The essential oil of Melaleuca alternifolia (tea tree) exhibits broad-spectrum antimicrobial activity. Its mode of action against the Gram-negative bacterium Escherichia coli AG100, the Gram-positive bacterium Staphylococcus aureus NCTC 8325, and the yeast Candida albicans has been investigated using a range of methods. We report that exposing these organisms to minimum inhibitory and minimum bactericidal/fungicidal concentrations of tea tree oil inhibited respiration and increased the permeability of bacterial cytoplasmic and yeast plasma membranes as indicated by uptake of propidium iodide. In the case of E. coli and Staph. aureus, tea tree oil also caused potassium ion leakage. Differences in the susceptibility of the test organisms to tea tree oil were also observed and these are interpreted in terms of variations in the rate of monoterpene penetration through cell wall and cell membrane structures. The ability of tea tree oil to disrupt the permeability barrier of cell membrane structures and the accompanying loss of chemiosmotic control is the most likely source of its lethal action at minimum inhibitory levels.

The minimum inhibitory concentrations (MIC) of 60 terpenoids against Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus and Candida albicans have been determined. Hierarchical cluster analysis was used to group the compounds into five groups according to their activity patterns against the four microorganisms. K‐Means cluster analysis was then used to confirm these groupings and to show the differences in the activity patterns of the groups. Ten molecular properties of the terpenoids, either calculated via molecular modelling or determined by direct measurement, were then used as variables in a forward stepwise discriminant analysis to identify which variables discriminated between groups. Low water solubility of Group IV compounds, mainly hydrocarbons and acetates, was found to be associated with their relative inactivity. The remaining groups, all containing oxygenated terpenoids, showed characteristic but distinct activity patterns towards the four test organisms. Hydrogen bonding parameters were found to be associated with antimicrobial activity in all cases. Activity against Gram‐negative E. coli and P. aeruginosa was associated with a combination of a hydrogen bonding and size parameters. This was not found to be the case for the Gram‐positive S. aureus or the yeast C. albicans. Copyright © 1999 John Wiley & Sons, Ltd.

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Research into the mode of action of the essential oil of Melaleuca alternifolia (Tea tree oil) is briefly reviewed. Its mode of action is interpreted in terms of the membrane-toxicity of its monoterpenoid components and different approaches for determining cell membrane damage are discussed.

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Substrates controlling the formation of branched chain volatile esters in ripening bananas were investigated by the application of alcohol and amino acid precursors to whole fruit and tissue samples. The resulting changes in the profile of the volatile esters were determined using SPME and GC. These changes revealed the selectivity characteristics of the esterification enzyme AAT, the availability of acyl CoA's for ester formation, and the role of substrate supply on volatile production. The results obtained suggest that substrate supply is a major determinant of the quantitative and qualitative composition of the resulting aroma profile.

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