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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