Inhibitory Effect of Thymus daenensis Essential Oil on Staphylococcus aureus NorA Efflux Pump

Antibiotic resistant phenotypes of bacteria have been shown to be related to efflux pumps. Research for finding compounds with an ability to inhibit these pumps seems worthwhile due to their ability to increase bacterial sensitivity to antibiotics or return sensitivity to resistant strains. The aim of this study was to evaluate inhibitory effect of Thymus daenensis essential oil (EO) on NorA efflux pump of staphylococcus aureus.
Thymus daenensis essential oil was extracted from dried thyme leaves by hydrodistillation method and then the effect of the essential oil on bacterial efflux pump was evaluated using ethidium bromide-agar cartwheel method with respect to bacterial efflux capacity of ethidium bromide. Minimum inhibitory concentration (MIC) of Thymus daenensis essential oil was measured using microdilution test followed by the evaluation of the effect of sub-MIC concentration of essential oil on fluoroquinolone antibiotics. The efflux level of ethidium bromide, ciprofloxacin and norfloxacin (substances transmitted by NorA) was considered as the NorA efflux pump activity.
The cartwheel method showed that Thymus daenensis essential oil could reduce the bacterial efflux capacity of ethidium bromide. The results of checkerboard test indicated that the interactions between sub-MIC concentration of the essential oil and ethidium bromide, ciprofloxacin and norfloxacin were totally synergistic.
Synergistic effect of Thymus daenensis essential oil with the other components demonstrated the potential of Thymus daenensis to enhance the antimicrobial activity of fluoroquinolone antibiotics against staphylococcus aureus. As these antibiotics are effluxed by NorA pump, this increased sensitivity to antibiotics is possibly due to NorA efflux pump inhibitors.