Exposure of Listeria monocytogenes to sublethal amounts of Origanum vulgare L. essential oil or carvacrol in a food-based medium does not induce direct or cross protection

In this study, the effects of Origanum vulgare L. essential oil (OVEO) and carvacrol (CAR) on the growth and survival of Listeria monocytogenes ATCC 7644 were evaluated. The induction of direct protection against OVEO and CAR and of cross protection against various stresses (high temperature 45°C; lactic acid, pH 5.2; NaCl 10g/100mL) after exposure to sublethal amounts of OVEO and CAR was also evaluated. Both OVEO and CAR decreased the cell viability of L. monocytogenes in meat broth over 120min of exposure at all assayed concentrations (MIC, 1/2 MIC and 1/4 MIC). The overnight exposure of L. monocytogenes to sublethal amounts of OVEO or CAR did not induce direct protection or cross protection against high temperature, lactic acid or NaCl. Cells that were subcultured (24h cycles) in meat broth containing progressively increasing amounts of the antimicrobials were able to survive exposure to up to 2×MIC (two-fold increase) of OVEO and up to the MIC (one-fold increase in MIC) of CAR, suggesting that there were only minor changes in the antimicrobial susceptibility to these substances. Overall, these data indicate that OVEO and CAR have little effect on the acquisition of direct resistance or cross resistance by L. monocytogenes. ► O. vulgare essential oil and carvacrol inhibited the viability of L. monocytogenes. ► Exposure of cells to sublethal amounts of the substances induced no protection. ► Cells grown in increasing amounts of the substances showed minor changes in tolerance.