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Habituation to sub-lethal concentrations of tea tree oil (Melaleuca alternifolia) is associated with reduced susceptibility to antibiotics in human pathogens.

McMahon MA, Blair IS, Moore JE, McDowell DA

Food Microbiology Research Group, University of Ulster, Newtownabbey, Northern Ireland BT37 OQB, UK. ma.mcmahon@ulster.ac.uk

OBJECTIVES: To investigate the effect of sub-lethal challenge with tea tree oil (TTO) on the antibiotic susceptibility profiles of significant human pathogens and commensals. METHODS: The study compared the antibiotic susceptibility (Etest) patterns of Escherichia coli, Staphylococcus aureus/methicillin-resistant S. aureus (MRSA) and Salmonella spp. after broth culture for 72 h in the presence or absence of sub-lethal concentrations of TTO (0.25%, 0.25% and 0.1%). RESULTS: All habituated cultures (exposed to sub-lethal concentrations of TTO) displayed reduced susceptibility to a range of clinically relevant antibiotics compared with non-habituated (control) cultures. CONCLUSIONS: Although TTO may be an effective antimicrobial agent when appropriately used at bactericidal concentrations, its application at sub-lethal concentrations may contribute to the development of antibiotic resistance in human pathogens.

Published 18 December 2006 in J Antimicrob Chemother, 59(1): 125-7.
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