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

• A. H
• B. M
• C. O
• D. S

Answer: (D)

The main idea is that the hemolysis seen on blood agar in the presence of air is driven by a toxin that remains active in oxygen. Streptolysin S fits this, as it is oxygen-stable and rapidly damages red blood cell membranes to produce a clear beta-hemolysis zone around the colonies under aerobic incubation. In contrast, streptolysin O is oxygen-labile and requires reduced, low-oxygen conditions to be active, so it does not primarily drive beta-hemolysis on standard aerobic plates. Thus, streptolysin S is the principal cause of the observed beta-hemolysis in this setting.