Quantitative comparison of Acanthamoeba castellanii adherence to rigid versus soft contact lenses

We evaluated quantitative differences in adherence of Acanthamoeba castellanii trophozoites and cysts to new unworn contact lenses as a function of contact lens type and duration of exposure. Daily wear soft (polymacon), disposable soft (etafilcon A), rigid gas permeable (RGP) (siloxane acrylate), and polymethylmethacrylate (PMMA) contact lenses were exposed to 1 x 10(5)/mL log phase A. castellanii (trophozoite:cyst ratio 90:10) for 10 seconds, 30 minutes, 60 minutes, and 120 minutes. The number of adherent organisms was determined by light microscopy and confirmed by scanning electron microscopy. Rates of cyst adherence were similar among contact lens types. We observed significantly greater adherence of trophozoites to RGP and PMMA lenses compared with soft contact lenses (P < 0.05). This difference in trophozoite adherence was significant at all four time periods (P < 0.05). Our findings show that A. castellanii trophozoites and cysts adhere to both soft and rigid contact lens types. We also found that although trophozoite adherence increases as a function of duration of exposure, adherent organisms were observed with exposure times as brief as 10 seconds. This suggests that all contact lens types may be at risk for Acanthamoeba attachment and that even minimal exposure times may be sufficient for lens contamination.