Nicole Parrish,1,*Stefanie L. Fisher,1Ashlea Gartling,1David Craig,1Nicholas Boire,1Joshua Khuvis,1Stefan Riedel,1,2 and Sean Zhang1Author informationArticle notesCopyright and License informationDisclaimer
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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.Keywords: dermatophytes, essential oils, antifungal, synergy, mechanismGo to:
Introduction
Dermatophytoses account for 20–25% of fungal infections worldwide, with a prevalence approaching one billion as of 2010 (Havlickova et al., 2008). In 2004, it was estimated that there were over 29.4 million cases of cutaneous fungal infections in the US accounting for over 51 million physician visits (Ghannoum et al., 2000; Bickers et al., 2004; Suh et al., 2006; Gupta et al., 2017). Infections caused by dermatophytes affect the keratinized layers of the, skin, hair and nails and the causative agents belong to one of the following three genera: Tricophyton, Microsporum, or Epidermophyton. While most dermatophyte infections are not life-threatening and respond well to currently available topical treatment with over-the-counter (OTC) fungal agents, some dermatophyte infections can, however, can be difficult to treat, require prolonged therapeutic regimens, and are increasingly resistant to conventional antifungal therapies. Previous estimates put the total cost for treatment of dermatophyte infections in the US at $1.67 billion (Degreef and DeDoncker, 1994; Mukherjee et al., 2003; Santos and Hamdan, 2007; Gupta et al., 2017). In addition to costs associated with treatment, many currently used antifungal agents have significant side effects thus 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). Essential oils are complex mixtures which typically consist of a variety of low molecular weight compounds which can range in number up to 100 or more with a select few being the most abundant (Raut and Karuppayil, 2014; Sharma and Malik, 2015; Lopes et al., 2016). The composition of essential oils can vary due to a number of factors including the extraction method used, the type and species of plant from which they are derived, the composition of the soil, and the exact stage of growth at the time of harvest. For this reason, it is important that careful chemical analyses be performed using methods such as gas chromatography-mass spectrometry (GC-MS) to verify and standardize the composition of essential oils to ensure batch to batch consistency over time. Previous investigators have evaluated the use of EO’s against dermatophytes focusing largely on melaleuca, thyme, eucalyptus, oregano, and lavender (Zuzarte et al., 2009, 2011; Lopes et al., 2017). Additional studies have been conducted which investigated the antifungal effects of specific components of these and other EOs including mono-, di-, and sesquiterpenes, phenolic terpenes, phenylpropanoids, hydrocarbons, and other cyclic compounds (Tullio et al., 2007; Jantan et al., 2008; Miron et al., 2014a,b). 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. In the current study, 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.
