Tolerance of Pseudomonas aeruginosa to Melaleuca alternifolia (tea tree) oil is associated with the outer membrane and energy-dependent cellular processes

<it>Objectives</it>: The essential oil of <it>Melaleuca alternifolia</it> (tea tree oil) and its components have antimicrobial activity against a wide range of Gram-positive and Gram-negative bacteria, fungi and viruses. The mechanism(s) by which <it>Pseudomonas aeruginosa</it> NCTC 10662 maintains a decreased susceptibility to tea tree oil and components was investigated. <it>Results</it>: Ethylene diamine tetraacetic acid enhanced the antimicrobial activity of tea tree oil and terpinen-4-ol against stationary phase <it>P. aeruginosa</it> while polymyxin B nonapeptide enhanced the activity of tea tree oil and γ-terpinene. Pre-treatment with the protonophore carbonyl cyanide <it>m</it>-chlorophenylhydrazone increased the susceptibility of exponential phase cells to sub-inhibitory concentrations of tea tree oil, terpinen-4-ol and γ-terpinene, indicating that intrinsic tolerance to tea tree oil and components is substantially energy dependent. <it>Conclusions</it>: Increased tolerance to tea tree oil in <it>P. aeruginosa</it> is directly related to the barrier and energy functions of the outer membrane, and may involve efflux systems.