Antimicrobial Efficacy and Chemical Properties of Caryophyllus aromaticus and Origanum majorana Essential Oils Against Foodborne Bacteria Alone and in Combination

Background: Food products need to be protected against pathogenic and non-pathogenic microorganisms. One method is adding antimicrobial agents. Consumers’ tendency to use synthetic additives is drastically decreasing due to their side effects and also the emergence of multidrug resistant microorganisms. Plant essential oils (EOs) are natural antimicrobial compounds which are widely used in food industry. Objective: The objectives were to determine the chemical compositions of Caryophyllus aromaticus and Origanum majorana EOs and also to assess their antimicrobial activities against foodborne bacteria alone and in combination. Materials and Methods: The EOs were analyzed by gas chromatography-mass spectrometry. Antibacterial activities of the EOs against foodborne bacteria were assessed using disc diffusion method. The minimum inhibitory concentration (MIC) values of the EOs were determined by microdilution broth method and then minimum bactericidal concentration (MBC) values were determined. Checkerboard synergy testing was performed to determine the fractional inhibitory concentration index. Then time-kill curves were drawn based on the bacterial population (CFU/mL) against time (h). Results: The major constituents of C. aromaticus were eugenol and carvacrol, while O. majorana had carvacrol, thymol, trans-caryophyllene, and cymene as the main constituents. Zone of inhibition for O. majorana EO was greater than that for C. aromaticus EO. The inhibition zone of O. majorana EO against all the tested bacteria except for Bacillus subtilis was significantly greater than that of streptomycin (P ˂ 0.05). MIC value of the EOs against bacteria was 0.1% except for O. majorana EO against B. subtilis (0.3%). MBC values of C. aromaticus and O. majorana EOs ranged from 0.5% to 1.0% and 0.3% to 0.5% (v/v), respectively. The EOs were more effective on gram-positive bacteria than gram-negative ones. The combination of EOs revealed synergistic activity against Listeria monocytogenes, partial synergistic activity against B. subtilis, and additive effect against Salmonella typhimurium, Escherichia coli O157:H7 and Staphylococcus aureus were indifferent against the combination of EOs. Time–kill curves of the EOs demonstrated strong bactericidal effect against all foodborne bacteria at 6 and 24 hours either alone or in combination. Conclusion: The synergistic, partial synergistic, and additive effects of the combination of C. aromaticus and O. majorana EOs strengthen the antimicrobial activity, expand the spectrum of activity, reduce the concentrations required, decrease the side effects, and prevent the alteration of organoleptic properties of food