Investigating the effects of nisin and free fatty acid combined treatment on Listeria monocytogenes inactivation

The FDA-approved antimicrobial peptide nisin has been used to prevent contamination of food products from Gram-positive pathogens including Listeria monocytogenes (L. monocytogenes). However, some L. monocytogenes strains are tolerant or resistant to nisin treatment at a dose that would normally be lethal. Therefore, alternative or combined antimicrobial treatment strategies are required to effectively control L. monocytogenes contaminations. In this study, we investigate whether combination treatment with nisin and fatty acids can overcome the nisin resistance of L. monocytogenes by monitoring cell survival and biofilm inactivation of two strains (ATCC15313 serotype 1/2a and ATCC19115 serotype 4b). Lauric acid and N-tridecanoic acid were chosen because they exhibit antimicrobial activities. First, we observed that nisin (22.5 mu g/mL) rapidly killed cells with up to a 6-log reduction after 3 h, but treatment for prolonged periods (i.e., up to 24 h) caused the cells to recover growth via the development of either inherent nisin resistance or temporary nisin tolerance. However, cotreatment of nisin and each fatty acid significantly decreased the cell survival and hindered the development of nisin resistance. In addition, the co-treatment effectively inactivated cells in biofilms. The combined antimicrobial treatment may be promising approach in the food processing environment, where Listeria contamination occurs.