Studies of antigenic components in acid extracts of group C streptococci with special reference to Streptococcus equi

For the determination of a species-specific antigen of Streptococcus (S.) equi, acid extracts of group C streptococcal strains from horses (S. equi, S. zooepidemicus, S. equisimilis) were investigated using polyacrylamide gel electrophoresis and the immunoblotting technique. Using sera of horses suffering from strangles as well as sera from horses with respiratory infection of unknown etiology, Western blotting yielded more or less multiple banding reactions with bands in the 70, 54, 42, 40, and 31-28 kd molecular weight ranges against extracts of all of the 3 different bacterial species. However, an antigen found in this study at 18-16 kd which was highly sensitive to trypsin, proved to react specifically and regularly only with the serum of horses exposed to S. equi. The specificity of the reaction was assured by antisera of rabbits and horses vaccinated against S. equi, S. zooepidemicus or S. equisimilis, respectively, and by cross absorption of a serum originating from a mare recovered from strangles with Lancefield group C and group G streptococci as well as a strain of S. pyogenes. According to Western blot results on 180 serum pairs from horses with clinical signs of respiratory infection, 15.6% of which gave positive reactions, the 18-16 kd antigen appears as a marker suitable for qualitative testing of horse sera for antibodies to S. equi.