Salmonella enterica and Listeria monocytogenes inactivation dynamics after treatment with selected essential oils

Twenty one essential oils (EOs) were screened against 10 Salmonella enterica and 10 Listeria monocytogenes strains, by Disk Diffusion and Minimal Inhibitory Concentration (MIC) determination. The most effective EOs were: Origanum vulgare (oregano) > Cinnamomum zeylanicum (cinnamon) approximate to Caryophillus aromaticus (clove) > Thymus vulgaris (red thyme) > Melaleuca alternifolia (tea tree), with MIC values ranging from 0.6 for oregano and 20.0 mu L/mL for tea tree. The survival/inactivation dynamics of S. Typhimurium S4 and L monocytogenes ATCC 7644, selected as the most resistant strains, in presence of increasing concentrations (MIC/4, MIC/2, MIC value, MIC x 2, MIC x 4) of the five most active EOs were determined. All EOs were effective, showing bacteriostatic or bactericidal effects, depending on concentration. Overall, when cells survived in presence of sub-MIC concentrations, lag phase was significantly extended, while cells death was promptly evidenced, immediately after exposure to the highest EOs concentrations. Particularly oregano showed the best antimicrobial activity on both bacteria at low concentrations (0.15-5.0 mu L/mL). The five EOs showed a total phenols concentration between 1.85 (tea tree) and 17.75 mg GAE/g for clove, which gave also the highest radical-scavenging activity, with 1362 mu M Trolox equivalent/mL. Phenols, terpenes and aldehydes were the major compounds detected by double GC/MS, evidencing carvacrol, borneol, eugenol, terpinen-4-ol and trans-cinnamaldehyde as principal components. In conclusion, our results highlighted a good activity of the five selected EOs on S. enterica and L monocytogenes, with bactericidal or bacteriostatic effect at very low concentrations and were particularly encouraging, with regard to the Gram negative pathogen. Information on the inactivation dynamics provided useful details to be taken into account with the aim of developing control strategies based on natural antimicrobial agents. (C) 2014 Elsevier Ltd. All rights reserved.