The effect of Acanthamoeba concentration on adherence to four types of unworn soft contact lenses

Contact lens wear is a predominant risk factor for Acanthamoeba keratitis. The exact nature of the relationship between organism concentration and contact lens adherence is poorly understood. We investigated the effect of Acanthamoeba inoculation concentration on adherence to four categories of contact lenses of varying polymers and water content. Acanthamoeba polyphaga was harvested in log growth phase at 5 days subculture and suspended in PBS at concentrations of 1 x 10(2), 10(3), 10(4), 10(5), and 10(6) organisms/mL (trophozoite:cyst ratio 90:10 +/- 2). Sterile unworn polymacon, etafilcon, lidofilcon, and bufilcon contact lens segments were exposed to Acanthamoeba for 2 hours. Acanthamoeba adherence was quantified using phase contrast microscopy. For all lens types, trophozoite adherence increased as the concentration of inoculum increased, but the relationship was not directly proportional. In all cases the minimal adherence was observed at 10(2). Trophozoite adherence increased disproportionate to cysts for all contact lens types. The greatest adherence was to lidofilcon lenses. At all concentrations adherence was greater to lidofilcon than etafilcon or polymacon, and greater to bufilcon than etafilcon or polymacon at the P < 0.01 level. Adherence was significantly greater to lidofilcon than bufilcon only at 1 x 10(5) and 10(6); P < 0.05 (ANOVA). This study suggests that adherence of A. polyphaga to contact lenses increases with the number of organisms in the inoculum, but the relationship is not directly proportional. The number of adherent organisms varies by contact lens type, with the greatest adherence to lidofilcon and the least to etafilcon lenses.