their capsule (cps) loci compared to examine their diversity and determine the impact on the production of capsular polysaccharide (CPS) sugar precursors and CPS shedding. We discovered 79 types with a unique cps locus sequence. Most variation was observed in the rmlB and rmlD genes of the TDP-Rha synthesis pathway and in the wzg gene, which is of unknown function. Interestingly, gene variation in the cps locus was conserved in multiple alleles. Using RmlB and RmlD protein models, we predict that enzymatic function is not affected by the single-nucleotide polymorphisms as identified. To determine if RmlB and RmlD function was affected, we analyzed nucleotide sugar levels using ultrahigh-performance liquid chromatography–mass spectrometry (UHPLC-MS). CPS precursors differed between 19A cps locus subtypes, including TDP-Rha, but no clear correlation was observed. Also, significant differences in multiple nucleotide sugar levels were observed between phylogenetically branched groups. Because of indications of a role for Wzg in capsule shedding, we analyzed if this was affected. No clear indication of a direct role in shedding was found. We thus describe genotypic variety in rmlB, rmlD, and wzg in serotype 19A in the Netherlands, for which we have not discovered an associated phenotype.