Anti-candida and Anti-virulence Activity of Essential Oils and Their Components
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Candida albicans is one of the most common fungi associated with lifethreatening infections. Treatment of the associated infections is often ineffective in the light of resistance, and so there is an urgent need to discover novel antifungals. Rather than killing the fungal cells, which requires quite high specificity and can lead to the emergence of further resistance, inhibiting growth and virulence factors in fungal cells represents a good alternative for the development of new antifungal drugs. Recently, there has been a resurged interest in essential oils and their active components, in relation to their pharmacological properties. The primary objective of this research was to evaluate the antifungal activity of cinnamon bark and rosemary essential oils, along with its major components cinnamadehyde, and 1,8-cineole, α-pinene, respectively, and two other common essential oil components, namely eugenol and citral, against C. albicans. Despite many reports on the antimicrobial activities of essential oils, the results have been diverse and the evaluation methods inconsistent. At higher essential oil concentrations, reactive oxygen species were generated, impacting a wide range of processes including cell membrane depolarization, vacuolar segregation, mitochondrial dysfunction, cell-cycle checkpoint deficiency, and mitotic catastrophe, which resulted in C. albicans death. At lower fractional MICs, essential oils had a ROS-independent response that inhibited mycelial growth and biofilm formation, which may be attributable to defects at the cell membrane. However, microtubule inhibition also plays a role in limiting hyphal growth. I explored the microtubule defects and how they relate to Kar3p, a member of the kinesin-14 family shown to be linked to microtubule stability. Interestingly, both tubulin and Kar3 protein was delocalized with essential oil components exposure at levels for which endogenous ROS levels were normal. I further demonstrate that the level of Kar3 is associated with resistance and susceptibility of C. albicans to essential oil components in both liquid and on solid growth media. All mutant strains grown on hyphae-inducing media in the presence of essential oil components exhibited an enhanced tendency to form pseudohyphal cells, a common phenotype for the homozygous and heterozygous deletion strains. Towards these ends, microtubule defects were linked with theoretical binding between essential oil components and α-tubulin and Kar3p adjacent to cofactor binding sites, consistent with experimentally observed hyphal defects and biofilm inhibition. This study for the first time uncovers a new mode of essential oilmediated microtubule defects, which does not follow the known mechanisms of conventional microtubule inhibition. The essential oil components appear to impact C. albicans eventually giving rise to psuedohyphal formation, microtubule loss, hyphal and biofilm reduction. Since those essential oil components impact multiple Candida targets, they should be less susceptible to resistance. Further, many components showed antivirulence, and may represent an effective approach for inhibiting Candida, especially in the context of physically preventing Candida from entering its host through catheters and prosthetics.