The Impact of Subinhibitory Concentrations of Ɛ-polylysine, Hydrogen Peroxide, and Lauric Arginate on Listeria monocytogenes Virulence

Recent studies on the use of plant-derived and other bioactive compounds and antimicrobials in food have challenged the idea that exposure to antimicrobials at sublethal or subinhibitory concentrations (SICs) increases the virulence potential of bacterial pathogens including Listeria monocytogenes. The objective of this study was to determine the effect of exposure to SICs of Ɛ-polylysine (EPL), hydrogen peroxide (HP), and lauric arginate (LAE) on L. monocytogenes virulence. For all assays, L. monocytogenes strains Scott A and 2014L-6025 were grown to mid-log phase in the presence of SICs of EPL, HP, or LAE. Motility was determined by spot inoculating cultures on soft brain heart infusion agar (0.3% agar). Cultures grown in SICs of antimicrobials were also inoculated onto Caco-2 cells (10:1 MOI) to determine the effects on subsequent adhesion and invasion. Last, the relative expression of key virulence genes (prfA, plcB, hlyA, actA, inlA, inlB, sigB, and virR) following growth in SICs was determined by RT-qPCR. Results indicate that L. monocytogenes growth in the presence of SICs of EPL, HP, or LAE did not affect the motility, adhesion, or invasion capacity of either strain. Changes in gene expression were observed for both L. monocytogenes strains. More specifically, SICs of EPL and LAE reduced hlyA expression in Scott A, whereas SICs of EPL and HP increased the expression of virR. The upregulation of sigB and actA in the presence of EPL and LAE, respectively, was observed in strain 2014L-6025. These findings indicate that exposure to SICs of these antimicrobials has varying effects on L. monocytogenes that differ by strain. Although no phenotypic effects were observed in terms of motility, adhesion, and invasion, the observed changes in virulence gene expression warrant further investigation.