Two haemolysins are produced by Streptococcus pyogenes. One cross-reacts antigenically with cardiac myosin and is strongly immunogenic; the other produces more intense haemolysis on blood agar but evokes no detectable antibodies. What distinguishes them?

• A Streptolysin O is oxygen stable, streptolysin S is oxygen labile
• B Both are oxygen labile, but only streptolysin S is immunogenic
• C Streptolysin O is oxygen labile and immunogenic, streptolysin S is oxygen stable and non-immunogenic ✓
• D Streptolysin O acts only on red cells of ruminants, streptolysin S on human cells

Explanation

Streptolysin O is oxygen labile, hence haemolysis appears only in subsurface colonies, and it is highly immunogenic so antistreptolysin O titres rise after pharyngeal infection. Streptolysin S is oxygen stable, produces surface haemolysis around colonies, and is not immunogenic, which is why it cannot be measured serologically. Option A simply reverses the oxygen lability property.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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