Flow cytometric assessment of the antimicrobial activity of essential oils against Listeria monocytogenes

Flow cytometry was applied to assess the antimicrobial activity of oregano, thyme and cinnamon essential oils against Listeria monocytogenes ATCC19114, using combined staining with propidium iodide (PI) for membrane damage evaluation and carboxyfluorescein diacetate (cFDA) for esterase activity detection. The antimicrobial activity of the essential oils was also tested at different NaCl concentrations. Significant differences were observed between plate count results and flow cytometric data, which suggested the presence of a sublethally stressed subpopulation, not able to form colonies on agar plates. Following treatments, flow cytometric assessment clearly discriminated three different subpopulations: viable, dead and injured cells. Cinnamon essential oil exerted a different impact on the cellular subpopulations, with a lower overall activity and a large percentage of cells having minimally damaged membranes. On the contrary, membrane disintegration seemed to be the primary inactivation mechanism of oregano and thyme essential oils. The antimicrobial activity of the essential oils increased with NaCl concentration increase, but higher NaCl concentrations were necessary following treatments with cinnamon essential oil. Our findings suggest differences in the mode of action of cinnamon essential oil against L. monocytogenes, in comparison with thyme and oregano essential oils. (C) 2008 Elsevier Ltd. All rights reserved.