Synergistic antibacterial effects of carvacrol and ε-polylysine

This study aimed to evaluate the antimicrobial efficacy of the combination of ɛ-polylysine (ɛ-PL) and carvacrol (Car) against foodborne pathogens, Escherichia coli and Staphylococcus aureus. The minimum inhibitory concentrations (MICs) of ɛ-PL (Car) against E. coli and S. aureus were 25 μg/mL (320 μg/mL) and 12.5 μg/mL (320 μg/mL), respectively. Checkerboard assays showed that the combination of ɛ-PL and Car exerted synergistic effects against E. coli and S. aureus with fraction inhibitory concentration index (FICI) of 0.375 and 0.5, respectively. It demonstrated that the combination of ɛ-PL and Car significantly inhibited the growth of the two strains compared to single treatment. Furthermore, the mode of action of ɛ-PL (6.25 μg/mL) or Car (80 μg/mL) in inhibiting E. coli and S. aureus was researched by assessing their changes with regard to cellular membrane integrity, membrane permeability, respiratory activity, and membrane structure. A combination of ɛ-PL and Car increased the damage to cell membranes and their permeability and led to the release of 260 nm absorbing materials, decreased respiratory-chain dehydrogenase activity compared with ɛ-PL or Car treatment alone. These results demonstrated that the combination of ɛ-PL and Car could be used as a new promising naturally sourced food preservative.