Inhibition of streptolysin O by allicin - an active component of garlic.

Streptolysin O (SLO) is a potent cytolytic toxin produced by almost all strains of group A streptococci and is considered an important virulence factor for this organism. In this study we investigated the effect of allicin and aqueous garlic extracts on the haemolytic activity of SLO. All tested materials potentially inhibited the SLO haemolytic activity. Allicin neutralized SLO in a dose- and time-dependent manner. A 15 min incubation of SLO with 35 microg allicin totally inhibited the haemolytic activity of SLO [IC(50) (concentration necessary to reach half maximum inhibition)=5.97 microg]. The inhibitory activity of an old extract of garlic was equipotent to pure allicin (IC(50)=6.27 microg; P<0.05). In contrast, fresh extract of garlic inhibited the SLO haemolytic activity at lower concentrations (IC(50)=1.59 microl; 1.9 microg allicin). The inhibitory effect of the allicin was restored by addition of reducing agent DTT at 2 mM, suggesting that allicin likely inhibits the SLO by binding to the cysteine residue in the binding site. These results indicate a new activity for allicin and allicin may be a potential alternative drug against streptococcal diseases.