Characterization of virulence of the Streptococcus suis serotype 2 reference strain Henrichsen S 735 in newborn gnotobiotic pigs.

Strain Henrichsen S 735 (NCTC 10234) of Streptococcus suis serotype 2 reference and three other such strains (strains S 4005, S 3921 and T 141) were tested for virulence by inoculating pigs intranasally and intravenously. The taxonomical properties of each strain were determined. Phenotypes were determined by Western blotting based on MRP and EF protein expression and genotypes were determined by Southern hybridization analysis of the mrp and epf genes. Reference strain S 735 and strain S 3921 produced the 136 kDa MRPh and a 180 kDa form of EF, and hence these strains belong to the MRP + EF phenotype. In accordance with previous experiments with this phenotype, strains S 735 and S 3921 appeared to be only weakly virulent for newborn gnotobiotic pigs. Strain S 4005 produced the 136 kDa MRP and the 110 kDa form of EF, hence it belongs to the MRP + EF + phenotype. This strain was highly virulent for pigs. Strain T 141 did not produce MRP or EF, and hence belongs to the MRP-EF- phenotype. It was nonvirulent for pigs. The route of inoculation did not influence the frequency or severity of clinical signs of disease or lesions, which demonstrated that the 110 kDa EF is not essential during invasion. Southern blot analysis showed that all four S. suis type 2 strains contain sequences that are homologous to the epf and mrp genes. For studies on pathogenesis of S. suis type 2 infections in pigs, we recommend the use of strains that have been tested in a standardized pig model and that belong to the MRP + EF + phenotype, such as strain S 4005.