Antibacterial Activity of Essential Oils

As a preliminary step, the antibacterial activities of the essential oils were determined by using paper disk diffusion method to screen the efficacy of essential oils among all samples. The essential oils were diluted with analytical grade ethanol at the following concentration 1, 1/1, 1/10, 1/20, and 1/40 (v/v). A volume of 20 μL of each concentration was, respectively, impregnated into the paper disk with 6 mm diameter (Biomérieux, Marcy-l'Etoile, France), and then placed onto Mueller-Hinton agar (MHA) plates (Oxoid, Badhoevedorp, Netherlands), which were previously inoculated on the surface agar with 200 μL of 10⁶ cfu/mL suspension for each tested bacterium. Ethanol was used as a control. Some individual components (carvacrol, cinnamaldehyde, eugenol, linalool, and thymol), frequently present as major component in essential oils, were also tested. Three standard reference antibiotics, ampicillin (10 μg/disk), chloramphenicol (30 μg/disk), and streptomycin (10 μg/disk), were used as reference controls for the tested bacteria. The plates were then incubated at 37°C for 24 h for L. monocytogenes, S. Typhimurium and E. coli O157:H7, and at 30°C for 24 h for P. fluorescens, and at 22°C for 48 h for B. thermosphacta. The antibacterial activity was evaluated by measuring the diameter of inhibitory zones in millimeters using digital calliper Top Craft (Globaltronics GmbH & Co. KG, Hamburg, Germany) and the means were expressed as the results of five determinations.