The Saprophytic Lifestyle of Listeria monocytogenes and Entry Into the Food-Processing Environment

Targeting motility of Listeria monocytogenes: Alternative strategies to control foodborne illness

Listeria monocytogenes, a gram-positive facultative anaerobic bacterium, demonstrates remarkable adaptability to various environmental stressors in food processing environments. It can survive and grow under extremely challenging environmental conditions such as low pH and temperatures, high salinity, and UV radiation. Its ability to generate biofilms at multiple stages of the food processing chain poses significant food safety issues. This bacterium is known for causing severe listeriosis, making it a major problem in microbiology and food safety. L. monocytogenes relies on motility to explore surfaces, attach, and build biofilms. It comprises actin-based motility, which is used for cell-to-cell propagation inside host tissues, and flagellar-driven motility, which assists in surface colonization and infection spread. Flagellar motility also plays an important function in increasing virulence throughout infection cycles. L. monocytogenes motility is regulated by a complex network of regulatory proteins that govern the expression of motility-associated genes. These proteins directly impact pathogenicity by influencing motility and biofilm formation, as well as an indirect impact via regulatory pathways. Efforts to control L. monocytogenes infections and decrease food safety impact include a variety of procedures. Natural compounds, synthetic agents, nanomaterials, and conjugates have emerged as intriguing options for inhibiting motility, disrupting biofilm formation, and reducing virulence. These strategies focus on vital elements of the L. monocytogenes life cycle and pathophysiology to improve food safety and public health. This review provides a comprehensive discussion of the regulatory mechanisms governing L. monocytogenes motility, emphasizing their role in pathogenicity, and explores potential strategies for attenuating the motility and virulence properties. Understanding these mechanisms is essential for developing targeted therapeutic approaches against L. monocytogenes infections and improving food safety practices.

Listeria monocytogenes and Listeriosis: The Global Enigma

Listeria monocytogenes is an intracellular, Gram-positive, non-spore-forming, non-encapsulated, facultative anaerobic, rod-shaped, and psychrotrophic food-borne pathogen that causes the infection, listeriosis, thus it attracts great attention following listeriosis outbreaks, which are often associated with high mortality rates. The prevalence of listeriosis is quite low globally; however, the most recent and deadliest outbreak occurred in South Africa, during which 216 persons lost their lives. L. monocytogenes is endowed with the potential to multiply through a wide range of harsh environmental conditions, forming biofilms on varying surfaces in the food industry, as well as having persistent and antibiotic-resistant cells, which pose a major threat and burden to the ready-to-eat food industry. A more frustrating characteristic of this bacterium is its strain divergence, alongside an increased level of antibiotic resistance registered among the strains of L. monocytogenes recovered from food, humans, and environmental sources, especially to those antibiotics involved in the treatment of human listeriosis. Antibiotic resistance exerted by and among pathogenic food-borne microbes is an ongoing public health menace that continues to be an issue. Against this background, a thorough search into different databases using various search engines was performed, which led to the gathering of salient information that was organised, chronologically, based on Listeria monocytogenes and listeriosis. Altogether, the findings elaborated in this study present up-to date knowledge on different aspects of this pathogen which will improve our understanding of the mystery associated with it and the ways to prevent and control its dissemination through ready-to-eat foods. In addition, constant monitoring of the antibiotic resistance profiles of strains of L. monocytogenes from varying sources detected changes, giving an update on the trend in antibiotic resistance. Overall, monitoring of bacterial contamination serves as the key aspect in the control of the food safety output in the food industry.

Impact of Nisin on Proliferation of Background Microbiota, Pressure-Stressed and Wild-Type Listeria monocytogenes, and Listeria innocua During a Real-Time Shelf-Life Study

With the rapid implementation of high-pressure processing in many sectors of the food industry, considerations associated with pressure-stressed microorganisms are emerging. Nisin was utilized in this study for controlling the proliferation of Listeria monocytogenes and L. innocua inoculated on cold-smoked trout during a 4-week refrigerated shelf-life trial. Wild-type and pressure-stressed phenotypes of Listeria were compared in this study. The pressure-stressed phenotypes were prepared by treating the surrogate strain and pathogen mixture at 103.4 MPa (15K PSI) for 20 min. L. monocytogenes multiplied extensively during the 4-week refrigerated trial and counts were increased (p < 0.05) from 3.68 ± 0.1 log CFU/g on the first week to 6.03 ± 0.1 log CFU/g. Both phenotypes and the surrogate microorganisms illustrated similar (p ≥ 0.05) multiplication trends. Unlike samples subjected to water treatment, nisin was effective (p < 0.05) in keeping the microbial counts lower compared with the controls, particularly earlier during the shelf-life trial. Our study illustrates that the selected surrogate microorganism has comparable sensitivity to nisin relative to L. monocytogenes and thus could be used interchangeably in future public health microbiology challenge studies with similar scope. Additionally, we observed that pressure-stressed L. monocytogenes has proliferation and sensitivity to nisin comparable to wild-type pathogen.

Determination of an Effective Sanitizing Procedure for Listeria innocua in Personal Protective Equipment Used in Dairy Facilities

The presence of Listeria monocytogenes in the dairy environment remains a food safety challenge. The source of microbial contamination may include employees and their personal protective equipment (PPE). This study investigated the effectiveness of cleaning protocols (i.e., detergents and mechanical action) and three chemical sanitizers commonly employed at dairy facilities against Listeria innocua contamination on different types of gloves, aprons, and boots. Coupons made of PPE material were inoculated with a two-strain cocktail of L. innocua suspended in either Phosphate Buffered Saline (PBS) or skim milk to determine the potential effect of organic matter. In general, peroxyacetic acid (0.20% (v/v)) was more effective at reducing Listeria counts on aprons and gloves compared to chlorine and quaternary ammonium at 200 ppm. Depending on the type of sanitizer, Listeria reductions ranged from 1.95 to 4.72 and 1.52 to 4.60 log CFU/in2 on aprons and gloves, respectively. In comparison, sanitizers achieved a 0.93-2.32 log CFU/in2 reduction on boot soles, with no significant differences observed among sanitizers. PVC (vinyl) gloves achieved lower Listeria log reductions than nitrile and latex gloves. Sanitizers were less effective on boots with wider and deeper lugs than those with shallow lugs. The presence of organic matter significantly reduced the antimicrobial efficacy of all sanitizers (<1 log CFU/in2 reduction). However, the inclusion of cleaning protocols with and without mechanical action achieved a ≥3 log CFU/in2 reduction in the different types of PPE. This study highlights the importance of scrubbing as an essential step to reduce Listeria on PPE.

Novel primers drive accurate SYBR Green PCR detection of Listeria monocytogenes and Listeria innocua in cultures and mushrooms

The close genetic resemblance between Listeria monocytogenes and Listeria innocua, combined with their presence in similar environments, poses challenges for species-specific detection in food products. Ensuring food safety through microbiological standards necessitates reliable detection of pathogens like L. monocytogenes and L. innocua throughout the food chain using appropriate analytical techniques. This study aims to develop, identify, and validate a SYBR Green qPCR-based genetic marker designed to detect L. monocytogenes and L. innocua. By performing a comparative analysis of the complete genome sequences of L. monocytogenes (ATCC 12392) and L. innocua (CFSAN044836), a unique gene region encoding a hypothetical protein with an LPXTG cell wall anchor domain (GCF_003031895.1) in L. monocytogenes and leucine-rich repeats (GCF_009648575.1) in L. innocua was identified. Primers targeting these specific region were designed and validated for their effectiveness in detecting L. monocytogenes/L. innocua using both conventional PCR and qPCR techniques. These primers exhibited high sensitivity and specificity in amplifying L. monocytogenes and L. innocua among different Listeria species. The sensitivity and specificity of the primers were further confirmed through standard curve analysis using three different templates: cloned DNA (as a positive control), genomic DNA, and bacterial cell suspension. Additionally, the primers were rigorously tested and validated for their accuracy in directly detecting the targeted strains in live enoki mushroom samples. This direct qPCR method offers significant advantages for the rapid and precise detection of L. monocytogenes and L. innocua, potentially enhancing the efficiency of diagnostic and monitoring processes within food and vegetable distribution systems.

Prevalence, antibiogram and molecular characterization of Listeria monocytogenes from ruminants and humans in New Valley and Beheira Governorates, Egypt

BACKGROUND

Listeriosis is a global health threat to both animals and humans, especially in developing countries. This study was designed to isolate Listeria monocytogenes from faeces; environmental samples; and cow, sheep and goat milk, as well as human stool, to study its molecular characteristics and antibiotic sensitivity in the New Valley and Beheira Governorates, Egypt. The isolation and identification of L. monocytogenes were carried out using traditional culture and biochemical methods, followed by antibiography, genus confirmation of some isolates and detection and sequencing of InlB genes via PCR.

RESULTS

Out of 2097 examined samples, the prevalence of L. monocytogenes was 13.4% in animals; the prevalence was 9.2%, 2.4%, 25.4%, 4%, 42.4%, and 6.4% in cattle faeces, cattle milk, sheep faeces, sheep milk, goat faeces, and goat milk, respectively. However, the prevalence of L. monocytogenes was 8.3% in human samples. Both animal and human isolates showed 100% resistance to trimethoprim-sulfamethoxazole, and the isolates showed the highest sensitivity to flumequine (100%), amikacin (99.2%), gentamicin (97.6%), and levofloxacin (94.6%). Multidrug resistance (MDR) was detected in 86.9% of the tested isolates. The 16 S rRNA and inlB genes were detected in 100% of the randomly selected L. monocytogenes isolates. Phylogenetic analysis of three isolates based on the inlB gene showed 100% identity between faecal, milk and human stool isolates.

CONCLUSIONS

Faeces and milk are major sources of listeriosis, and the high degree of genetic similarity between animal and human isolates suggests the possibility of zoonotic circulation. The high prevalence of MDR L. monocytogenes in both animal and human samples could negatively impact the success of prevention and treatments for animal and human diseases, thereby imposing serious risks to public health.

Comparative genomics analysis to explore the biodiversity and mining novel target genes of Listeria monocytogenes strains from different regions

As a common foodborne pathogen, infection with L. monocytogenes poses a significant threat to human life and health. The objective of this study was to employ comparative genomics to unveil the biodiversity and evolutionary characteristics of L. monocytogenes strains from different regions, screening for potential target genes and mining novel target genes, thus providing significant reference value for the specific molecular detection and therapeutic targets of L. monocytogenes strains. Pan-genomic analysis revealed that L. monocytogenes from different regions have open genomes, providing a solid genetic basis for adaptation to different environments. These strains contain numerous virulence genes that contribute to their high pathogenicity. They also exhibit relatively high resistance to phosphonic acid, glycopeptide, lincosamide, and peptide antibiotics. The results of mobile genetic elements indicate that, despite being located in different geographical locations, there is a certain degree of similarity in bacterial genome evolution and adaptation to specific environmental pressures. The potential target genes identified through pan-genomics are primarily associated with the fundamental life activities and infection invasion of L. monocytogenes, including known targets such as inlB, which can be utilized for molecular detection and therapeutic purposes. After screening a large number of potential target genes, we further screened them using hub gene selection methods to mining novel target genes. The present study employed eight different hub gene screening methods, ultimately identifying ten highly connected hub genes (bglF_1, davD, menE_1, tilS, dapX, iolC, gshAB, cysG, trpA, and hisC), which play crucial roles in the pathogenesis of L. monocytogenes. The results of pan-genomic analysis showed that L. monocytogenes from different regions exhibit high similarity in bacterial genome evolution. The PCR results demonstrated the excellent specificity of the bglF_1 and davD genes for L. monocytogenes. Therefore, the bglF_1 and davD genes hold promise as specific molecular detection and therapeutic targets for L. monocytogenes strains from different regions.

A novel multiplex PCR based method for the detection of Listeria monocytogenes clonal complex 8

Listeria monocytogenes is an important foodborne pathogen worldwide, which could cause listeriosis with a 20-30 % fatality rate in immunocompromised individuals. Listeria monocytogenes MLST clonal complex (CC) 8 strain is a common clone in food and clinical cases. The aim of this study was to develop multiplex PCR (mPCR) and high-resolution melting (HRM) qPCR to simultaneously detect L. monocytogenes CC8 and the other L. monocytogenes strains based on pan-genome analysis. A novel multiplex PCR and HRM qPCR targeted for the genes LM5578_1180 (specific for CC8) and LM5578_2262 (for L. monocytogenes) were developed. The specificity of this multiplex PCR and HRM qPCR were verified with other CCs of L. monocytogenes and other species strains. The detection limit of this multiplex PCR and HRM qPCR is 2.1 × 103 CFU/mL and 2.1 × 100 CFU/mL, respectively. This multiplex PCR and HRM qPCR could accurately detect CC8 strains with the interference of different ratios of L. monocytogenes CC9, CC87, CC121, CC155, and L. innocua strains. Subsequently, the detection ability of mPCR and HRM qPCR were also evaluated in spiked samples. The mPCR method could successfully detect 6.2 × 103 CFU/mL of CC8 L. monocytogenes after 6 h enrichment while the multiplex HRM qPCR method could successfully detect 6.2 × 104 CFU/mL of CC8 L. monocytogenes after 3 h enrichment. The feasibility of these methods were satisfactory in terms of sensitivity, specificity, and efficiency after evaluating 12 mushroom samples and was consistent with that of the National Standard Detection Method (GB4789.30-2016). In conclusion, the developed assays could be applied for rapid screening and detection of L. monocytogenes CC8 strains both in food and food production environments, providing accurate results to adopt monitoring measures to improve microbiological safety.

Characterisation of the growth behaviour of Listeria monocytogenes in Listeria synthetic media

The foodborne pathogen Listeria monocytogenes can grow in a wide range of environmental conditions. For the study of the physiology of this organism, several chemically defined media have been developed over the past decades. Here, we examined the ability of L. monocytogenes wildtype strains EGD-e and 10403S to grow under salt and pH stress in Listeria synthetic medium (LSM). Furthermore, we determined that a wide range of carbon sources could support the growth of both wildtype strains in LSM. However, for hexose phosphate sugars such as glucose-1-phosphate, both L. monocytogenes strains need to be pre-grown under conditions, where the major virulence regulator PrfA is active. In addition, growth of both L. monocytogenes strains was observed when LSM was supplemented with the amino acid sugar N-acetylmannosamine (ManNAc). We were able to show that some of the proteins encoded in the operon lmo2795-nanE, such as the ManNAc-6-phosphate epimerase NanE, are required for growth in the presence of ManNAc. The first gene of the operon, lmo2795, encodes a transcriptional regulator of the RpiR family. Using electrophoretic mobility shift assays and quantitative real-time PCR analysis, we were able to show that Lmo2795 binds to the promoter region of the operon lmo2795-nanE and activates its expression.

Listeria monocytogenes an Emerging Pathogen: a Comprehensive Overview on Listeriosis, Virulence Determinants, Detection, and Anti-Listerial Interventions

Listeria monocytogenes, the third most deleterious zoonotic pathogen, is a major causative agent of animal and human listeriosis, an infection related to the consumption of contaminated food products. Even though, this pathogen has been responsible for the outbreaks of foodborne infections in the early 1980s, the major outbreaks have been reported during the past two decades. Listeriosis infection in the host is a rare but life-threatening disease with major public health and economic implications. Extensive reports on listeriosis outbreaks are associated with milk and milk products, meat and meat products, and fresh produce. This bacterium can adapt to any environmental and stress conditions, making it a prime causative agent for major foodborne diseases. The pathogen could survive an antibiotic treatment and persist in the host cell, thereby escaping the standard diagnostic practices. The current review strives to provide concise information on the epidemiology, serotypes, and pathogenesis of the L. monocytogenes to decipher the knowledge on the endurance of the pathogen inside the host and food products as a vehicle for Listeria contaminations. In addition, various detection methods for Listeria species from food samples and frontline regimens of L. monocytogenes treatment have also been discussed.

Rapid separation and detection of Listeria monocytogenes with the combination of phage tail fiber protein and vancomycin-magnetic nanozyme

In this work, a lateral flow assay for Listeria monocytogenes was developed based on phage tail fiber protein (TFP) and triple-functional nanozyme probes with capture-separation-catalytic activity. Inspired by interaction between phage and bacteria, TFP of L. monocytogenes phage was immobilized on test line as capture molecule, which replaced traditional antibody and aptamer. After Gram-positive bacteria was captured and separated from samples by nanozyme probes modified with vancomycin (Van), TFP specifically recognized L. monocytogenes and overcame non-specific binding of Van. Special color reaction between Coomassie Brilliant Blue and bovine serum albumin which was an amplification carrier on probe was simply utilized as control zone to replace traditional control line. Relying on enzyme-like catalytic activity of nanozyme, this biosensor realized improved sensitivity and colorimetric quantitative detection with a detection limit of 10 CFU mL^-1. Analytic performance results suggested this TFP-based biosensor provided a portable, sensitive and specific strategy to detect pathogen.

Cross-contamination of mature Listeria monocytogenes biofilms from stainless steel surfaces to chicken broth before and after the application of chlorinated alkaline and enzymatic detergents

The objectives of this study were, firstly, to compare a conventional (i.e., chlorinated alkaline) versus an alternative (chlorinated alkaline plus enzymatic) treatment effectivity for the elimination of biofilms from different L. monocytogenes strains (CECT 5672, CECT 935, S2-bac and EDG-e). Secondly, to evaluate the cross-contamination to chicken broth from non-treated and treated biofilms formed on stainless steel surfaces. Results showed that all L. monocytogenes strains were able to adhere and develop biofilms at approximately the same growth levels (≈5.82 log CFU/cm²). When non-treated biofilms were put into contact with the model food, obtained an average transference rate of potential global cross-contamination of 20.4%. Biofilms treated with the chlorinated alkaline detergent obtained transference rates similar to non-treated biofilms as a high number of residual cells (i.e., around 4 to 5 Log CFU/cm²) were present on the surface, except for EDG-e strain on which transference rate diminished to 0.45%, which was related to the protective matrix. Contrarily, the alternative treatment was shown to not produce cross-contamination to the chicken broth due to its high effectivity for biofilm control (<0.50% of transference) except for CECT 935 strain that had a different behavior. Therefore, changing to more intense cleaning treatments in the processing environments can reduce risk of cross-contamination.