Discovery and description of a new serogroup 7 Streptococcus pneumoniae serotype, 7D, and structural analysis of 7C and 7D

Streptococcus pneumoniae is characterised into 92 serotypes based on antigenic reactions of commercial rabbit sera to the capsular polysaccharides. During development of a bioinformatic serotyping tool (PneumoCaT), an isolate exhibited a novel codon at residue 385 of the glycosyltransferase gene wcwK encoding a distinct amino acid, which differentiates genogroup 7. Investigation by repeat serotyping and Quellung reaction revealed a novel pattern of factor sera with the isolate reacting very strongly with 7f, but also with 7e factor sera. The structure of the capsular polysaccharide was determined by NMR spectroscopy to be an approximately 5:1 combination of the structures of 7C and 7B, respectively, and the structure of 7C was also elucidated. All data from whole genome sequencing, NMR spectroscopy, production of antisera and serotyping of the novel 7 strain shows that it is a new serotype, which will be named in the Danish nomenclature as 7D.