Sub-inhibitory concentration of essential oils induces antibiotic resistance in Staphylococcus aureus

Fourteen Staphylococcus aureus wild strains were stressed with sub-inhibitory concentration of five essential oils: Leptospermum scoparium (manuka), Origanum majorana (marjoram), Origanum vulgare (oregano), Satureja montana (winter savoury) and Thymus vulgaris (thyme). Antibiotics susceptibility profiles of the strains were determined by agar disk diffusion method before and after EOs treatment. The following antibiotics were employed: amoxicillin-clavulanic acid, amikacin, ampicillin, amoxicillin, aztreonam, ceftazidime, cephalothin, ciprofloxacin, colistin, cefotaxime, doxycycline, enrofloxacin, erythromycin, gentamicin, cephalexin, neomycin, piperacillin, rifampin, streptomycin, trimethoprim-sulphamethoxazole, tetracycline and tobramycin. Before EOs treatment, strains were susceptible to all antibiotics except for aztreonam and colistin. After exposure to sub-inhibitory EOs concentration of manuka, marjoram and oregano, several modifications in antibiotics susceptibility profiles were detected. Conversely, few modifications were induced by winter savoury and thyme EOs. Moreover, occurrence of resistances seems uncorrelated with drug classes as low concentration of EO could induce phenotypic changes in susceptible bacteria leading to antibiotic resistance phenomena.