First evidence of antibacterial and synergistic effects of Thymus riatarum essential oil with conventional antibiotics

• Thymus riatarum essential oil is mainly composed by borneol (41.67%), terpinen-4-ol (8.65%), and trans-caryophyllene (7.59%).
• T. riatarum essential oil possesses a noticeable antibacterial activity.
• Bacterial cell wall and cytoplasmic membrane could be disrupted by T. riatarum essential oil, which provoke the leakage of cytoplasmic constituents.
• Active compounds of T. riatarum essential oil could be substrates of the efflux pump involved in the antibiotic resistance.
• T. riatarum essential oil is able to restore chloramphenicol efficacy through specific interaction with this drug efflux pumps.

The in vitro antibacterial activity of Thymus riatarum essential oil was evaluated in the present study using different assays toward human pathogenic isolates collected during serious infections.The phytochemical analysis of T. riatarum essential oil (EO) indicated the presence of 20 components, including mainly borneol (41.67%), terpinen-4-ol (8.65%), and trans-caryophyllene (7.59%). Determination of antibacterial activity showed that the EO possesses a noticeable potential of inhibiting the growth of tested strains with Minimum Inhibitory Concentrations (MICs) ranging from 3.57 mL/L to 7.5 mL/L. However, Pseudomonas aeruginosa showed the least sensitivity and was only inhibited with high concentrations (MIC > 7.5 mL/L). The release of cytoplasmic material absorbing at 260 nm, measured in the presence of T. riatarum EO, increased in response to oil concentration.The ability of T. riatarum EO to block efflux pumps systems of resistant Gram-negative bacteria was also studied. The results indicated that the EO activity was significantly enhanced in the presence of an efflux pump inhibitor such as phenylalanine arginyl ß-naphthylamide (PAßN). In addition, the tested EO, used at MIC/4, was able to increase chloramphenicol susceptibility of several resistant isolates.These results demonstrated that T. riatarum EO could permeabilize bacterial membrane impairing efflux pump activity and may be used in combination with conventional antibiotics against multidrug-resistant bacteria. To our knowledge, this is the first report about the antibacterial activity and the mode of action of T. riatarum EO.