Investigating the Potential of Plant-derived Antimicrobials and Probiotic Bacteria for Controlling Listeria monocytogenes

Listeria monocytogenes is a major foodborne pathogen resulting in multiple foodborne outbreaks in United States. In this dissertation, the efficacy of six plant-derived antimicrobials (PDAs), namely trans-cinnamaldehyde, carvacrol, thymol, eugenol, β-resorcylic acid and caprylic acid, and five probiotic bacteria, namely Bifidobacterium bifidum (NRRL-B41410), Lactobacillus reuteri (B-14172), L. fermentum (B-1840), L. plantarum (B-4496), and Lactococcus lactis subspecies lactis (B-633) was investigated for controlling L. monocytogenes. Specifically, the efficacy of aforementioned PDAs in reducing L. monocytogenes biofilms on abiotic surfaces, and pathogen populations on popular high-risk foods such as frankfurter and cantaloupe was investigated. In addition, the efficacy of PDAs and probiotic bacteria in attenuating L. monocytogenes virulence was studied in vitro and in an invertebrate model, Galleria mellonella. Low concentrations of trans-cinnamaldehyde, carvacrol, thymol, and eugenol significantly reduced L. monocytogenes biofilm formation, while concentrations greater than the minimum bactericidal concentrations of PDAs rapidly inactivated pre-formed biofilms. Trans-cinnamaldehyde and β-resorcylic acid also significantly reduced L. monocytogenes counts on frankfurters and cantaloupes. In addition, trans-cinnamaldehyde, carvacrol, thymol, and eugenol either alone or in combination with probiotic bacteria reduced L. monocytogenes motility, invasion of intestinal epithelial cells, production of virulence factor listeriolysin O, and the expression of virulence genes critical for motility, adhesion-invasion, intracellular survival, and cell-to-cell spread in the host. Moreover, the PDA-probiotic treatments significantly enhanced the survival of G. mellonella infected with lethal doses of L. monocytogenes. The above results underscore the potential use of aforementioned PDAs and probiotics in reducing L. monocytogenes on foods, and controlling listeriosis in humans