Identification of two proteins associated with virulence of Streptococcus suis type 2

The protein profiles of various cell fractions of 180 strains of Streptococcus suis type 2, which were isolated from diseased pigs, from healthy pigs when they were slaughtered, and from human patients, were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. The isolates from diseased pigs contained two proteins that were absent in most of the isolates from healthy pigs. One of these proteins was a 136-kDa protein that was previously identified as the muramidase-released protein (MRP). This protein was predominantly detected in protoplast supernatants and culture supernatants. The second protein was a 110-kDa protein that was detected only in culture supernatants and therefore was provisionally called extracellular factor (EF). Three phenotypes of S. suis type 2 strains were recognized. Isolates from organs of diseased pigs mainly belonged to the MRP+ EF+ phenotype (77%), while isolates from tonsils of healthy pigs mainly had the MRP- EF- phenotype (86%). Most of the isolates from human patients contained MRP (89%); 74% had the MRP+ EF- phenotype. These findings confirm the results of previous investigations which demonstrated that S. suis type 2 strains differ in virulence. Monoclonal antibodies raised against the 110-kDa EF recognized proteins with higher molecular weights in culture supernatants of all of the strains with the MRP+ EF- phenotype. However, none of the strains with the MRP+ EF+ phenotype produced these high-molecular-weight proteins. Our results demonstrate that MRP and EF are associated with virulence. This suggests that one or both of these proteins are virulence factors that play a role in the pathogenesis of S. suis type 2 infections in pigs and human patients.