Reappraisal of the taxonomy of Streptococcus suis serotypes 20, 22, 26, and 33 based on DNA-DNA homology and sodA and recN phylogenies

To date, Streptococcus suis was divided into thirty-three serotypes based on its polysaccharide capsular antigens. Although 16S rRNA sequence similarities of serotypes 20, 22, 26, and 33 reference strains to the type strain NCTC 10234T were below the threshold value of 98.5% to assign them to S. suis species, no strong evidence support to reclassification. Here, their taxonomic identities were determined by DNA-DNA hybridization assays and by partial sequencing of the sodA and recN genes. Our results confirmed that the serotype 20, 22, 26, and 33 reference strains were distantly related to the type strain NCTC 10234T and the whole sequence strain P1/7 of S. suis. Moreover, the reference strains of serotypes 20, 22, and 26 were closely related to each other but distinct from the serotype 33 reference strain. Sequencing analyses of sodA and recN of a total 33 serotype reference strains showed that the serotype 20, 22, and 26 reference strains and the serotype 33 reference strain did not fall with not only other serotypes of S. suis, but also other streptococcal species (63 strains of 56 species for sodA and 87 strains of 55 species for recN). The evidence further substantiates the view that the reference strains of serotypes 20, 22, 26 and 33 should be taxonomically removed from S. suis, although their taxonomic designations and determinative phenotypic characteristics are yet to be addressed.