Streptolysin

Streptolysin is a set of toxin proteins produced by certain Streptococcus species, most notably Streptococcus pyogenes (group A Streptococcus). The two best characterized streptolysins are streptolysin O (SLO) and streptolysin S (SLS). They are virulence factors that damage host cells and tissues, but differ in chemical nature, stability, and regulation.

Streptolysin O is an oxygen-labile, cholesterol-dependent cytolysin encoded by the slo gene. It is secreted as an exoprotein and binds to cholesterol-containing membranes, where it oligomerizes and forms pores that disrupt cell integrity. SLO activity is inhibited by cholesterol in membranes and by oxidative conditions. It is highly hemolytic on blood agar and is immunogenic, meaning the immune system can produce antibodies against it. Antibodies to streptolysin O (ASO) are used clinically as a marker of recent group A Streptococcus infection, particularly in the evaluation of post-streptococcal diseases such as rheumatic fever.

Streptolysin S is an oxygen-stable, non-immunogenic peptide cytolysin produced by Streptococcus species, encoded within the sag operon (sagA, sagB, sagC, sagD, and related genes). It is processed by post-translational modification and contributes to beta-hemolysis on blood agar. Unlike SLO, SLS is not a cholesterol-dependent cytolysin and is not neutralized by cholesterol. It acts in concert with SLO to enhance tissue damage and immune evasion during infection.

Clinical and research relevance: measuring ASO titers helps infer recent GAS exposure; streptolysins are studied as models of pore-forming toxins and as targets for understanding streptococcal virulence.