Electronic paramagnetic resonance investigation of the activity of Origanum vulgare L. essential oil on the Listeria monocytogenes membrane

Aims: To evaluate the effect of oregano essential oil on Listeria monocytogenes cytoplasmic membrane. Methods and Results: Nitroxide free‐radical Electron Paramagnetic Resonance was applied on L. monocytogenes after 30 min exposure to oregano essential oil concentrations ranging from 0 to 1·25%. The impact of essential oil on the number of viable cells was evaluated by plate count. Growth dynamics of survivors in BHI and TSB were evaluated by turbidometry. After exposure to essential oil concentrations up to 0·50%, the membrane fluidity was changed and its order increased. When L. monocytogenes was exposed to higher concentrations, membrane order parameters slightly returned to the values of untreated cells. However, when the cells were exposed to EO in the presence of sodium azide, which impairs energy metabolism, the membrane fluidity was progressively enhanced, even at the lowest EO concentration (0·25%). Microbiological analyses confirmed a progressive reduction of viable count, at increasing essential oil concentrations. Both in BHI and TSB, the Lag phase length increased in treated cells with respect to controls, suggesting a cell damage recovery. Conclusions: The combined approach including microbiological and EPR analyses provided relevant information on membrane modification and cell response to essential oils. Significance and Impact of the Study: EPR approach was demonstrated to be an effective and helpful tool to comprehend the modifications exerted by essential oil on the bacterial membrane.