Streptococcus suis is an important zoonotic pathogen. However, identification of virulent S. suis strains is complicated because of the high diversity of the species. Here we evaluated the genetic difference among S. suis strains using comparative genomic hybridization (CGH) and virulence variation in vivo and in vitro. We showed that different clades differed in their ability to activate TLR2/6 in vitro and their capacity to induce cytokine production in vivo as well as their resistance to phagocytosis and survival in vivo. Our data showed the S. suis strains tested can be classified into three groups having differing levels of virulence: epidemic and highly virulent strains were clustered into clade Ia (epidemic and highly virulent group, E/HV group), virulent strains were clustered into clade Ib (virulent group, V group), and intermediately or weakly virulent strains were clustered into other clades (intermediately or weakly virulent group, I/WV group). Our study provided further insight into the genomic and virulence variation of S. suis.
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