Serological grouping of streptococci by slide agglutination.

Streptococcal grouping sera for groups A, B, C, and G prepared for conventional testing by precipitation were made specific by absorption and used to identify streptococci by slide agglutination with and without staphyloccocal coagglutination. Trypsinised suspensions of 1055 strains, identified by precipitation as belonging to group A, B, C, or G, were tested by slide agglutination. Of these, 998 were correctly identified using a streptococcal suspension and antisera alone and a further 65 were identified when a loopful of protein A-positive staphylococci was added. Suspensions of 88 strains not of groups A, B, C, or G gave no reaction in the agglutination test with or without the addition of staphylococci. Group polysaccharide extracted by conventional methods also caused agglutination of staphylococci on a slide when specific antiserum was added. Growth from primary or secondary cultures digested in streptomyces enzyme for only 15-30 minutes provided an excellent antigen for a quick and simple method of streptococcal grouping using non-sensitised staphylococcal suspension and specific antisera for coagglutination.