Development of Standardized Methods for Assessing Biocidal Efficacy of Contact Lens Care Solutions Against Acanthamoeba Trophozoites and Cysts

PURPOSE: To investigate experimental variables in the development of standardized methods to assess the efficacy of contact lens disinfection systems against the trophozoite and cysts of Acanthamoeba spp. METHODS: A. castellanii (ATCC 50370), A. polyphaga (ATCC 30461), and A. hatchetti (CDC: V573) were adapted to axenic culture and used to produce cysts either with Neff's encystment medium (NEM) or starvation on nonnutrient agar (NNA). Challenge test assays and a most probable number approach were used to compare the trophozoite and cysticidal efficacy of four multipurpose disinfectant solutions (MPDSs) and a one-step hydrogen peroxide system (with and without the neutralizing step). RESULTS: With trophozoites, four of four MPDSs and the one-step peroxide system gave ≥3 log₁₀ kill for all strains 6 hours, regardless of culture medium used. Greater resistance was found against cysts, with results for MPDSs varying by species and method of cyst production. Here, 1-3 log₁₀ kill was found with NEM cysts for three of four MPDSs compared with one of four for the NNA cysts at 6 hours (A. castellanii and A. polyphaga, only). The one-step peroxide system gave 1-1.9 log₁₀ kill with NEM cysts and 0.8-1.1 for NNA cysts. Only 3% hydrogen peroxide gave total kill (>3 log₁₀) of NNA cysts at 6 hours. CONCLUSIONS: A reproducible method for determining the susceptibility of Acanthamoeba trophozoites and cysts to contact lens care systems has been developed. This will facilitate assay standardization for assessing the efficacy of such products against the organism and aid development of improved disinfectant and therapeutic agents.