On the virulence of group A streptococci: evidence for virulence factors other than M-protein and capsule.

Group A streptococci from parent cultures (PC) of six different serotypes were selected by rotation with human blood (RHB) or by serial passage via intraperitoneal inoculation in mice (MP). M-protein content of PC, RHB and MP streptococci of each serotype was determined in quintuplicate by radial immunodiffusion against type-specific antisera. Hyaluronic acid content was determined in quintuplicate colorimetrically after treatment of streptococci with hyaluronidase. Data were subjected to variance analysis. LD(50) for mice of PC, RHB and MP streptococci, inoculated intraperitoneally, was also determined. It was observed that MP streptococci virulent for mice were rich in M-protein and hyaluronic acid. However, RHB streptococci of M-protein and hyaluronic acid content similar to MP streptococci were not virulent for mice. This dichotomy with respect to mouse virulence infers the existence of as yet unidentified streptococcal virulence factors, in addition to M-protein and capsule.