Assay of type-specific M antigens on whole group A streptococci.

A novel radioimmunoassay of type-specific M antigens on whole group A streptococcal cells is described. Absorbed rabbit anti-M antisera directed against M types 12 and 49 were used for determining M antigens on intact bacterial organisms. Staphylococcal protein A labelled with 125I was used as an anti-antibody reagent. The absorbed antisera were tested against ten homologous and 48 heterologous serotypes. All homologous serotypes gave an unequivocal reaction distinct from the weaker reaction with the heterologous serotypes. The type-specificity of the reaction was confirmed by the removal of type-specific antibodies after absorption to purified M protein coupled to Sepharose 4B. The results indicate that the described method is a simple and reliable technique for the recognition of M types of group A streptococci and offers a valuable tool for studies of M antigen in situ.