streptolysin S

S. pyogenes produces two haemolysins with complementary properties. Streptolysin S is oxygen-stable and non-immunogenic, and it produces the clear zone of beta-haemolysis visible on the aerobic surface of the plate. Streptolysin O is oxygen-labile, so it is inactivated at the aerobic surface and produces haemolysis only in the deeper, anaerobic parts of a stab culture, and it is strongly antigenic, generating the anti-streptolysin O antibodies measured by the ASO titre. The practical consequence is that the ASO titre detects prior streptococcal infection but is unaffected by streptolysin S. ASO titres rise after pharyngeal infection and are used as evidence of antecedent streptococcal disease in acute rheumatic fever, whereas anti-DNase B is more reliable after skin infection and therefore in post-streptococcal glomerulonephritis following impetigo. Both haemolysins lyse erythrocytes, leukocytes, and platelets. Beta-haemolysis with bacitracin sensitivity and a positive PYR test identifies group A Streptococcus, distinguishing it from bacitracin-resistant group B Streptococcus agalactiae.