Abstract
Dermatophytoses account for nearly a quarter of all fungal infections worldwide. These difficult to treat infections of the skin, hair, and nails, are growing more resistant to conventional antifungal treatments, and when treatable, often require prolonged therapeutic regimens. For centuries, essential oils have been used to treat a variety of ailments. In this study, we evaluated the clinical effects in vitro of 65 essential oils and 21 essential oil blends against various clinical species/strains of dermatophytes from two primary genera, Microsporum and Trichophyton. Our aim: To determine the overall activity of a wide range of essential oils against a number of clinical strains of dermatophytes. For all assays, 16 clinically derived species/strains of dermatophytes were used. The activity of each essential oil was assessed using a modified disk-diffusion assay over a period of 21 days of incubation vs. standard antifungal drugs. Subsequently, we determined the minimum inhibitory dilution possible for the most potent essential oils and performed combination testing to determine if synergy could be demonstrated with sub-inhibitory concentrations. We also assessed the effect of repeated vs. single applications. Of all the essential oils tested, cassia, cilantro, cinnamon, thyme, and oregano were the most potent along with one blend, DDR Prime; all genera/species tested were completely inhibited for 21 days following a single application. Many of the other oils tested exhibited temporal differences in activity where significant inhibition was observed ≤10 days of incubation which declined by day 21. Synergistic combinations were achieved with oregano and cilantro, cassia, or cinnamon bark; rose and cassia were also synergistic. Repeat application maintained complete inhibition for citronella, lemon myrtle, and litsea out to 21 days, but not lemon grass or On Guard. More study is necessary to understand the ways essential oils inhibit the growth of dermatophytes. Comprehensive research aimed at understanding the mechanism of action of essential oils and their components may provide the basis for a natural alternative to topical antifungal drugs. Such research could be envisioned to target optimal combinations and determine the timing between applications to provide for maximum inhibition of recurrence or growth.
Highlights
Dermatophytoses account for 20–25% of fungal infections worldwide, with a prevalence approaching one billion as of 2010 (Havlickova et al, 2008)
MO) and terbinafine (30 μg/ml, Cambridge Reagents, East Yorkshire, UK) were used as a comparative reference for essential oil mediated zones of inhibition where a distribution curve for these two antifungal drugs was established. This distribution curve was used to determine the range for each antifungal drug, known to vary in potency from high to low against various dermatophyte genera/species
Other compounds or components in lesser abundance may contribute to the inhibitory activity observed in this study. This is in contrast to what Chee and coworkers observed in their study (2009) in which limonene alone in concentrations of 0.5% v/v inhibited the dermatophyte T. rubrum in a broth microdilution assay (Chee and Lee, 2009)
Summary
Dermatophytoses account for 20–25% of fungal infections worldwide, with a prevalence approaching one billion as of 2010 (Havlickova et al, 2008). In addition to costs associated with treatment, many currently used antifungal agents have significant side effects underscoring the need for identification of therapeutic alternatives including those from natural products such as plant-based bioactive compounds, essential oils, and/or their components (Gupta et al, 2016; Lopes et al, 2016). Some investigations have led to the conclusion that the anti-dermatophytic activity resulted from synergy between major and minor components rather than the result of the presence of a single compound (Elaissi et al, 2012) In this regard, there is much to learn about these complex mixtures, including expansion of the number of EOs tested. We evaluated a large number of EOs for which GC-MS data was available and determined their activity in vitro against Trichosporum and Microsporum species of clinical importance
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