Post by David Guymer, Newcastle University
The term ‘essential oils’ is frequently used and often represents a catch-all for a variety of volatile, water-insoluble compounds extracted from plants. While potentially harmful, undiluted, these oils have been used throughout history in the treatment of illnesses and remain popular, particularly in complementary therapies such as aromatherapy. The efficacy of many of these remedies are questionable or unproven, but many have established benefits: thymol (a component of thyme oil), for example, is a potent antiseptic, while clove oil has been used for centuries – and is still sold in chemists – as an antiseptic and analgesic for the relief of dental pain.
Recent scientific studies have indicated that consumption of certain essential oils may be of benefit to gut health. However the gut is a complex ecosystem populated by thousands of different microbial species both bad and – as more of us are becoming increasingly conscious – good, and the effect of essential oils on this balance was, until now, unevaluated. In this study, published in Microbiology, the effects of essential oils and purified essential oil compounds were assessed against a group of human gut commensals as well as a range of human pathogens including Clostridium difficile and Salmonella enteritidis.
The growth of these different strains was tested on agar plates in the presence of clove oil, and two pure essential oil compounds, thymol and eugenol. The results showed that thymol was highly toxic to all tested strains, even at low doses. Clove oil, though less effective than thymol at any given concentration, inhibited both pathogens and commensals alike. Surprisingly, while eugenol, the major constituent of clove oil, showed similar harmful effects as clove oil towards the tested pathogens, the commensals were affected much less severely.
The authors then extended the range of species tested, as well as the variety of essential oils and compounds with which to challenge them. They found other essential oils that effectively inhibited pathogens, but not commensals, at the same concentration. Those oils registering the most beneficial and selective effects included eugenol, geraniol (e.g. from geranium oil), linalool, curcuma oil and Agolin blend.
This work marks an encouraging step on the path towards the possible application of essential oils or, more likely, essential oil derivatives, for the benefit of human health. Although the use of ‘natural’ extracts will always carry a certain allure, this study highlights the differences in activity between a complex oil, in this case clove oil, and its primary constituent, eugenol, as well as pointing out the seasonal and geographic variations inherent to natural plant products.
Although positive overall, these results did bear one crucial caveat – while generally more tolerant of essential oils, one commensal in particular, Faecalibacterium prausnitzii, was more sensitive to the tested compounds than most of the tested pathogens. F. prausnitzii is one of the most beneficial gut commensals, playing an important anti-inflammatory role, and reduced populations of this bacterium have correlated with instances of Crohn’s disease. This study suggests that bacteria with differently structured cell envelopes are affected by essential oils in different ways, but the mechanisms underlying this remain an open question. Further research is required to ensure that essential oils can be precisely targeted to their target and/or site of action before they can be employed in any future therapeutic use.