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.
To determine whether the virulence of Streptococcus suis type 2 is associated with the phenotype of the strain, we infected newborn germfree pigs with 10 strains of S. suis type 2 categorized by three phenotypes. In an earlier study, the phenotypes were distinguished by the presence or absence of the muramidase-released protein (MRP) and an extracellular factor (EF) and were designated MRP+ EF+, MRP+ EF-and MRP-EF-. Pigs were first inoculated with Bordetella bronchiseptica to predispose them to infection and were then intranasally inoculated with the streptococci. Strains of the MRP+ EF+ phenotype induced fever and increased the number of polymorphonuclear leukocytes in blood. Specific clinical signs of disease such as nervous disorders and lameness were also observed. At necropsy bacteriologic and pathologic examination disclosed meningoencephalitis, polyserositis, and polyarthritis. Strains of the MRP+ EF-phenotype induced only nonspecific clinical signs of disease such as recumbency, lack of appetite, and fever; only slight pathologic changes were detected in the serosae. The four strains of the MRP-EF-phenotype induced no signs of disease. These findings indicate that the 110-kDa EF and, to a lesser degree, the 136-kDa MRP may be associated with the virulence of the bacterium. The results demonstrated that S. suis type 2 strains producing both MRP and EF are pathogenic for pigs.
Streptococcus suis type 2 strains that are pathogenic for pigs produce a 110-kDa extracellular protein factor (EF). Nonpathogenic and weakly pathogenic strains do not produce EF or produce a protein (EF*) that is immunologically related to EF. To study the pathogenesis of S. suis type 2 in pigs and to develop tools and methods for the control of S. suis type 2 infections, we cloned and characterized the genes encoding EF and various EF* proteins. Analysis of the deduced amino acid sequences showed that the first 833 amino acids at the N terminus of the EF and EF* proteins were nearly identical. The proteins differed, however, at their C termini. Unlike the 110-kDa EF protein, the EF* proteins contained several repeated units of 76 amino acids. The number and arrangement of the repeats in the EF* proteins varied. The data suggest that the gene encoding EF could have evolved from an epfp gene by a specific deletion event. The lack of repeated amino acid units in the EF protein may be related to virulence.
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