The antimicrobial activity of plant oils and extracts has been recognized for many years. However, few investigations have compared large numbers of oils and extracts using methods that are directly comparable. In the present study, 52 plant oils and extracts were investigated for activity against Acinetobacter baumanii, Aeromonas veronii biogroup sobria, Candida albicans, Enterococcus faecalis, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Salmonella enterica subsp. enterica serotype typhimurium, Serratia marcescens and Staphylococcus aureus, using an agar dilution method. Lemongrass, oregano and bay inhibited all organisms at concentrations of ≤2·0% (v/v). Six oils did not inhibit any organisms at the highest concentration, which was 2·0% (v/v) oil for apricot kernel, evening primrose, macadamia, pumpkin, sage and sweet almond. Variable activity was recorded for the remaining oils. Twenty of the plant oils and extracts were investigated, using a broth microdilution method, for activity against C. albicans, Staph. aureus and E. coli. The lowest minimum inhibitory concentrations were 0·03% (v/v) thyme oil against C. albicans and E. coli and 0·008% (v/v) vetiver oil against Staph. aureus. These results support the notion that plant essential oils and extracts may have a role as pharmaceuticals and preservatives.

SUMMARY Complementary and alternative medicines such as tea tree (melaleuca) oil have become increasingly popular in recent decades. This essential oil has been used for almost 100 years in Australia but is now available worldwide both as neat oil and as an active component in an array of products. The primary uses of tea tree oil have historically capitalized on the antiseptic and anti-inflammatory actions of the oil. This review summarizes recent developments in our understanding of the antimicrobial and anti-inflammatory activities of the oil and its components, as well as clinical efficacy. Specific mechanisms of antimicrobial and anti-inflammatory action are reviewed, and the toxicity of the oil is briefly discussed.

The essential oil of Melaleuca alternifolia (tea tree) has broad-spectrum antimicrobial activity. The mechanisms of action of tea tree oil and three of its components, 1,8-cineole, terpinen-4-ol, and ␣-terpineol, against Staphylococcus aureus ATCC 9144 were investigated. Treatment with these agents at their MICs and two times their MICs, particularly treatment with terpinen-4-ol and ␣-terpineol, reduced the viability of S. aureus. None of the agents caused lysis, as determined by measurement of the optical density at 620 nm, although cells became disproportionately sensitive to subsequent autolysis. Loss of 260-nm-absorbing material occurred after treatment with concentrations equivalent to the MIC, particularly after treatment with 1,8-cineole and ␣-terpineol. S. aureus organisms treated with tea tree oil or its components at the MIC or two times the MIC showed a significant loss of tolerance to NaCl. When the agents were tested at one-half the MIC, only 1,8-cineole significantly reduced the tolerance of S. aureus to NaCl. Electron microscopy of terpinen-4-ol-treated cells showed the formation of mesosomes and the loss of cytoplasmic contents. The predisposition to lysis, the loss of 260-nm-absorbing material, the loss of tolerance to NaCl, and the altered morphology seen by electron microscopy all suggest that tea tree oil and its components compromise the cytoplasmic membrane.The essential oil derived by steam distillation from the leaves of Melaleuca alternifolia is also known as tea tree oil (TTO) or Melaleuca oil. TTO is well characterized and contains approximately 100 terpenes and their related alcohols (6). The physical and chemical properties of commercial TTO are regulated by an international standard (23). TTO has antibacterial (8, 17), antifungal (18, 19), antiviral (4), and anti-inflammatory (5) properties in vitro, suggesting that it may have a role in the treatment of cutaneous infection. Clinical trials have demonstrated that TTO may be efficacious in the treatment of acne (2) and oral candidiasis (24) and in the decolonization of methicillin-resistant Staphylococcus aureus carriers (7). Although the in vitro antimicrobial activity and in vivo efficacy of TTO have been reported, less is known about its mechanism of action. Since this will have implications for its spectrum of activity, selective toxicity, and the development of resistance, we examined the mechanism of action of TTO and its components against S. aureus. MATERIALS AND METHODSTTO and components. TTO (batch 971) was kindly provided by Australian Plantations Pty. Ltd., Wyrallah, New South Wales, Australia. The levels of the components, determined by gas chromatographic analysis according to the international standard (23), were as follows: 41.5% terpinen-4-ol, 21.2% ␥-terpinene, 10.2% ␣-terpinene, 3.5% terpinolene, 2.9% ␣-terpineol, 2.5% ␣-pinene, 2.1% 1,8-cineole, 1.5% -cymene, 1% aromadendrene, 1% ␦-cadinene, 0.9% ledene, 0.9% limonene, 0.6% globulol, 0.4% sabinene, and 0.3% viridiflorol. Terpinen-4-ol and ␣-terpineol (Aldrich Chem...

Tea tree oil, or the essential oil of Melaleuca alternifolia, is becoming increasingly popular as a naturally occurring antimicrobial agent. The antimicrobial activity of eight components of tea tree oil was evaluated using disc diffusion and broth microdilution methods. Attempts were also made to overcome methodological problems encountered with testing compounds which have limited solubility in aqueous media. After assessing media with and without solubilizing agents, the disc diffusion method was used to determine the susceptibility of a range of micro-organisms to 1,8-cineole, 1-terpinen-4-ol, rho-cymene, linalool, alpha-terpinene, gamma-terpinene, alpha-terpineol and terpinolene. While the disc diffusion method lacked reproducibility, it was considered useful as a procedure for screening for antimicrobial activity. Terpinen-4-ol was active against all the test organisms while rho-cymene demonstrated no antimicrobial activity. Linalool and alpha-terpineol were active against all organisms with the exception of Pseudomonas aeruginosa. Minimum inhibitory and minimum cidal concentrations of each component against Candida albicans, Escherichia coli and Staphylococcus aureus were determined using a broth microdilution method. Modifications to this method overcame solubility and turbidity problems associated with the oil components and allowed the antimicrobial activity of each of the components to be quantified reproducibly. There was reasonable agreement between minimum inhibitory concentrations and zones of inhibition. These results may have significant implications for the future development of tea tree oil as an antimicrobial agent.

Aims: To investigate the in vitro antifungal activity of the components of Melaleuca alternifolia (tea tree) oil. Methods and Results: Activity was investigated by broth microdilution and macrodilution, and time kill methods.Components showing the most activity, with minimum inhibitory concentrations and minimum fungicidal concentrations of £0AE25%, were terpinen-4-ol, a-terpineol, linalool, a-pinene and b-pinene, followed by 1,8-cineole. The remaining components showed slightly less activity and had values ranging from 0AE5 to 2%, with the exception of b-myrcene which showed no detectable activity. Susceptibility data generated for several of the least watersoluble components were two or more dilutions lower by macrodilution, compared with microdilution. Conclusions: All tea tree oil components, except b-myrcene, had antifungal activity. The lack of activity reported for some components by microdilution may be due to these components becoming absorbed into the polystyrene of the microtitre tray. This indicates that plastics are unsuitable as assay vessels for tests with these or similar components. Significance and Impact of the Study: This study has identified that most components of tea tree oil have activity against a range of fungi. However, the measurement of antifungal activity may be significantly influenced by the test method.