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

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.