Purpose: There is some evidence that povidone iodine and natamycin may have activity against acanthamoeba, although these agents have not been well-studied and are not commonly used in clinical practice. We tested a series of acanthamoeba isolates obtained from the corneas of patients seen in San Francisco in order to confirm the cysticidal activity of these two medications.

Methods: We selected 9 acanthamoeba isolates obtained from corneal scraping at the Proctor Foundation from 2008-2012, and performed a previously described broth microdilution assay to determine the minimum cysticidal concentration (MCC) of povidone iodine and natamycin. Cysts were exposed to different concentrations of the drugs for 48 hours, then inoculated with E. coli, and then observed after 7 days. The lowest concentration of drug at which no trophozoites grew was classified as the minimum cysticidal concentration (MCC). An identical assay was performed for chlorhexidine as a control. All assays were performed in triplicate.

Results: We performed the assay on 1 ATCC acanthamoeba strain and 9 keratitis specimens. The MCCs for chlorhexidine ranged from 3.13 to 25 μg/ml, with a median of 12.5 μg/ml (IQR 6.25 to 12.5). The MCCs for natamycin ranged from 390.6 to 3125 μg/ml, with a median of 390.6 μg/ml (IQR 390.6 to 781.3). The MCCs for povidone iodine ranged from 0.31 to 78.1 μg/ml, with a median of 2.4 μg/ml (IQR 0.61 to 9.8).

Conclusion: Both natamycin and povidone iodine were cysticidal against clinical acanthamoeba isolates in this in vitro assay. A 5% natamycin suspension would be more than 100-fold higher than the MCC found in this study, and a 1% povidone iodine solution would be more than 4000-fold higher than the MCC found in this study. Although the clinical effectiveness of these agents remains to be studied, they may be considered as adjunctive therapy for acanthamoeba keratitis.

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