In Vitro Evaluation of the Combination of Melaleuca alternifolia (Tea Tree) Oil and Dimethyl Sulfoxide (DMSO) against Trophozoites and Cysts of Acanthamoeba Strains. Oxygen Consumption Rate (OCR) Assay as a Method for Drug Screening

Ameobae belonging to the genus Acanthamoeba are responsible for the human diseases Acanthamoeba keratitis (AK) and granulomatous amoebic encephalitis (GAE). The treatment of these illnesses is hampered by the existence of a resistance stage (cysts). In an attempt to add new agents that are effective against trophozoites and cysts, tea tree oil (TTO) and dimethyl sulfoxide (DMSO), separately and in combination, were tested In Vitro against two Acanthamoeba isolates, T3 and T4 genotypes. The oxygen consumption rate (OCR) assay was used as a drug screening method, which is to some extent useful in amoebicide drug screening; however, evaluation of lethal effects may be misleading when testing products that promote encystment. Trophozoite viability analysis showed that the effectiveness of the combination of both compounds is higher than when either compound is used alone. Therefore, the TTO alone or TTO + DMSO in combination were an amoebicide, but most of the amoebicidal activity in the combination’s treatments seemed to be caused mainly by the TTO effect. In fact, DMSO alone seems to be a non-amoebicide, triggering encystment. Regarding cytotoxicity, these compounds showed toxicity in human corneal epithelial cells (HCEpiC), even at low concentrations when tested in combination. In conclusion, the use of TTO and DMSO, in combination or alone, cannot be recommended as an alternative for AK treatment until more cytotoxicity and cyst adhesion tests are performed.