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.