The cell envelope subtilisin-like proteinase is a virulence determinant for Streptococcus suis

Two mutants (G6G and M3G) possessing a single Tn917 insertion were isolated. The affected gene coded for a protein (SSU0757) that shared a high degree of identity with Streptococccus thermophilus PrtS (95.9%) and, to a lesser extent, with Streptococcus agalactiae CspA (49.5%), which are cell surface serine proteinases. The SSU0757 protein had a calculated molecular mass of 169.6 kDa and contained the catalytic triad characteristic of subtilisin family proteinases: motif I (Asp200), motif II (His239), and motif III (Ser568). SSU0757 also had the Gram-positive cell wall anchoring motif (Leu-Pro-X-Thr-Gly) at the carboxy-terminus, which was followed by a hydrophobic domain. All the S. suis isolates tested, which belonged to different serotypes, possessed the gene encoding the SSU0757 protein. The two mutants devoid of subtilisin-like proteinase activity had longer generation times and were more susceptible to killing by whole blood than the wild-type parent strain P1/7. The virulence of the G6G and M3G mutants was compared to the wild-type strain in the CD1 mouse model. Significant differences in mortality rates were noted between the P1/7 group and the M3G and G6G groups (p < 0.001).In summary, we identified a gene coding for a cell surface subtilisin-like serine proteinase that is widely distributed in S. suis. Evidences were brought for the involvement of this proteinase in S. suis virulence.The swine pathogen Streptococcus suis is transmitted via the respiratory route and colonizes the palatine tonsils and nasal cavities of pigs from where it can disseminate throughout the animal and cause infections [1], mainly septicemia, meningitis, and endocarditis, as well as arthritis [1]. Zoonotic infections occur mainly in individuals who work in close contact with pigs or pork by-products [2]. In fact, S. suis is considered one of the most important etiologic agents of adult meningitis in Asian countries [3]. While thirty-five serotypes (1 to 34 and 1/2) have been identifi