Acanthamoeba spp. are free-living protozoans that cause a serious human eye disease called Acanthamoeba keratitis (AK). Several new and effective medical therapies for AK patients remain highly debated, and therefore, chlorhexidine digluconate (CHG) is still considered one of the first lines of treatment for AK patients. We hypothesized that ocular microenvironmental factors are responsible for Acanthamoeba drug resistance and clinical AK treatment failure. To investigate the influence of the ocular surface on CHG treatment, we tested the effect of several ocular elements on the antiamoeba activity of CHG. The suspected inhibitory elements, including mucin, albumin, human and amoeba cell lysates, live and heat-killed bacteria, and cornea, were added to the amoebicidal activity platform, where amoebae were incubated with CHG at various concentrations. Mucin showed a significant inhibitory effect on CHG activity against Acanthamoeba castellanii. In contrast, albumin did not affect CHG treatment. Furthermore, human and amoeba cell lysates and live and heat-killed bacterial suspensions also significantly inhibited CHG activity. Additionally, we found that pig corneas also reduced CHG activity. In contrast, dry-eye drops and their major component, propylene glycol, which is commonly used as eyewash material, did not have an impact on CHG activity. Our results demonstrate the effect of ocular microenvironmental factors on CHG activity and suggest that these factors may play a role in the development of amoeba resistance to CHG and treatment failure.
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