Streptococcus pneumoniae serotype 33H: a novel serotype with frameshift mutations in the acetyltransferase gene wciG.

Background: Streptococcus pneumoniae (the pneumococcus) is a leading cause of community-acquired pneumonia. Pneumococci are categorised into serotypes, based on the type of capsular polysaccharide produced, which has important implications for virulence, vaccine impact and global surveillance. Recently, we identified a novel serotype, which we named 33G, that is comprised of an O-acetylated hexasaccharide repeat unit. In this study, we report and describe variants of 33G, designated 33G-like, which we isolated from the nasopharynx of two adults hospitalised with pneumonia in Mongolia.

Methods: Serological comparison of 33G and 33G-like pneumococci were conducted by Quellung serotyping. Genetic analysis of the capsular polysaccharide loci was performed using whole genome sequencing. Polysaccharide composition was determined using ¹H nuclear magnetic resonance.

Results: By Quellung serotyping, 33G pneumococci type as both 10B and 33B whereas 33G-like pneumococci type as both 10B and 33F. Genomic analysis of the capsular polysaccharide locus revealed 33G-like loci are identical to 33G, except for frameshift mutations in the wciG gene which encodes an acetyltransferase responsible for the O-acetylation of beta-galactofuranose (β-Galf) in the capsular polysaccharide repeat unit. We constructed an artificial 33G-like by deleting wciG in a 33G strain and confirmed this gene was responsible for the serological differences between 33G and 33G-like pneumococci. Lastly, ¹H nuclear magnetic resonance confirmed the O-acetylation present in the 33G polysaccharide is absent in the 33G-like polysaccharide.

Conclusions: Here, we have provided serological, genetic and biochemical evidence that the 33G-like capsule differs to 33G and all other pneumococcal serotypes, meeting the requirements to be designated as a new serotype, which we have named 33H.