Background: In an ongoing project to evaluate essential oils as modulators of antibiotic resistance, the essential oil from Origanum vulgare L. (OVEO), as well as its individual constituents carvacrol (CAR) and thymol (THY), were investigated using Staphylococcus aureus strains possessing efflux mechanisms of resistance to norfloxacin, erythromycin and tetracycline. Methods: The minimum inhibitory concentration (MIC) values of the antibiotics were determined by agar dilution method, in the absence and in the presence of subinhibitory concentrations of OVEO, CAR or THY. Results: Along with relevant antistaphylococcal activity, OVEO, CAR and THY modulated the activity of tetracycline, i.e. in combination with antibiotics a reduction in the MIC was observed (up to fourfold). Conclusions: The results presented here represent, as far as we know, the first report of OVEO, CAR and THY as putative efflux pump inhibitors. Broadly, these findings indicate that essential oils could serve as potential sources of compounds capable of modulating drug resistance.

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