The bacterial multiple antibiotic resistance (Mar)phenotype leads to increased tolerance to tea tree oil

Mutants of Escherichia coli strain AG100 exhibiting the multiple antibiotic resistance (Mar) phenotype demonstrated a greater level of tolerance to tea tree oil (TTO) compared with the parent strain. The ability of TTO to kill all E. coli strains studied was greater at 37 than at 30°C. Growth of parent strain AG100 in the presence of salicylate, which induces the mar operon leading to the Mar phenotype, also increased tolerance to TTO. Escherichia coli Mar mutant YL1 demonstrated greater tolerance to antimicrobial terpenes found in TTO and did not leak K+ as rapidly in the presence of TTO when compared with its parent strain AG100. Attempts to isolate Mar mutants of Staphylococcus aureus using tetracycline gradients proved unsuccessful. However, when grown in the presence of salicylate, S. aureus strain BB255 demonstrated greater tolerance to TTO and did not leak K+ as rapidly in the presence of TTO compared with this strain grown without additions. This evidence demonstrates that bacterial Mar phenotypes increase tolerance to the killing action of TTO. This work also adds indirect evidence that the target of TTO is the cell membrane