Virulence genotyping and population analysis of Streptococcus suis serotype 2 isolates from China

Streptococcus suis is one of the most important swine pathogens worldwide. In this study, a total of 22 virulence-related genes in 101 strains of S. suis serotype 2 (SS2) were detected by PCR, namely, mrp, epf, sly, fbps, rgg, ofs, srtA, pgdA, gapdh, iga, endoD, ciaRH, salKR, manN, purD, rgg, DppIV, neuB, dltA, SMU_61-like, SpyM3_0908 (Permease) and the SspA gene. The distribution of virulence-related genes among isolates was visualized using BioNumerics software to study similarities among the isolates. Two clusters of SS2 were apparent on the phylogenetic tree, namely, Clusters A and B. Both mouse and zebrafish infection models revealed that strains in Cluster B were more virulent than those in Cluster A. Statistical comparison between the two clusters was performed, and structure analysis demonstrated that epf, sly, rgg, endoD, SMU_61-like and SpyM3_0908 were possible predictive markers for SS2 virulence. The transcription levels of highly prevalent genes in both clusters were detected by qRT-PCR in representative strains. For Cluster A isolates, the transcription levels of neuB, dltA, fbps and pgdA were significantly lower, but the transcription level of iga was significantly higher than that in Cluster B isolates. Although encoded in the genomes of the selected Cluster A isolates, DppIV and mrp genes were not expressed. These results revealed the genetic differences between virulent and low-virulence SS2 isolates from China and provide a better understanding of the SS2 pathogenic mechanism.