The mechanisms of action of carvacrol and its synergism with nisin against Listeria monocytogenes on sliced bologna sausage

Consumption of contaminated food appears to be the main route of acquiring listeriosis and has been estimated as the source of L. monocytogenes infection for up to 99% of cases. The interest in and combined use of natural food preservatives to control Listeria contamination of food products has dramatically increased in recent years. The aim of this study was to investigate the effect of carvacrol, a major component of oregano essential oil, against L monocytogenes. Changes in membrane permeability, membrane depolarization, respiratory activity and membrane structure were determined to elucidate possible mechanisms of action. Carvacrol had potent antibacterial activity against all strains studied, including 10403S and food isolates, with a MIC of 250 mu g/ml and MBC ranging from 250 to 500 mu g/ml. Bacterial cells exposed to carvacrol showed increased membrane permeability and depolarization, and changes in respiratory activity. Transmission electron microscopic analysis showed degenerative changes of cell wall and cytoplasmic membrane, and structural disruption to L. monocytogenes cells treated with carvacrol. These results indicated that carvacrol inactivates the bacterium through a multi-target action that disrupts cell membranes leading to cell lysis, as well as inhibits the respiratory activity. The synergistic interaction of carvacrol and nisin against L. monocytogenes 10403S and three food isolates (CM2, CM8 and CM11) was shown in vitro by checkerboard assay with FICI values ranging from 0.375 to 0.500. The synergic effect of carvacrol and nisin on the survival of L monocytogenes 10403S was examined during 4 degrees C storage of sliced bologna sausages. For up to seven days, the presence of carvacrol combined with nisin resulted in significant growth rate reductions compared to those of controls (p < 0.05). These results indicated that the combination of carvacrol and nisin might be an effective natural antimicrobial application with potential use as a preservative to control L. monocytogenes in foods.