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Project

Striking Essential Oil: Novel tools and new insights to study the biological activities of essential oils (EOs) and their components (EOCs)

The increasing level of antimicrobial resistance poses the threat that no suitable antimicrobials will be available for systematic treatment of common diseases. More and more Candida infections are caused by intrinsically and multi-resistant Candida isolates. There is clearly renewed scientific interest in  discovering drugs from natural sources, although natural product-based drug discovery is considered intrinsically complex and requires a highly integrated interdisciplinary approach. Medicinal plants that are used to treat infectious diseases appear to be an abundant source of new bioactive secondary metabolites. One such class of plant-based molecules are essential oils which have been used therapeutically for millennia as they have a broad range of biological activities.

Reanalyzing the data of a large antimicrobial screening (2012) of essential oils in microtiter plates systematically showed false-positive hits. We discovered that these false-positives were mediated by the vapor-phase of some essential oils in adjacent wells. The challenge was to circumvent this problem before we could start screening our latest essential oil (component) collection against two potentially disease-causing fungi i.e. C. albicans and C. glabrata. We developed an alternative plate set-up for the new screening in a way that it intrinsically does not modify the standardized protocol, while controlling for false positive results caused by the vapor-phase-mediated antimicrobial activity of the tested essential oils and their components. However, the alternative plate set-up and its derivatives are not limited to the testing of essential oils and their components but can be used with any volatile compound; nor is the set-up limited to testing antimicrobial activity but can essentially be extended to any biological activity tested in a microtiter plate. Therefore, we recommend these adaptations as good laboratory practices when working with volatiles in microtiter plates.

An in vitro assay to test the vapor-phase-mediated antimicrobial activity of volatiles did not exist. From a drug discovery point such an assay is interesting as a model for testing compounds for e.g. the treatment of tract-related infections. We hypothesized that if we can detect false-positives in microtiter plates caused by the vapor-phase of volatiles, it should be possible to develop a microtiter plate-based assay. We developped the qualitative vapor-phase-mediated patch assay, a relatively simple test to detect the vapor-phase-mediated activity of volatiles such as essential oils and their components. We conceptualized a quantitative version of the aforementioned assay, i.e. the vapor-phase-mediated susceptibility assay, which we then developed further. Subsequently we quantified with this assay the vapor-phase-mediated antimicrobial activity of a collection of 175 commercial essential oils and 37 reference essential oil components against C. albicans and C. glabrata. About half of the essential oils and their components had a growth-inhibitory vapor-phase-mediated antimicrobial activity. On average, a stronger activity was observed against the intrinsically more resistant C. glabrata, with essential oil component citronellal showing the largest  significant difference in vapor-phase-mediated antimicrobial activity. In contrast, representatives of each class of antifungals currently used in clinical practice did not exhibit any vapor-phase-mediated antimicrobial activity. The vapor-phase-mediated susceptibility assay can advance the search for novel (applications of existing) antimicrobials. Furthermore, our study is the first comprehensive characterization of essential oils and their components as a unique class of antifungals with properties distinct from existing antifungal classes. We conducted a pairwise comparison between the inhibitory vapor-phase-mediated anti-Candida activity of 33 certified organic essential oils and as many equivalent essential oils without such certification. It is believed among essential oil consumers that essential oils from organic cultivation, which includes plants that are harvested in the wild, are therapeutically more potent than those from conventional cultivation. However, we did not discern a significant difference in inhibitory vapor-phase-mediated anti-Candida activity between certified organic essential oils and those without such certification.

Although essential oils have been used therapeutically for millennia they continue to be considered as products mainly used in complementary and alternative medicine. Few have made the transition to drugs despite a shared Nobel Prize in 2015 for essential oil-related research of Dr. Youyou Tu for her discoveries concerning a novel therapy against malaria. In the past decades, natural products such as essential oils and their components have somehow been avoided in drug discovery because of inter alia possible technical problems during processing. However, is this still justified with our current understanding of drug discovery? Therefore, we studied selected physicochemical parameters, used in conventional drug discovery, from the components of a collection of 142 essential oils. We demonstrated that, contrary to generally held belief, most essential oil components satisfy current-day requirements of medicinal chemistry for good drug candidates. Therefore, their therapeutic potential remains vastly under-used and should be more vigorously explored with modern methods. Furthermore, it seems that they offer striking opportunities for lead optimization or even fragment-based drug discovery.

Date:1 Oct 2013 →  1 Mar 2019
Keywords:Essential oil, Essential oil components, vapor-phase, antimicrobial activity, biological activity
Disciplines:Genetics, Systems biology, Molecular and cell biology, Microbiology, Laboratory medicine, Biomaterials engineering, Biological system engineering, Biomechanical engineering, Other (bio)medical engineering, Environmental engineering and biotechnology, Industrial biotechnology, Other biotechnology, bio-engineering and biosystem engineering
Project type:PhD project