ORIGINAL ARTICLE Electronic paramagnetic resonance investigation of the activity of Origanum vulgare L. essential oil on the Listeria monocytogenes membrane A. Serio 1 , M. Chiarini 1 , E. Tettamanti 2 and A. Paparella 1 1 Department of Food Science, University of Teramo, Mosciano Sant’Angelo (TE), Italy 2 Department of Biomedical Comparative Sciences, University of Teramo, Piazza Aldo Moro, Teramo (TE), Italy Introduction Today, different strategies are applied to control patho- gens in food, and particular interest has been focused on the application of essential oils (EOs), as a safe and effec- tive alternative to chemical preservatives (Lambert et al. 2001; Skandamis and Nychas 2001). According to Euro- pean laws regarding Microbiological Criteria for food products (Commission Regulation EC 2073 ⁄ 2005), among pathogens, special attention has to be paid to Listeria monocytogenes, a ubiquitous bacterium, able to adapt to stressing conditions and therefore present in many food products. Essential oils (EOs) are well known to be a good source for active compounds in biological systems, and their antimicrobial properties have been widely studied in the past (Burt 2004). Among many others, Origanum vulgare L. (oregano) EO showed prominent results against Escher- ichia coli (Skandamis and Nychas 2001), Pseudomonas aeruginosa, Staphylococcus aureus (Lambert et al. 2001) and L. monocytogenes (Burt 2004) at variable concentra- tions. Although EOs antimicrobial action is established, their mechanism of action has not been completely explained in detail (Lambert et al. 2001). In fact, considering the large number of different chemical compounds present in Keywords EPR, essential oil activity, growth dynamics, Listeria monocytogenes, nitroxide spin- labelling. Correspondence Antonello Paparella, Department of Food Science, University of Teramo, Via Carlo R. Lerici no. 1, I-64023, Mosciano Sant’Angelo (TE), Italy. E-mail: apaparella@unite.it 2009 ⁄ 1966: received 13 November 2009, revised 23 April 2010 and accepted 5 May 2010 doi:10.1111/j.1472-765X.2010.02877.x Abstract 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 1Æ25%. The impact of essential oil on the number of viable cells was evaluated by plate count. Growth dynamics of survi- vors in BHI and TSB were evaluated by turbidometry. After exposure to essen- tial oil concentrations up to 0Æ50%, 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 (0Æ25%). Microbiological analy- ses 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 anal- yses 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 essen- tial oil on the bacterial membrane. Letters in Applied Microbiology ISSN 0266-8254 ª 2010 The Authors Journal compilation ª 2010 The Society for Applied Microbiology, Letters in Applied Microbiology 51 (2010) 149–157 149