Sodium chloride significantly enhances the bactericidal actions of carvacrol and thymol against the halotolerant species Escherichia coli O157:H7, Listeria monocytogenes, and Staphylococcus aureus

Abstract

This study investigated the synergistic effects of sodium chloride and essential oils (EOs) on three representative halotolerant species of foodborne pathogens, Escherichia coli O157:H7, Listeria monocytogenes, and Staphylococcus aureus. When bacterial cells were treated with carvacrol or thymol alone (0.5–2.0 mM), NaCl alone (1–15%, w/w), or their combinations at 22 °C for 1, 5, or 10 min, the combinations of EOs with NaCl showed obvious synergistic effects. For instance, the combination of carvacrol or thymol (2.0 mM) plus NaCl (≥3%) completely inactivated all tested bacterial cells (initial population: 7.2–7.4 CFU/ml; detection limit: 1 CFU/ml), while there were negligible bacterial reductions in the individual treatments (maximum 0.7 log reduction). The mechanisms of the synergistic effects were primarily derived from carvacrol- or thymol-induced membrane disruption (19.6% of cells were damaged by 2.0 mM carvacrol treatment for 10 min) and the loss of the ability of the bacterial cells to maintain an osmotic balance in the presence of NaCl. The results of the present study will contribute to overcoming the disadvantages of using EOs such as decreased organoleptic quality and high costs by reducing their working concentrations in food matrices.