Determination of amoebicidal activities of multipurpose contact lens solutions by using a most probable number enumeration technique

Six multipurpose contact lens solutions [All-in-One, All-in-One (Light), ReNu MultiPlus, Optifree Express, Complete, and Solo-care soft] were tested for their efficacies against Acanthamoeba castellanii trophozoites and cysts by using a most probable number (MPN) technique for amoebic enumeration. Against trophozoites, All-in-One, ReNu Multiplus, and Optifree Express achieved total kill (log reduction of >3) after the manufacturer's minimum recommended disinfection time (MMRDT), with the remaining solutions failing to reach a log reduction of 1. After 24 h of exposure, all solutions proved trophozoiticidal, achieving, with the exception of Complete (log reduction of 3.13), total kill. Against cysts, All-in-One gave a log reduction of >3 within the MMRDT, with all other solutions failing to achieve a log reduction of 1. After 24 h of exposure, All-in-One achieved total kill of cysts (log reduction of 3.74), ReNu MultiPlus gave a log reduction of 3.15, and the remaining solutions reached log reductions of between 1.09 and 2.27. The MPN technique provides a simple, reliable, and reproducible method of amoebic enumeration that depends on simply establishing the presence or absence of growth on culture plates inoculated with a series of dilutions and determining the MPN of amoebae present from statistical tables. By use of this technique, two of the multipurpose solutions tested, ReNu MultiPlus and Optifree Express, demonstrated effective trophozoiticidal activities within the recommended disinfection times; however, only All-in-One proved effective against both trophozoites and cysts over the same time period. This MPN technique, which uses axenically produced trophozoites and mature, double-walled cysts, has the potential to form the basis of a national standard for amoebicidal efficacy testing of multipurpose contact lens disinfecting solutions.