Outbreaks of multidrug-resistant bacteria are increasingly reported at the clinical setting. The antimicrobial, anti-biofilm, anti-quorum sensing, and anti-oxidant activities of four essential oils extracted from Cinnamomum verum, Origanum majorana, Thymus vulgaris, and Eugenia caryophyllata against Gram-positive and Gram-negative multidrug-resistant bacteria were evaluated in vitro.

This study was conducted on 105 multidrug resistant clinical strains. Inhibition diameter zone, minimum inhibitory concentration, and minimum bactericide concentration of the oils were determined using agar disc diffusion method and microdilution. The ability of the 4 essential oils to inhibit the production of bacterial biofilms was tested on polystyrene plates, as well as their inhibitory effect on the production of violacein by Chromobacterium violaceum CV026. The anti-oxidant activity was evaluated by the 2,2-diphenyl-1-picrylhydrazyl scavenging method.

Essential oils of Cinnamomum verum, Thymus vulgaris and Eugenia caryophyllata showed an important antibacterial activity. The inhibition diameter zone was higher than 20 mm for 90.24 %, 85.71 % and 60.95 % of strains respectively.

These essential oils have a remarkable anti-biofilm and anti-quorum sensing activities against almost all the species studied. Clove extract revealed the highest anti-oxidant activity (Pourcentage of inhibtion of DPPH = 90.3 %).

These results supported the use of the 4 essential oils as alternative or complementary agents to treat infections caused by multidrug-resistant bacteria, and to prevent biofilm formation and quorum sensing signaling. They might be used as a safe anti-oxidants instead of harmful artificial ones.