Streptolysin O

Streptolysin O is a cholesterol-dependent cytolysin that inserts into host membranes to form pores; it is inactivated by oxygen, so on a blood agar plate it contributes to subsurface haemolysis while streptolysin S produces the surface beta-haemolysis. Its antigenicity is what gives it clinical utility: the anti-streptolysin O titre rises about one to three weeks after pharyngeal infection, peaks at three to five weeks, and supports a diagnosis of acute rheumatic fever, where laboratory evidence of preceding group A streptococcal infection is required alongside the Jones criteria. ASO is less reliable after skin infection because skin lipids inhibit streptolysin O, so anti-DNase B is preferred when investigating post-streptococcal glomerulonephritis following impetigo. The two post-streptococcal sequelae differ importantly: rheumatic fever follows pharyngitis only and is prevented by prompt antibiotic treatment, whereas glomerulonephritis follows either pharyngitis or skin infection and is not prevented by antibiotics. Related cholesterol-dependent cytolysins include pneumolysin and listeriolysin O.