Antimicrobial synergism and antibiofilm activities of , , and essential oils against extreme drug-resistant clinical isolates.

L., L., and L., essential oils are used by complementary medicine specialists simultaneously with traditional antibiotics for treatment purposes. The chemical composition of essential oils was analyzed by the gas chromatography-mass spectrometry method. antibacterial and antibiofilm activities of the essential oils were tested against extreme drug-resistant (XDR) colistin-resistant and colistin susceptible clinical strains. The synergistic activities between essential oils and colistin antibiotics were investigated by the checkerboard method. The highest antibacterial effect was detected in mint essential oil (2.5-5 μl/ml), followed by pelargonium essential oil (5-20 μl/ml) and rosemary essential oil (5-20 μl/ml). The combination of rosemary essential oil or pelargonium essential oil with colistin showed strong synergistic activity in most of the bacterial strains tested (fractional inhibitory concentration index ≤ 0.5; synergy). As a result of the combination of mint essential oil and colistin, an indifferent effect was observed in only two bacterial strains, and other strains could not be evaluated. No antagonistic effects were observed in any of the tested essential oils. As a result of the effectiveness of the combination, the minimum inhibitory concentration (MIC) values of colistin in XDR- clinical isolates decreased 2-32 fold. Additionally, the sub-MIC concentration of essential oils exhibited an inhibitory effect (48-90%) against the biofilm layer of tested strains.