This study aimed to characterize physiological differences between persistent and presumed nonpersistent Listeria monocytogenes strains isolated at processing facilities and to investigate the molecular basis for this by transcriptomic sequencing. Full metabolic profiles of two strains, one persistent and one nonpersistent, were initially screened using Biolog's Phenotype MicroArray (PM) technology. Based on these results, in which major differences from selected antimicrobial agents were detected, another persistent strain and two nonpersistent strains were characterized using two antimicrobial PMs. Resistance to quaternary ammonium compounds (QACs) was shown to be higher among persistent strains. Growth of persistent and nonpersistent strains in various concentrations of the QACs benzethonium chloride (BZT) and cetylpyridinium chloride (CPC) was determined. Transcriptomic sequencing of a persistent and a presumed nonpersistent strain was performed to compare gene expression among these strains in the presence and absence of BZT. Two strains, designated “frequent persisters” because they were the most frequently isolated at the processing facility, showed overall higher resistance to QACs. Transcriptome analysis showed that BZT induced a complex peptidoglycan (PG) biosynthesis response, which may play a key role in BZT resistance. Comparison of persistent and nonpersistent strains indicated that transcription of many genes was upregulated among persistent strains. This included three gene operons: pdu , cob-cbi , and eut . These genes may play a role in the persistence of L. monocytogenes outside the human host.

This study extends current understanding of the genetic diversity among L. monocytogenes from various food products and food processing environments. Application of WGS-based strategies facilitated tracking of this pathogen of importance to human health along the production chain while providing insights into the pathogenic potential for some of the L. monocytogenes isolates recovered. These analyses enabled the grouping of selected isolates into three putative virulence categories according to their genotypes along with informing selection for phenotypic assessment of their pathogenicity using the zebrafish embryo infection model. It has also facilitated the identification of those isolates with genes conferring tolerance to commercially used biocides. Findings from this study highlight the potential for the application of WGS as a proactive tool to support food safety controls as applied to L. monocytogenes .

High mortality and hospitalization rates have seen Listeria monocytogenes as a foodborne pathogen of public health importance for many years and of particular concern for high-risk population groups. Food manufactures face an ongoing challenge in preventing the entry of L. monocytogenes into food production environments (FPEs) due to its ubiquitous nature. In addition to this, the capacity of L. monocytogenes strains to colonize FPEs can lead to repeated identification of L. monocytogenes in FPE surveillance. The contamination of food products requiring product recall presents large economic burden to industry and is further exacerbated by damage to the brand. Poor equipment design, facility layout, and worn or damaged equipment can result in Listeria hotspots and biofilms where traditional cleaning and disinfecting procedures may be inadequate. Novel biocontrol methods may offer FPEs effective means to help improve control of L. monocytogenes and decrease cross contamination of food. Bacteriophages have been used as a medical treatment for many years for their ability to infect and lyse specific bacteria. Endolysins, the hydrolytic enzymes of bacteriophages responsible for breaking the cell wall of Gram-positive bacteria, are being explored as a biocontrol method for food preservation and in nanotechnology and medical applications. Antibacterial proteins known as bacteriocins have been used as alternatives to antibiotics for biopreservation and food product shelf life extension. Essential oils are natural antimicrobials formed by plants and have been used as food additives and preservatives for many years and more recently as a method to prevent food spoilage by microorganisms. Competitive exclusion occurs naturally among bacteria in the environment. However, intentionally selecting and applying bacteria to effect competitive exclusion of food borne pathogens has potential as a biocontrol application. This review discusses these novel biocontrol methods and their use in food safety and prevention of spoilage, and examines their potential to control L. monocytogenes within biofilms in food production facilities.

Listeria monocytogenes is a potentially lethal foodborne pathogen commonly found in the environment. European Union hygiene legislation places responsibility for safety on primary production facili...