Identification of six novel capsular polysaccharide loci (NCL) from Streptococcus suis multidrug resistant non-typeable strains and the pathogenic characteristic of strains carrying new NCLs.

Streptococcus suis is a major swine pathogen and an important zoonotic agent worldwide. At least nine serotypes can infect human so far. Although 29 serotypes (1-19, 21, 23-25, 27-31 and 1/2) strains are considered as authentic S. suis, a novel variant serotype Chz and strains carrying 20 novel capsular polysaccharide loci (NCL) have been identified recently. However, information about pathogenic and antimicrobial resistance characteristics of strains carrying NCLs is still unavailable. In this study, we identified six new NCLs (designated as NCL21-26) from 35 non-typeable S. suis strains by agglutination tests and whole genome sequencing analysis. Further analysis of the genetic context of NCL25 and NCL26 showed a mosaic structure of the capsular polysaccharide loci. NCL25 exhibited considerable similarity to that of serotypes 10 and 11, and NCL26 shared similarity to that of serotype 9 and NCL4. Antimicrobial susceptibility testing demonstrated that strains carrying NCL21-26 were all resistant to clindamycin, lincomycin, erythromycin, tilmicosin and tetracycline. Animal infection experiments showed that the virulence of NCL26 strain NJ1112 isolated from a disease pig was similar to that of S. suis serotype 2 virulent strain SC070731 in both zebrafish and mouse infection models, highlighting the necessity for surveillance of strains belonging to NCL26. We also developed a multiplex PCR assay to detect NCL21-26 strains. Our findings expand the views of genetic diversity of S. suis capsular polysaccharide loci and S. suis pathogenic characteristic.