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Streptolysin O

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(1) An oxygen-labile β-hemolytic enzyme belonging to a family of thiol-activated toxins, produced by and isolated from groups A, C and G of streptococci. This cytolysin is capable of causing lysis in animal cells especially WBCs and RBCs by binding with the membrane cholesterol where it polymerizes to form a transmembrane channel (about 30 nm in diameter), which then causes cell lysis.

(2) An antigenic compound used as a basis in antistreptolysin O (ASO) test to diagnose infection with group A Streptococcus bacteria.


Streptolysin O is a potent cell poison responsible for the lysis of WBCs and RBCs. It is also cardiotoxic. It causes changes in myocardial contractility and affects the cardiac conduction system.

Compare: streptolysin S
See also: haemolytic streptococci.