The novel virulence-related gene stp of Streptococcus suis serotype 9 strain contributes to a significant reduction in mouse mortality.

Streptococcus suis is an important swine pathogen responsible for a diverse range of diseases. S. suis serotype 2 (SS2) and S. suis serotype 9 (SS9) are the prevalent serotypes in diseased Chinese pigs. Little is known about SS9 virulence factors. Two strains, GZ0565 and SH040917, were isolated from a diseased pig and a healthy pig, respectively. Suppression subtractive hybridization (SSH) was used to identify SS9 virulence genes associated with pathogenicity. We identified 30 gene fragments unique to GZ0565, including stp which encodes a serine/threonine protein phosphatase known to affect the virulence and morphology of bacteria. To investigate the role of stp in pathogenesis of SS9, an isogenic stp mutant (Δstp) and a complementation strain (CΔstp) were constructed. The results demonstrated that the stp affected the expression of a few genes involving in adhesion and virulence for bacteria. The Δstp exhibited a significant decrease in HEp-2 cell adherence, compared with the wild type, and a reduced survival ratio in whole blood. The Δstp was attenuated in a CD1 murine model of infection and its LD50 values was seven-fold higher than the wild type. Our data suggest that stp is involved in the pathogenesis of SS9.