A streptolysin is a type of hemolysin (hemolytic exotoxin, in particular) produced by or derived from some strains of streptococci. A streptolysin may be streptolysin O or streptolysin S. In 1930s, E. W. Todd was able to identify these two toxins responsible for the ability of group A streptococcus to lyse the erythrocytes of mammals.1 Streptolysin O is oxygen-labile whereas streptolysin S is oxygen-stable. 2
Streptolysin O is an oxygen-labile β-hemolytic enzyme belonging to a family of thiol-activated toxins. It is produced by and isolated from groups A, C and G streptococci. This cytolysin is capable of causing lysis in animal cells especially WBCs and RBCs by binding to the membrane cholesterol where it polymerizes to form a transmembrane channel (about 30 nm in diameter), which then leads to cell lysis. Thus, 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.
Word origin: strepto- from Streptococcus + (hemo)lysin)
1 Wessels, M. R. (2005). Streptolysin S. The Journal of Infectious Diseases, 192 (1): 13–15. https://doi.org/10.1086/430625
2 Sierig, G., Cywes, C., Wessels, M. R., & Ashbaugh, C. D. (2003). "Cytotoxic effects of streptolysin o and streptolysin s enhance the virulence of poorly encapsulated group a streptococci". Infect. Immun. 71 (1): 446–55