Determination of Amoebicidal Activities of Multipurpose Contact Lens Solutions by Using a Most Probable Number Enumeration Technique

Abstract

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, doublewalled cysts, has the potential to form the basis of a national standard for amoebicidal efficacy testing of multipurpose contact lens disinfecting solutions.

The Association of Contact Lens Solution Use and Acanthamoeba Keratitis

“…In addition, Complete multipurpose solution is less effective than other multipurpose solutions against cysts and trophozoites when tested using multiple Acanthamoeba genotypes, 39 as well as alternative amoebicidal efficacy testing techniques. 30,31 Analysis was restricted to soft contact lens use because the percentage of soft contact lens use between cases and controls was statistically different, reducing sample size. This differential soft lens use between cases and controls was unexpected based on previous AK series and laboratory studies, which suggest insufficient evidence to indicate differential AK risk among soft and rigid lens users.…”

“…While the underlying mechanism for this risk remains unknown, none of the widely-used multipurpose solutions have been shown to be highly efficacious against Acanthamoeba; however they have generally been adequate to suppress large AK outbreaks in the past. 8,[30][31][32][33][34][35][36] This suggests the risk presented by use of Complete MoisturePlus may be a relative inability to withstand an Acanthamoeba challenge compared to that of other solutions. Results similarly demonstrate non-statistically significant patterns of risk suggesting hygiene-and potentially water-related factors such as showering with contact lenses may contribute to AK.…”

How Could Contact Lens Wearers Be at Risk of Acanthamoeba Infection? A Review

“…28 The adherence of Acanthamoeba trophozoites and cysts to contact lenses can be detected and quantified using different methods and techniques. 9,[29][30][31] The ability of Acanthamoeba to attach to contact lenses is influenced by several factors:…”

“…Acanthamoeba has a great affinity for the attachment to the corneal epithelium 8 and to contact lenses. 9 The tendency for Acanthamoeba to adhere to surfaces is a key first step in the pathogenesis of Acanthamoeba keratitis, particularly in contact lens wearers. Contact lenses affect corneal epithelium integrity in two different ways: directly, through the associated fitting-related abrasions 10 and indirectly, by altering the normal physiological and metabolic cellular activities.…”

Development of Colorimetric Microtiter Plate Assay for Assessment of Antimicrobials against Acanthamoeba

“…Other quantitative methods include the standard plaque assay (14,13) and a quantitative microtiter method for the enumeration of track-forming units (6). The number of viable organisms has also been estimated by most probable number enumeration methods (4). An ideal assay would measure only live parasites, the results would be read in a nonsubjective manner, and the assay would not rely on manual counting.…”