Virulence of Streptococcus suis type 2 for mice and pigs appeared host-specific.

A murine model for Streptococcus suis infection in pigs was validated by inoculating groups of 5 BALB/c and 5 CF1 mice with 10(7) CFU/ml of 13 different S. suis serotype 2 strains. The pathogenicity of these strains had been established in a standardized pig model of S. suis infection using one-week-old gnotobiotic pigs. We inoculated groups of mice intraperitoneally with 4 strains that were highly virulent for pigs and belonged to the phenotype MRP+EF+, with 4 strains, that were weakly virulent for pigs and belonged to the phenotype MRP+EF+, and with 5 strains that were non-virulent for pigs and belonged to phenotype MRP-EF-. The S. suis strains that were highly virulent for pigs caused high morbidity and an intermediate mortality in mice, the S. suis strains that were weakly virulent for pigs caused high morbidity but low mortality, and the strains that were non-virulent for pigs, induced highest morbidity and mortality. These results were comparable in both breeds of mice. In contrast to the pathology of S. suis infection in pigs with specific lesions, lesions in mice were histologically often characterized as non-specific, i.e., necrotizing encephalitis and focal or diffuse hepatitis sometimes with abscesses. Irrespective of breed (BALB/c vs. CF1), the murine model used for S. suis infection was incompatible with the pig model. This indicates that virulence of S. suis type 2 for mice and pigs is host-specific. Therefore, we regard the presently available murine models unsuitable for studying S. suis infections in pigs.

Purification of a heat labile dermonecrotic toxin from culture fluid of Pasteurella multocida.

A highly pure heat-labile dermonecrotic toxin (DNT) of Pasteurella multocida was isolated from bacterium-free broth culture fluid. The protocol for the isolation included the following steps: ammonium sulphate precipitation, gel filtration, ion exchange chromatography and preparative polyacrylamide gel electrophoresis (PAGE). About 1 mg of purified DNT was recovered from 3 l of broth culture fluid. The final product was toxic for embryonic bovine lung (EBL) cells, lethal for mice, dermonecrotic in the guinea pig skin test and inactivated by heating at 56 degrees c. The recovery of biological activity was about 5% that of the original culture fluid and the specific activity had increased about 4000 times. After sodium dodecyl sulphate (SDS)-PAGE and silver staining a single band appeared, indicating that the purified DNT was free from contaminating proteins. The molecular weight of the toxin was approximately 125,000 daltons. The minimal toxic dose of DNT protein for embryonic bovine lung cells was about 2 ng, the minimal dermonecrotic dose in the guinea pig skin test was about 80 ng and the 50% lethal dose for mice about 300 ng.