Synergistic Antimicrobial Combinations Inhibit and Inactivate Listeria monocytogenes in Neutral and Acidic Broth Systems

Abstract

The use of antimicrobial compounds can be an effective approach to control Listeria monocytogenes in ready-to-eat foods, but it can also be limited by cost, restrictions on concentrations in foods, and potential changes to organoleptic properties. Combinatorial approaches that produce additive or synergistic effects allow for reductions in individual antimicrobial concentrations while achieving the same level of control. The present study determined the MIC and MBC of an antimicrobial compound when used alone or in binary combinations against L. monocytogenes in growth media adjusted to pH values 7.4 and 5.5 and characterized interactions as synergistic, additive, or antagonistic. Inhibitory and bactericidal concentrations were defined as changes in L. monocytogenes counts of ≤1.0 or ≥3.0 log CFU/mL compared with the starting inoculum, respectively. Individually, lauric arginate (LAE), hydrogen peroxide (HP), and ε-polylysine (EPL) inhibited L. monocytogenes growth at the lowest concentrations when applied alone in broth adjusted to pH 7.4. Similarly, LAE, EPL, and HP had the lowest MBCs in broth adjusted to both pH levels. The inhibitory efficacy of both caprylic acid and sodium caprylate (SC) increased at the lower pH, with reductions in MICs of >98%. In total, 35 and 19 additive or synergistic inhibitory and bactericidal combinations were identified at pH values 7.4 and 5.5, respectively. Combinations of acidified calcium sulfate with lactic acid (ACSL) and SC were among the most synergistic inhibitory groupings at both pH levels, whereas EPL+LAE were the most effective bactericides at pH 7.4. Combinations of SC with EPL or ACSL were also among the most effective bactericides at pH 5.5. These data serve as a foundation for developing more effective antimicrobial approaches for the control of L. monocytogenes in foods with different pH levels.