Analysis of essential oils used in dermatological infections

Abstract

Essential oils are natural volatile oils that have previously been shown to display antimicrobial activity. The use and popularity of complementary and alternative therapies such as aromatherapy are on the rise due to the misconception that these products are safer and more cost effective. More than 90 essential oils and at least 1500 combinations (consisting of two oils) could be identified as being recommended for topical use, yet scientific evidence validating the effectiveness of the majority of the oils and at least 95% of combinations was lacking. The application of essential oils on the skin requires dilution of the oil into a base, such as a carrier oil. This is used with the belief of decreasing the toxicity of the essential oils, a claim yet to be scientifically validated. Thus, this study set out to investigate the acclaimed antimicrobial activity of essential oils recommended for skin infections (78 oils) and in combination (essential oil with essential oil, and essential oil with carrier oil) using the broth microdilution method, against pathogen reference strains involved in bromodosis (foot odour), Brevibacterium agri (ATCC 51663), B. epidermidis (DSM 20660), B. linens (DSM 20425); pathogens involved in wounds, such as Staphylococcus aureus (ATCC 25923), methicillin-resistant S. aureus (MRSA) (ATCC 43300), gentamicin-methicillin-resistant S. aureus (GMRSA) (ATCC 33592), Escherichia coli (ATCC 25922) and Pseudomonas aeruginosa (ATCC 27858); acne pathogens including S. epidermidis (ATCC 2223) and Propionibacterium acnes (ATCC 11827); and fungal pathogens with one yeast Candida albicans (ATCC 10231), and two dermatophytes Trichophyton mentagrophytes (ATCC 9533) and Microsporum canis (ATCC 36299). The minimum inhibitory concentration (MIC) was recorded for each oil alone, and for the combinations. Gas chromatography coupled to mass spectrometry (GC-MS) was utilised to confirm purity of the essential oils and to identify the chemical profiles. Slight variations were observed for the chemical profiles compared to other studies, and even between chemotypes, this can be explained by the differences in harvesting and environment of the plants used for essential oils. The data from the GC-MS and antimicrobial studies was then analysed using multivariate tools. Orthogonal projections to latent structures models for seven of the pathogens was created and the compounds responsible for antimicrobial activity identified. The toxicity of 23 essential oils and six carrier oils alone and in combination was determined using the brine shrimp lethality assay. The interactive profiles of the combinations (antimicrobial and toxicity) were determined by calculating the fractional inhibitory concentration (ΣFIC) where either synergy, additivity, indifference or antagonism could be identified. The antimicrobial synergistic interactions were further investigated at different ratios and interpretation displayed on isobolograms. The dermatophytes, Brevibacterium spp., and GMRSA were observed to be the most susceptible to essential oil inhibition. Cinnamomum verum (cinnamon bark), Santalum austrocaledonicum (sandalwood) and Vetiveria zizanioides (vetiver) were the three oils that on average displayed the most noteworthy inhibition with MIC values of 0.13-1.00 mg/ml, 0.01-1.00 mg/ml and 0.05-1.00 mg/ml respectively. Eugenol could be identified as the compound responsible for antimicrobial activity. Numerous essential oil combinations were found to display noteworthy antimicrobial activity, and certain combinations, for example Santalum austrocaledonicum with Commiphora myrrha (myrrh), displayed noteworthy antimicrobial activity against more than one group of pathogens. This same combination also displayed synergy against four of the five wound reference strains. Although the carrier oils were found to display poor antimicrobial activity, the combinations with essential oils showed no antimicrobial antagonism. In fact, an enhancement of antimicrobial activity could be observed against B. epidermidis, B. linens and P. aeruginosa when essential oils were combined with Aloe vera and Simmondsia chinensis. The overall toxicity was reduced for at least 90% of the oils. Aloe vera and S. chinensis could be identified as the carrier oils resulting in the most decreased essential oil toxicity while having the highest antimicrobial activity. This study could conclude that essential oils display antimicrobial potential alone and in combination against skin reference strains. Chemometric analysis was proven to be an effective tool for identifying active compounds that could potentially become novel lead compound for new antimicrobials. There is no doubt that the importance of studying interactions is critical to demonstrate and better understand the therapeutic potential of essential oils used in combination therapy. One needs to keep in mind that only an antimicrobial model was pronominally studied. The potential to include other pharmacological models such as anti-inflammatory, anti-oxidant and further toxicity effects may provide data that validates oil combinations targeting different therapeutic outcomes. This study further provides validation to aromatherapists, dermatologists and the layman on essential oil blend selection and broadens the global knowledge of essential oil and carrier research.