Combinations of Multiple Natural Antimicrobials with Different Mechanisms as an Approach to Control Listeria monocytogenes

Introduction: To achieve synergistic interaction of antimicrobials likely requires that individual compounds have different mechanisms of inhibition or inactivation. Natural antimicrobials vary in reported mechanisms of inhibition of bacterial cells. Finding a combination of natural antimicrobials that acts synergistically would allow a reduction in the use concentration which is important for essential oils (EO) or EO components as they may contribute undesirable sensory effects to foods.

Purpose: The objective of this study was to attempt to achieve synergistic antimicrobial interactions and reduce use concentrations by combining EOs and a naturally occurring hydroxycinnamic acid with reported different mechanisms.

Methods: Oregano essential oil (OEO), basil essential oil (BEO), coriander essential oil (CEO), and ferulic acid (FA) were evaluated alone and in combination against Listeria monocytogenes at pH 6.0 and 25°C. A broth dilution assay was used to determine the minimum inhibitory concentrations (MIC) of individual and combined antimicrobials. Fractional inhibitory concentrations (FIC) were calculated and the interactions interpreted as synergistic (FIC ≤0.5), additive (FIC ≥0.5), or antagonistic (FIC >1.5).

Results: MICs of EOs and FA alone were 250 ppm OEO, 2500 ppm CEO, 7500 ppm BEO, and 5000 ppm FA. Binary combinations of OEO+BEO, CEO+BEO, and BEO+FA resulted in a synergistic effect (FIC ≤ 0.5). An additive effect was observed with binary combinations of OEO+CEO, OEO+FA, and CEO+FA. For tertiary combinations, synergistic antimicrobial effects were noted for all combinations except OEO+CEO+FA which had an FIC of 1.0. The quaternary combination of OEO+CEO+BEO+FA was inhibitory at 31.25, 312.5, 937.5, and 625 ppm, respectively, and had a synergistic effect.

Significance: Combining natural antimicrobials with suggested different mechanisms may be a solution for controlling foodborne pathogens and reducing use concentrations. A quaternary antimicrobial blend reduced the concentration of each compound needed for inhibition by 87.5% which could also reduce the potential for negative sensory effects.