The mode of antimicrobial action of the essential oil of Melaleuca alternifolia (tea tree oil)

The essential oil of Melaleuca alternifolia (tea tree) exhibits broad-spect rum antimicrobial activity. Its mode of action against the Gram-negative ba cterium Escherichia coli AG100, the Gram-positive bacterium Staphylococcus aureus NCTC 8325, and the yeast Candida albicans has been investigated usin g a range of methods. We report that exposing these organisms to minimum in hibitory and minimum bactericidal/fungicidal concentrations of tea tree oil inhibited respiration and increased the permeability of bacterial cytoplas mic 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 potassiu m ion leakage. Differences in the susceptibility of the test organisms to t ea tree oil were also observed and these are interpreted in terms of variat ions in the rate of monoterpene penetration through cell wall and cell memb rane structures. The ability of tea tree oil to disrupt the permeability ba rrier of cell membrane structures and the accompanying loss of chemiosmotic control is the most likely source of its lethal action at minimum inhibito ry levels.