Essential Oils

ESSENTIAL OILS

Essential oils, sometimes termed ethereal or volatile oils, are contained within many plants and impart their characteristic odour. They can be obtained from the plant material by distillation, expression or solvent extraction and have been used throughout the centuries for medicinal and other purposes. In recent times attention has focused on a few of these oils, particularly tea tree oil, because of their reported antimicrobial activity and this will be used as an exemplar of the group as a whole. As the name suggests, they are hydrophobic, which makes them difficult to work with, and being natural products they frequently contain a large number of chemical components.

Tea tree oil (TTO) is the essential oil obtained from the Australian plant Melaleuca alternifolia (although other species may be used). Of its numerous chemical constituents (mainly monoterpenes, sesquiterpenes and their associated alcohols) terpinen-4-ol has been shown to have the major antimicrobial activity. It has general antimicrobial activity against viruses, bacteria and fungi but the main interest in this compound is centred on its activity against meticillin-resistant Staphylococcus aureus (MRSA). TTO has been suggested as a potential agent for the nasal and skin decolonization of MRSA in carriers. It has been formulated in a variety of topical preparations and is available over the counter for the treatment of a range of conditions.

One of the many issues surrounding the use of any natural material, especially multicomponent substances such as plant extracts, is that of batch variability and quality control. In the case of TTO there is a British Standard which regulates the composition of the 15 components of the oil and this is identical to ISO 4730:2004 (Table 27.3). The standard demands a lower limit of 30% for the main antimicrobial compound terpinen-4-ol, and other minor components are included in order to make the formulation of an artificial oil more difficult. Unless all preparations conform to the required standards of composition it is very difficult to judge the value of any in vitro or in vivo tests. The situation is further confused because the nature of the oil makes it problematic to evaluate. Oils are only sparingly soluble in the media used to test antimicrobial activity and when mixed with aqueous solutions result in turbid suspensions; consequently, end-points based on turbidity are difficult to determine, and the hydrophobic nature of the compounds limits their diffusion in agar. Thus in the literature a range of methodologies have been described to evaluate antimicrobial activity, and comparison between them is difficult.

Those in vitro studies which have been conducted indicate activity against a broad range of bacteria with similar minimum inhibitory concentrations (MICs) reported regardless of whether the isolate was antibiotic-sensitive or resistant. Generally bacteria are susceptible to concentrations below 1% (v/v) with the main exception being Ps.aeruginosawhich requires concentrations approaching 8% (v/v). The mechanism of action of TTO is thought to involve disruption of cellular membranes which will result in loss of intracellular constituents and inhibition of enzyme function. The more complex outer membranes of Ps. aeruginosa render that organism more resistant to the effects of  TTO.

Clinical studies have evaluated TTO for the treatment of a range of bacterial and fungal infections. There is no doubt that it is effective, although in many cases the treatment was not found to be superior to conventional therapy. Despite being used for many years TTO has exhibited safety issues and is toxic if ingested. It has also been shown to have irritant effects on the skin, although these may be reduced if the oil is properly formulated. Essential oils such as TTO cannot be directly compared to conventional antibiotics since they are used primarily as topical antiseptics. However, in this regard they certainly have some value although more extensive, properly controlled trials are necessary to fully confirm their potential use.