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The increasing incidence of hospital-acquired infections caused by drug-resistant pathogens, host toxicity, the poor efficacy of drugs and high treatment costs has drawn attention to the potential of natural products as antifungals in mucocutaneous infections and combinational therapies. Moreover, cellular and subcellular targets for these compounds may provide better options for the development of novel antifungal therapies. Eugenol, methyl eugenol and estragole are phenylpropanoids found in essential oil. They are known to possess pharmacological properties including antimicrobial activity. Induction of oxidative stress characterized by elevated levels of free radicals and an impaired antioxidant defence system is implicated as a possible mechanism of cell death. An insight into the mechanism of action was gained by propidium iodide cell sorting and oxidative stress response to test compounds in Candida albicans. The extent of lipid peroxidation (LPO) of cytoplasmic membranes was estimated to confirm a state of oxidative stress. Activity levels of primary defence enzymes and glutathione were thus further determined. Whereas these compounds cause fungal cell death by disrupting membrane integrity at minimum inhibitory concentrations (MIC), sub-MIC doses of these compounds significantly impair the defence system in C. albicans. The study has implications for understanding microbial cell death caused by essential oil components eliciting oxidative stress in Candida. The formation of membrane lesions by these phenylpropanoids thus appears to be the result of free radical cascade-mediated LPO.

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Our studies suggest that mint EO and its lead compounds exert antifungal activity by reducing ergosterol levels, inhibiting PM-ATPase leading to intracellular acidification, and ultimately cell death. Our results suggest that mint EO and its constituents are potential antifungal agents and need to be further investigated.

The species Candida is a group of opportunistic pathogenic commensals in immunecompromised patients. Treatment of Candida infections is becoming increasingly difficult due to antifungal drug resistance, especially with fluconazole (FLC), which is a commonly used azole. In the present study the in vitro antifungal activity of eugenol (EUG) and methyleugenol (MEUG) alone and in combination against 64 FLC-sensitive and 34 FLC-resistant clinical Candida isolates is highlighted. All the strains were susceptible to both the naturally occurring phenyl propanoids. The nature of the interaction was studied from fractional inhibitory concentration indices (FICIs) for both EUG plus FLC, and MEUG plus FLC combinations calculated from chequerboard microdilution assays. FICI values depicted a high synergism of FLC with both compounds, which was greatest with MEUG. FLC-resistant Candida isolates showed high sensitivity to both compounds. No antagonistic activity was seen in the strains tested in the present study. From these results we suggest that EUG and MEUG have great potential as antifungals, and that FLC can be supplemented with EUG and MEUG to treat FLC-resistant Candida infections.

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Aqueous extracts and oils of five Indian medicinal plants, traditionally used for their antimicrobial activities, were evaluated against two of the most prevalent Candida species causing candidiasis, C. albicans and C. tropicalis. Of these plant materials, three showed varying degrees of antifungal activity against both species. Tulsi (Ocimum sanctum Linn.) essential oil (TEO) was found to be the most effective, followed by Peppermint essential oil, and Aloe vera aqueous leaf extract. The product with the lowest MIC was further studied along with its lead molecules to explore the possible mechanism of action of the most active constituents. Eugenol, methyl eugenol, linalool, and 1, 8-cineole, along with TEO were then evaluated at the same. The pattern and extent of inhibition was studied using growth and WST1 cytotoxicity assays. Proton pumps are important for growth and metabolism of Candida species and so H + extrusion studies were performed to explore the possible mechanism of the test compounds. Linalool was the most active constituent of TEO, whereas inhibition of H + extrusion appeared to be a synergistic function of the lead molecules.