Pneumolysin (PLY) is a key virulence factor contributes to the pathogenesis of Streptococcus
pneumoniae. In this study we investigated the effect of allicin and aqueous garlic extracts on
hemolytic activity of PLY both in prelysed and intact cells. Additionally the antimicrobial
activity of allicinwas tested against the bacteria. All tested materials potently inhibited the PLY
hemolytic activity. Allicin neutralizes PLY in a concentration- and time-dependent manner.
Twenty five minute incubation of PLY (2 HU/mL) with 0.61 mM/mL concentration of allicin,
totally inhibited hemolytic activity of PLY (IC50 ¼ 0.28 mM/mL). The inhibitory activity of old
extract of garlic was similar to pure allicin (IC50 ¼ 50.46 mL/mL; 0.31 mM/mL; P < 0.05). In
contrast fresh extract of garlic inhibits the PLY hemolytic activity at lower concentrations
(IC50¼13.96 mL/mL; 0.08 mM/mLallicin). Exposure of intact cells to allicin (1.8 mM)completely
inhibited hemolytic activity of PLY inside bacterial cells. The inhibitory effect of the allicinwas
restored by addition of reducing agent DTT at 5 mM, proposing that allicin likely inhibits the
PLY by binding to cysteinyl residue in the binding site. The MIC value of allicinwas determined
to be 512 mg/mL (3.15 mM/mL). These results indicate that PLY is a novel target for allicin and
may provide a new line of investigation on pneumococcal diseases in the future
