The beta-hemolysis produced by group A Streptococcus on sheep blood agar is primarily the result of which streptolysin?

The beta-hemolysis produced by group A Streptococcus on sheep blood agar is primarily the result of which streptolysin?

• A. H
• B. M
• C. O
• D. S

Answer: (D)

Beta-hemolysis on sheep blood agar by group A Streptococcus is mainly due to streptolysin S, an oxygen-stable cytolysin produced by the organism. This toxin diffuses and lyses red blood cells around the colonies under normal aerobic incubation, producing the characteristic clear zone. Streptolysin O, while a potent hemolysin, is oxygen-labile and contributes less to the visible beta-hemolysis in standard culture conditions; it is more active under low-oxygen environments or within tissues. Clinically, antibodies against streptolysin O (ASO) are used to indicate recent exposure to this toxin, not to reflect SLS activity. Thus, the primary cause of the observed beta-hemolysis is streptolysin S.

Beta-hemolysis on sheep blood agar by group A Streptococcus is mainly due to streptolysin S, an oxygen-stable cytolysin produced by the organism. This toxin diffuses and lyses red blood cells around the colonies under normal aerobic incubation, producing the characteristic clear zone. Streptolysin O, while a potent hemolysin, is oxygen-labile and contributes less to the visible beta-hemolysis in standard culture conditions; it is more active under low-oxygen environments or within tissues. Clinically, antibodies against streptolysin O (ASO) are used to indicate recent exposure to this toxin, not to reflect SLS activity. Thus, the primary cause of the observed beta-hemolysis is streptolysin S.