Characteristics of an R antigen common to Streptococcus equi and zooepidemicus

An R antigen of the group C streptococcus S. equi that cross reacts with a similar antigen of S. zooepidemicus has been identified and characterized. It is acid, heat and trypsin resistant, but pepsin sensitive and has an isoelectric point of 4.8. The amino acids in highest concentration are glutamic, aspartic, alanine, leucine, and valine. Bacterial components released in a French Press contain large amounts of R antigen, which is present also in culture supernatants and acid extracts. It has a molecular weight of about 82,000. Trypsin extraction of cells yields molecules of predominantly 56,000 and 25,000 molecular weight that appeared by immunoblotting to be similar to those obtained by trypsinization of purified R protein from preparations derived from the French Press. Although horses naturally infected with S. equi or S. zooepidemicus develop cross reacting R antibodies, the role of the R antigen in pathogenesis is unknown. Purified R protein does not stimulate bactericidal antibody in horses nor is it protective for mice. Its occurrence in antigen preparations in assays for S. equi antibody could be a source of interpretive errors because of the presence in many sera of antibody to the immunologically similar R antigen of S. zooepidemicus, a normal nasopharyngeal commensal of Equidae.