Comprehensive Review on the Biocontrol of Listeria monocytogenes in Food Products.

Listeria monocytogenes is a foodborne pathogen that causes listeriosis, a group of human illnesses that appear more frequently in countries with better-developed food supply systems. This review discusses the efficacy of actual biocontrol methods combined with the main types of food involved in illnesses. Comments on bacteriophages, lactic acid bacteria, bacteriocins, essential oils, and endolysins and derivatives, as main biological antilisterial agents, are made bearing in mind that, using them, food processors can intervene to protect consumers. Both commercially available antilisterial products and solutions presented in scientific papers for mitigating the risk of contamination are emphasized. Potential combinations between different types of antilisterial agents are highlighted for their synergic effects (bacteriocins and essential oils, phages and bacteriocins, lactic acid bacteria with natural or synthetic preservatives, etc.). The possibility to use various antilisterial biological agents in active packaging is also presented to reveal the diversity of means that food processors may adopt to assure the safety of their products. Integrating biocontrol solutions into food processing practices can proactively prevent outbreaks and reduce the occurrences of L. monocytogenes-related illnesses.

Deterministic Approach and Monte Carlo Simulation to Predict Listeria monocytogenes Time to Grow on Refrigerated Ham: A Study Supporting Risk-based Decisions for Consumers' Health.

This study assessed the growth of Listeria monocytogenes in ready-to-eat (RTE) ham during storage under conditions simulating domestic practices with the intention to offer support in the elaboration of food safety policies that should better protect consumers against food poisoning at home. RTE ham, artificially contaminated at either medium (102-103 CFU/g) or high (104-105 CFU/g) concentration, was stored at both isothermal (4℃ in a refrigerator able to maintain a relatively constant temperature and 5℃ and 7℃ in a refrigerator with fluctuating temperature) and dynamic (5℃ and 7℃ with intermittent exposure to ambient temperature, e.g. 25℃) conditions. Under isothermal conditions, the increasing storage temperature determined a significantly increased (p < 0.05) capacity of L. monocytogenes to grow. The kinetic growth parameters were derived by fitting the Baranyi and Roberts model to the experimental data and, based on the maximum specific growth rates, it was estimated the temperature dependence of L. monocytogenes growth in RTE ham. At medium contamination level, sanitary risk time calculation revealed that, unlike storage at 5℃ and 7℃, storage at 4℃ of the RTE ham extends the time period during which the product is safe for consumption by ∼40 and 52%, respectively. However, the real temperature fluctuations included in the Monte Carlo simulations at low L. monocytogenes counts (1, 5 and 10 CFU/g) have shortened the safety margins. Stochastic models also proved to be useful tools for describing the pathogen's behavior when refrigeration of the RTE ham alternates with periods of ham being kept at room temperature, considered dynamic conditions of growth.

Synthesis of ZnO/Au Nanocomposite for Antibacterial Applications.

Annually, antimicrobial-resistant infections-related mortality worldwide accelerates due to the increased use of antibiotics during the coronavirus pandemic and the antimicrobial resistance, which grows exponentially, and disproportionately to the current rate of development of new antibiotics. Nanoparticles can be an alternative to the current therapeutic approach against multi-drug resistance microorganisms caused infections. The motivation behind this work was to find a superior antibacterial nanomaterial, which can be efficient, biocompatible, and stable in time. This study evaluated the antibacterial activity of ZnO-based nanomaterials with different morphologies, synthesized through the solvothermal method and further modified with Au nanoparticles through wet chemical reduction. The structure, crystallinity, and morphology of ZnO and ZnO/Au nanomaterials have been investigated with XRD, SEM, TEM, DLS, and FTIR spectroscopy. The antibacterial effect of unmodified ZnO and ZnO/Au nanomaterials against Escherichia coli and Staphylococcus aureus was investigated through disc diffusion and tetrazolium/formazan (TTC) assays. The results showed that the proposed nanomaterials exhibited significant antibacterial effects on the Gram-positive and Gram-negative bacteria. Furthermore, ZnO nanorods with diameters smaller than 50 nm showed better antibacterial activity than ZnO nanorods with larger dimensions. The antibacterial efficiency against Escherichia coli and Staphylococcus aureus improved considerably by adding 0.2% (w/w) Au to ZnO nanorods. The results indicated the new materials' potential for antibacterial applications.

The complete genome sequence of Listeria monocytogenes strain S2542 and expression of selected genes under high-pressure processing.

OBJECTIVES: The study aims to generate the whole genome sequence of L. monocytogenes strain S2542 and to compare it to the genomes of strains RO15 and ScottA. In addition, we aimed to compare gene expression profiles of L. monocytogenes strains S2542, ScottA and RO15 after high-pressure processing (HPP) using ddPCR. RESULTS: The whole genome sequence of L. monocytogenes S2542 indicates that this strain belongs to serotype 4b, in contrast to the previously reported serotype 1/2a. Strain S2542 appears to be more susceptible to the treatment at 400 MPa compared to RO15 and ScottA strains. In contrast to RO15 and ScottA strains, viable cell counts of strain S2542 were below the limit of detection after HPP (400 MPa/8 min) when stored at 8 °C for 24 and 48 h. The transcriptional response of all three strains to HPP was not significantly different.

High-pressure processing-induced transcriptome response during recovery of Listeria monocytogenes.

BACKGROUND: High-pressure processing (HPP) is a commonly used technique in the food industry to inactivate pathogens, including L. monocytogenes. It has been shown that L. monocytogenes is able to recover from HPP injuries and can start to grow again during long-term cold storage. To date, the gene expression profiling of L. monocytogenes during HPP damage recovery at cooling temperature has not been studied. In order identify key genes that play a role in recovery of the damage caused by HPP treatment, we performed RNA-sequencing (RNA-seq) for two L. monocytogenes strains (barotolerant RO15 and barosensitive ScottA) at nine selected time points (up to 48 h) after treatment with two pressure levels (200 and 400 MPa). RESULTS: The results showed that a general stress response was activated by SigB after HPP treatment. In addition, the phosphotransferase system (PTS; mostly fructose-, mannose-, galactitol-, cellobiose-, and ascorbate-specific PTS systems), protein folding, and cobalamin biosynthesis were the most upregulated genes during HPP damage recovery. We observed that cell-division-related genes (divIC, dicIVA, ftsE, and ftsX) were downregulated. By contrast, peptidoglycan-synthesis genes (murG, murC, and pbp2A) were upregulated. This indicates that cell-wall repair occurs as a part of HPP damage recovery. We also observed that prophage genes, including anti-CRISPR genes, were induced by HPP. Interestingly, a large amount of RNA-seq data (up to 85%) was mapped to Rli47, which is a non-coding RNA that is upregulated after HPP. Thus, we predicted that Rli47 plays a role in HPP damage recovery in L. monocytogenes. Moreover, gene-deletion experiments showed that amongst peptidoglycan biosynthesis genes, pbp2A mutants are more sensitive to HPP. CONCLUSIONS: We identified several genes and mechanisms that may play a role in recovery from HPP damage of L. monocytogenes. Our study contributes to new information on pathogen inactivation by HPP.

Genomic characterization of the most barotolerant Listeria monocytogenes RO15 strain compared to reference strains used to evaluate food high pressure processing.

BACKGROUND: High pressure processing (HPP; i.e. 100-600 MPa pressure depending on product) is a non-thermal preservation technique adopted by the food industry to decrease significantly foodborne pathogens, including Listeria monocytogenes, from food. However, susceptibility towards pressure differs among diverse strains of L. monocytogenes and it is unclear if this is due to their intrinsic characteristics related to genomic content. Here, we tested the barotolerance of 10 different L. monocytogenes strains, from food and food processing environments and widely used reference strains including clinical isolate, to pressure treatments with 400 and 600 MPa. Genome sequencing and genome comparison of the tested L. monocytogenes strains were performed to investigate the relation between genomic profile and pressure tolerance. RESULTS: None of the tested strains were tolerant to 600 MPa. A reduction of more than 5 log10 was observed for all strains after 1 min 600 MPa pressure treatment. L. monocytogenes strain RO15 showed no significant reduction in viable cell counts after 400 MPa for 1 min and was therefore defined as barotolerant. Genome analysis of so far unsequenced L. monocytogenes strain RO15, 2HF33, MB5, AB199, AB120, C7, and RO4 allowed us to compare the gene content of all strains tested. This revealed that the three most pressure tolerant strains had more than one CRISPR system with self-targeting spacers. Furthermore, several anti-CRISPR genes were detected in these strains. Pan-genome analysis showed that 10 prophage genes were significantly associated with the three most barotolerant strains. CONCLUSIONS: L. monocytogenes strain RO15 was the most pressure tolerant among the selected strains. Genome comparison suggests that there might be a relationship between prophages and pressure tolerance in L. monocytogenes.

Characterization of the biofilm phenotype of a Listeria monocytogenes mutant deficient in agr peptide sensing.

Listeria monocytogenes is a food-borne human pathogen and a serious concern in food production and preservation. Previous studies have shown that biofilm formation of L. monocytogenes and presence of extracellular DNA (eDNA) in the biofilm matrix varies with environmental conditions and may involve agr peptide sensing. Experiments in normal and diluted (hypoosmotic) complex media at different temperatures revealed reduced biofilm formation of L. monocytogenes EGD-e ΔagrD, a mutant deficient in agr peptide sensing, specifically in diluted Brain Heart Infusion at 25°C. This defect was not related to reduced sensitivity to DNase treatment suggesting sufficient levels of eDNA. Re-analysis of a previously published transcriptional profiling indicated that a total of 132 stress-related genes, that is 78.6% of the SigB-dependent stress regulon, are differentially expressed in the ΔagrD mutant. Additionally, a number of genes involved in flagellar motility and a large number of other surface proteins including internalins, peptidoglycan binding and cell wall modifying proteins showed agr-dependent gene expression. However, survival of the ΔagrD mutant in hypoosmotic conditions or following exposure to high hydrostatic pressure was comparable to the wild type. Also, flagellar motility and surface hydrophobicity were not affected. However, the ΔagrD mutant displayed a significantly reduced viability upon challenge with lysozyme. These results suggest that the biofilm phenotype of the ΔagrD mutant is not a consequence of reduced resistance to hypoosmotic or high pressure stress, motility or surface hydrophobicity. Instead, agr peptide sensing seems to be required for proper regulation of biosynthesis, structure and function of the cell envelope, adhesion to the substratum, and/or interaction of bacteria within a biofilm.

Resistance of Listeria monocytogenes to Stress Conditions Encountered in Food and Food Processing Environments.

Listeria monocytogenes is a human food-borne facultative intracellular pathogen that is resistant to a wide range of stress conditions. As a consequence, L. monocytogenes is extremely difficult to control along the entire food chain from production to storage and consumption. Frequent and recent outbreaks of L. monocytogenes infections illustrate that current measures of decontamination and preservation are suboptimal to control L. monocytogenes in food. In order to develop efficient measures to prevent contamination during processing and control growth during storage of food it is crucial to understand the mechanisms utilized by L. monocytogenes to tolerate the stress conditions in food matrices and food processing environments. Food-related stress conditions encountered by L. monocytogenes along the food chain are acidity, oxidative and osmotic stress, low or high temperatures, presence of bacteriocins and other preserving additives, and stresses as a consequence of applying alternative decontamination and preservation technologies such high hydrostatic pressure, pulsed and continuous UV light, pulsed electric fields (PEF). This review is aimed at providing a summary of the current knowledge on the response of L. monocytogenes toward these stresses and the mechanisms of stress resistance employed by this important food-borne bacterium. Circumstances when L. monocytogenes cells become more sensitive or more resistant are mentioned and existence of a cross-resistance when multiple stresses are present is pointed out.