Insights into the Antimicrobial Mechanisms of Cinnamon (Cinnamomum verum) and Oregano (Origanum vulgare) Essential Oils Against Foodborne Pathogens

Essential oils (EOs) are promising natural antimicrobial agents, although their mechanisms of action are not yet fully understood. This study investigated the antimicrobial mechanisms of cinnamon (Cinnamomum verum) and oregano (Origanum vulgare) EOs against Listeria monocytogenes Scott A and Staphylococcus aureus DSM 20231T by combining conventional plate counting with multiparameter flow cytometry. Bacterial cells were exposed to different sub-lethal or lethal EO concentrations, and culturability and three complementary physiological endpoints—viability, membrane permeability, and membrane potential—were monitored during treatment and after stress removal. Oregano EO, rich in carvacrol, rapidly reduced bacterial culturability and induced extensive membrane damage and cell death in both species. In contrast, cinnamon EO, characterized by transcinnamaldehyde as its main constituent, produced a slower antimicrobial response, with more limited membrane permeabilization and a greater recovery after stress removal. Discrepancies between culturability and flow-cytometric viability revealed transient VBNC-like populations, while post-stress recovery demonstrated that loss of culturability was reversible under some conditions. Overall, cinnamon EO-induced injury was at least partially repairable, particularly in L. monocytogenes, whereas damage caused by oregano EO at the highest tested concentration was essentially irreversible. These findings clarify the distinct antimicrobial effect of the two EOs and support their rational application as natural food preservatives.