Streptolysin O (SLO)

Streptolysin O is a cholesterol-dependent, thiol-activated pore-forming exotoxin and major virulence factor secreted by Streptococcus pyogenes, encoded by the slo gene, with a mass around 57–63 kDa. It is oxygen-labile and antigenic, eliciting antistreptolysin O antibodies used clinically to document recent Group A streptococcal infection. SLO binds as a soluble monomer to cholesterol in target membranes, oligomerizes at the surface, and undergoes large conformational changes to insert transmembrane β-hairpins and form large pores that cause cytolysis and hemolysis. Crystal structures reveal four domains, with domain 4 mediating membrane recognition via an undecapeptide motif and cholesterol-sensing loops, and distinctive conformational features relative to perfringolysin O. Beyond pore formation, SLO contributes to GAS pathogenesis by translocating NAD-glycohydrolase into host cells, modulating neutrophil function, enhancing superantigen activity, impairing phagocytic clearance, and—recently—by directly binding plasminogen to accelerate its conversion to plasmin, promoting bacterial dissemination. Experimentally, human serum albumin can bind and neutralize SLO activity, and engineered double-mutant SLO toxoids abolish toxicity yet elicit protective antibodies in mice, supporting SLO as a vaccine antigen target.

Other names

Streptococcal hemolysin OCholesterol-dependent cytolysin (streptolysin O)Thiol-activated cytolysin (streptolysin O)slo gene productGroup A streptococcal pore-forming toxin

Binding to membrane cholesterol followed by oligomerization into large β-barrel pores leading to cell lysis. Oxygen-labile, thiol-activated hemolytic activity. Neutralization/antagonism: antibodies against SLO prevent binding/oligomerization (basis for toxoid vaccine protection). Human serum albumin binding can neutralize cytotoxic and hemolytic effects (experimental). Moonlighting activity: binding plasminogen to accelerate its activation (enhances bacterial spread).