A single exposure to a sublethal concentration of Origanum vulgare essential oil initiates response against food stressors and restoration of antibiotic susceptibility in Listeria monocytogenes

Understanding the bacterial behavior in response to sublethal stresses is crucial for the optimization of antimicrobial treatments, particularly in food processing. However, the data regarding the effect of sublethal concentrations of essential oils (EOs) on microbial cells are still scarce. In this study, we evaluated the response of Listeria monocytogenes ATCC 7644 to the sublethal concentration of 1.25 μL/mL Origanum vulgare L. EO at 30 °C for 1 h, by means of Phenotype Microarray, modelling the kinetic data obtained by inoculating the control and treated cells into GEN III microplates (Biolog Inc.) for 96 h after EO removal. The results showed important differences regarding the growth dynamics in the presence of carbohydrates and environmental conditions often encountered in food products and that normally sustain the growth of the pathogen (i.e. pH 6.0, pH 5.0, NaCl 1–8%, presence of lactic acid and sodium lactate). In detail, the lag phase of the treated cells was extended, the growth rate was slowed down and the maximum concentration was often significantly reduced. Interestingly, the susceptibility to antimicrobials such as vancomycin, lincomycin and rifamycin was restored after EO application and removal, suggesting a potential role in combating antibiotic resistance. Confocal laser scanner microscopy showed a diffuse aggregation of the treated cells, as a response to the stress encountered. In conclusion, a single exposure to a sublethal concentration of O. vulgare EO causes a stressing effect that persists after its removal. This outcome should be better investigated for potential applications both in food production, to combine low EO concentrations with other hurdles to guarantee food safety, and in medicine, to restore the sensitivity to antibiotics.