Enhanced killing of Acanthamoeba cysts with a plant peroxidase-hydrogen peroxide-halide antimicrobial system

The activity of H[subscript 2]O[subscript 2] against the resistant cyst stage of the pathogenic free-living amoeba Acanthamoeba was enhanced by the addition of KI and either horseradish peroxidase or soybean peroxidase or, to a lesser degree, lactoperoxidase. This resulted in an increase in the cysticidal activity of 3% (wt/vol) H[subscript 2]O[subscript 2], and there was >3-log killing in 2 h, compared with the 6 h required for comparable results with the peroxide solution alone (P < 0.05). With 2% H[subscript 2]O[subscript 2], enhancement was observed at all time points (P < 0.05), and total killing of the cyst inoculum occurred at 4 h, compared with 6 h for the peroxide alone. The activity of sublethal 1% H[subscript 2]O[subscript 2] was enhanced to give 3-log killing after 8 h of exposure (P < 0.05). No enhancement was obtained when KCl or catalase was used as a substitute in the reaction mixtures. The H[subscript 2]O[subscript 2] was not neutralized in the enhanced system during the experiments. However, in the presence of a platinum disk used to neutralize H[subscript 2]O[subscript 2] in contact lens care systems, the enhanced 2% H[subscript 2]O[subscript 2] system gave 2.8-log killing after 6 h or total cyst killing by 8 h, and total neutralization of the H[subscript 2]O[subscript 2] occurred by 4 h. In contrast, 2% H[subscript 2]O[subscript 2] alone resulted in <0.8-log killing of cysts in the presence of the platinum disk due to rapid (<1 h) neutralization of the peroxide. Our observations could result in significant improvement in the efficacy of H[subscript 2]O[subscript 2] contact lens disinfection systems against Acanthamoeba cysts and prevention of acanthamoeba keratitis.