Which organism’s beta-hemolysis on blood agar is primarily due to streptolysin S?

• A. Group A Streptococcus Heliobacter
• B. Group A Streptococcus M-protein
• C. Group A Streptococcus O antigen
• D. Group A Streptococcus S streptolysin

Answer: (D)

Beta-hemolysis on blood agar by Group A Streptococcus is mainly caused by streptolysin S, an oxygen-stable cytolysin produced by the organism. Because it remains active in the presence of oxygen, streptolysin S drives the clear beta-hemolysis zone seen on standard aerobic plates. Streptolysin O, the other toxin the bacteria can produce, is oxygen-labile and requires low-oxygen conditions to contribute, so its role is less prominent under typical lab conditions. The other options don’t fit because they are not the toxins responsible for the hemolysis pattern: M-protein is a virulence factor for adherence and immune evasion, not a hemolysin; the O antigen is part of Gram-negative LPS; and the mentioned organism for this scenario is Group A Streptococcus producing streptolysin S.