Antimicrobial activity of cinnamate-eugenol: Synergistic potential, evidence of efflux pumps and amino acid effects

Food safety is achieved mainly by the combination of preservation factors which results in additive or synergistic effect. Mixtures of cinnamate and eugenol were tested to determine bacteriostatic and bactericidal doses against Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, Listeria monocytogenes and Staphylococcus aureus. A combination of the two compounds was used to challenge these four bacteria, yielding a dose-dependent bactericidal or bacteriostatic effect. The synergistic effect was demonstrated in the three rod-shaped bacteria, L. monocytogenes, E. coli O157:H7 and S. Typhimurium, while for S. aureus, the mixture showed no synergistic activity. Notably, in the case of S. aureus, the interaction term was not significant (p>0.05), consistent with an additive but not synergistic effect between cinnamate and eugenol. The optimal combination of 1.35 mM cinnamate and 0.12 mM eugenol was bactericidal. Data showed that in response to this antimicrobial activity, these strains expressed efflux pumps. Additionally, the effect of three aminoacids on the bactericidal activity of these compounds was investigated. The bactericidal activity was interrupted by the presence of cysteine and proline, but not tyrosine. This effect was reversed when larger doses of eugenol and cinnamate were tested. We conclude that combining cinnamate and eugenol produces a synergistic bactericidal effect against L. monocytogenes, E. coli O157:H7 and Salmonella Typhimurium. This knowledge may be useful in the development of food products.