Modified coagglutination procedure for the serological grouping of streptococci

Cowan I staphylococci coated with antisera to streptococcal groups A, B, C, D, F, and G were used as coagglutination reagents in a modified coagglutination procedure (MCAP). Streptococcal group antigens were extracted with a Streptomyces albus-lysozyme enzyme mixture for 30 min at 55 degrees C and centrifuged, and the supernatant was tested by slide coagglutination. Positive coagglutination reactions occurred within 30 s. The cell pellets from overnight broth cultures and colonies taken directly from sheep blood agar plates were tested and compared with the results of the Lancefield capillary precipitin method. Of the 102 strains of broth-grown cells tested, 100 were grouped by the MCAP and the Lancefield capillary precipitin method. The remaining two isolates were serologically identified only by the MCAP. Of the original 102 strains, 97 were tested by MCAP after extraction of five well-isolated colonies from a sheep blood agar plate. When this latter method was used, 95.9% of the strains were correctly identified. Nonspecific reactions were observed only while testing the MCAP with the direct plate assay. These cross-reactions were remedied promptly by either absorption or dilution of the antisera involved. The MCAP was found to be a rapid and reliable technique for the serological grouping of streptococci.