Genetic variation in the M antigen of group A streptococci: reassortment of type-specific markers and possible antigenic drift.

The phylogenetic relationships of 73 strains of group A Streptococcus isolated over a six-year period from a population with a high prevalence of streptococcal infections were analyzed with use of four serotype-specific markers: the serum opacity factor (OF), the T antigen, the determinant of the M antigen that precipitates with antibody, and the M antiphagocytic determinant. OF inhibition tests divided the strains into five subtypes: a, b, c, d, and e. Strains within subtypes a, b, and c shared a previously unidentified M precipitin. The identification of this new common M precipitin was based on absorption studies, double agar immunodiffusion, and immunoelectrophoresis analysis. Antisera directed against representative strains from subtypes a, b, and c contained cross-reactive bactericidal antibody, and absorption studies indicated that determinants of the M precipitins were identical, whereas determinants of resistance to phagocytosis were closely related but not identical. The association of a common M precipitin with three different OF antigens, two different T antigens, and three different, yet similar, antiphagocytic determinants is discussed in light of the possibility that the genes that code for these antigens have reassorted or recombined. Moreover, the possibility that differences in the antiphagocytic determinants of these strains have resulted from antigenic drift in a common gene is considered.