The cell envelope subtilisin-like proteinase is a virulence determinant for Streptococcus suis.
BACKGROUND
Streptococcus suis is a major swine pathogen and zoonotic agent that mainly causes septicemia, meningitis, and endocarditis. It has recently been suggested that proteinases produced by S. suis (serotype 2) are potential virulence determinants. In the present study, we screened a S. suis mutant library created by the insertion of Tn917 transposon in order to isolate a mutant deficient in a cell surface proteinase. We characterized the gene and assessed the proteinase for its potential as a virulence factor.


RESULTS
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).


CONCLUSIONS
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.
Groupe de Recherche en Ecologie Buccale, Faculté de Médecine Dentaire, Université Laval, Quebec City, Quebec, Canada.
Laetitia
L
Bonifait
Maria
M
de la Cruz Dominguez-Punaro
Katy
K
Vaillancourt
Christian
C
Bart
Josh
J
Slater
Michel
M
Frenette
Marcelo
M
Gottschalk
Daniel
D
Grenier
D000595:Amino Acid Sequence; D000818:Animals; D015139:Blotting, Southern; D002473:Cell Wall / Q000737:chemistry / Q000201:enzymology / Q000378:metabolism; D003198:Computer Simulation; D004195:Disease Models, Animal; D005260:Female; D006801:Humans; D053208:Kaplan-Meier Estimate; D051379:Mice; D050296:Microbial Viability; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional; D017011:Streptococcus suis / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism / Q000472:pathogenicity*; D020860:Subtilisin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D037521:Virulence Factors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
D037521:Virulence Factors; D020860:Subtilisin