Group-A streptococci belonging to opacity-factor (OF)-positive M types were poorly haemolytic on horse-blood agar, but members of OF-negative M types, and M-negative variants of OF-positive strains gave good haemolysis. Horse-serum extracts of strains of OF-negative serotypes 6 and 12, and M-negative variant cultures of OF-positive serotypes 4 and 49, had higher titres of streptolysin S than did similar extracts of OF-positive, M-positive cultures of types 4 and 49. However, much larger amounts of streptolysin S could be extracted with ribonuclease (RNAase)-digested yeast ribonucleic acid (RNA) and M-positive OF-positive cultures treated in this way gave extracts at least as strong as did their M-negative variants or the OF-negative strains. Extraction of streptolysin S from OF-negative strains by serum could be inhibited by previous incubation of the serum with extracellular OF, suggesting that the production of diffusable OF by M-positive variants of OF-positive serotypes interferes with the extraction of streptolysin S by serum and leads to poor haemolysis on blood agar.
The haemolysis of all strains on blood agar was greatly improved by the incorporation of 0·1% (w/v) RNAase-digested yeast RNA into the medium, but the improvement was most marked in OF-positive serotypes.