Clinical streptococcal isolates, distinct from Streptococcus pneumoniae, containing the beta-glucosyltransferase tts gene and expressing serotype 37 capsular polysaccharide.

The major virulence factor of the pneumococcus, and target for conjugate vaccines, is the polysaccharide capsule, which is usually encoded by the highly variable cps locus. Serotype 37 is an unusual pneumococcal type in which the single processive β-glucosyltransferase gene responsible for serotype capsule production (tts) s located outside of the capsular operon region. Using a previously described automated whole genome sequence (WGS)-based serotyping bioinformatics stool, PneumoCaT (Pneumococcal Capsule Typing), we identified and further investigated seven clinical isolates (three from blood cultures) of non-pneumococcal streptococci, and included a further 10 clinical isolates of S.pneumoniae serotype 37, the Statens Serum Institut reference strain of serotype 37 and the S.pseudopneumoniae type strain BAA 960 to create a study panel of 20 isolates for full characterisation. Seven non-pneumococcal isolates generated novel alleles at all 7 pneumococcal MLST loci and gave low percentage similarity (99%)to the sequence reference for BAA-960. Twelve isolates gave high similarity (>77%) to the Streptococcus pneumoniae 5652-06 reference genome sequence and had previously reported MLST alleles. Each of the seven clinical non-pneumococcal strains and all of the 12 S.pneumoniae possessed a β-glycosyltransferase gene (tts) with >95% similarity to the pneumococcal tts reference sequence. All, but two strains in which the tts gene was detected gave positive reactions for serotype 37 in slide agglutination tests with serotype 37 typing sera. The two negative isolates were both identified as non-pneumococcal species. Phylogenetic analysis showed distinct clades corresponding to strains identified as pneumococcus or non-pneumococcus by kmer WGS analysis. Extended kmer database analysis and ribosomal MLST placed the isolates within the S. mitis group. Biochemical and bile solubility assays showed differences between the unusual isolates and S. pneumoniae. All isolates had detectable pneumolysin (ply) genes, but only those that identified as pneumococcus contained the genes for autolysin (lytA) or the ABC transporter lipoprotein A (piaA) with >80% coverage and >95% similarity. Here we report the existence of a novel group of strains separate from S.pneumoniae, but which can express a pneumococcal serotype 37 capsular polysaccharide and could be associated with clinical disease.