Antimicrobial effects of Melaleuca alternifolia (tea tree) essential oil against biofilm-forming multi-drug resistant cystic fibrosis-associated Pseudomonas aeruginosa as a single agent and in combination with commonly nebulised antibiotics.

Broth microdilution assays were used to determine minimum inhibitory concentrations (MICs) and fractional inhibitory concentration indices (FICIs) of tea tree oil (TTO), tobramycin, colistin, and aztreonam against clinical cystic fibrosis-associated P. aeruginosa (CFPA) isolates (n=20). The minimum biofilm eradication concentration (MBEC) and fractional biofilm eradication concentration index (FBECI) were also determined using a similar microbroth dilution checkerboard assay, with biofilms formed using the MBEC device®. TTO was effective at lower concentrations against multi-drug resistant (MDR) CFPA isolates (n=3) in a biofilm compared to in a planktonic state (MBEC 18.7-fold lower than MIC). CFPA within biofilm was less susceptible to aztreonam, colistin and tobramycin compared to planktonic cells (MBEC 6.3-fold, 9.3-fold, and 2.1-fold higher than MIC, respectively). All combinations of essential oil and antibiotic showed indifferent relationships (FICI 0.52 – 1.72) when tested against planktonic MDR CFPA isolates (n=5). Against CFPA isolates (n=3) in biofilm, combinations of TTO/aztreonam and TTO/colistin showed indifferent relationships (mean FBECI 0.85 and 0.60, respectively), whereas TTO/tobramycin showed a synergistic relationship (mean FBECI 0.42). The antibiofilm properties of TTO and the synergistic relationship seen between TTO and tobramycin against CFPA in vitro make inhaled TTO a promising candidate as a potential therapeutic agent.
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