Biofilm-isolated Listeria monocytogenes exhibits reduced systemic dissemination at the early (12-24 h) stage of infection in a mouse model

Listeria adhesion protein orchestrates caveolae-mediated apical junctional remodeling of epithelial barrier for Listeria monocytogenes translocation

The cellular junctional architecture remodeling by Listeria adhesion protein-heat shock protein 60 (LAP-Hsp60) interaction for Listeria monocytogenes (Lm) passage through the epithelial barrier is incompletely understood. Here, using the gerbil model, permissive to internalin (Inl) A/B-mediated pathways like in humans, we demonstrate that Lm crosses the intestinal villi at 48 h post-infection. In contrast, the single isogenic (lap- or ΔinlA) or double (lap-ΔinlA) mutant strains show significant defects. LAP promotes Lm translocation via endocytosis of cell-cell junctional complex in enterocytes that do not display luminal E-cadherin. In comparison, InlA facilitates Lm translocation at cells displaying apical E-cadherin during cell extrusion and mucus expulsion from goblet cells. LAP hijacks caveolar endocytosis to traffic integral junctional proteins to the early and recycling endosomes. Pharmacological inhibition in a cell line and genetic knockout of caveolin-1 in mice prevents LAP-induced intestinal permeability, junctional endocytosis, and Lm translocation. Furthermore, LAP-Hsp60-dependent tight junction remodeling is also necessary for InlA access to E-cadherin for Lm intestinal barrier crossing in InlA-permissive hosts.

IMPORTANCE

Listeria monocytogenes (Lm) is a foodborne pathogen with high mortality (20%-30%) and hospitalization rates (94%), particularly affecting vulnerable groups such as pregnant women, fetuses, newborns, seniors, and immunocompromised individuals. Invasive listeriosis involves Lm's internalin (InlA) protein binding to E-cadherin to breach the intestinal barrier. However, non-functional InlA variants have been identified in Lm isolates, suggesting InlA-independent pathways for translocation. Our study reveals that Listeria adhesion protein (LAP) and InlA cooperatively assist Lm entry into the gut lamina propria in a gerbil model, mimicking human listeriosis in early infection stages. LAP triggers caveolin-1-mediated endocytosis of critical junctional proteins, transporting them to early and recycling endosomes, facilitating Lm passage through enterocytes. Furthermore, LAP-Hsp60-mediated junctional protein endocytosis precedes InlA's interaction with basolateral E-cadherin, emphasizing LAP and InlA's cooperation in enhancing Lm intestinal translocation. This understanding is vital in combating the severe consequences of Lm infection, including sepsis, meningitis, encephalitis, and brain abscess.

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.

Survival strategies of Listeria monocytogenes to environmental hostile stress: biofilm formation and stress responses

Listeria monocytogenes is a critical foodborne pathogen that causes listeriosis and threatens public health. This pathogenic microorganism forms a transmission cycle in nature, food industry, and humans, expanding the areas of contamination among them and influencing food safety. L. monocytogenes forms biofilms to protect itself and promotes survival through stress responses to the various stresses (e.g., temperature, pH, and antimicrobial agents) that may be inflicted during food processing. Biofilms and mechanisms of resistance to hostile external or general stresses allow L. monocytogenes to survive despite a variety of efforts to ensure food safety. The current review article focuses on biofilm formation, resistance mechanisms through biofilms, and external specific or general stress responses of L. monocytogenes to help understand the unexpected survival rates of this bacterium; it also proposes the use of obstacle technology to effectively cope with it in the food industry.

The ribonuclease PNPase is a key regulator of biofilm formation in Listeria monocytogenes and affects invasion of host cells

Biofilms provide an environment that protects microorganisms from external stresses such as nutrient deprivation, antibiotic treatments, and immune defences, thereby creating favorable conditions for bacterial survival and pathogenesis. Here we show that the RNA-binding protein and ribonuclease polynucleotide phosphorylase (PNPase) is a positive regulator of biofilm formation in the human pathogen Listeria monocytogenes, a major responsible for food contamination in food-processing environments. The PNPase mutant strain produces less biofilm biomass and exhibits an altered biofilm morphology that is more susceptible to antibiotic treatment. Through biochemical assays and microscopical analysis, we demonstrate that PNPase is a previously unrecognized regulator of the composition of the biofilm extracellular matrix, greatly affecting the levels of proteins, extracellular DNA, and sugars. Noteworthy, we have adapted the use of the fluorescent complex ruthenium red-phenanthroline for the detection of polysaccharides in Listeria biofilms. Transcriptomic analysis of wild-type and PNPase mutant biofilms reveals that PNPase impacts many regulatory pathways associated with biofilm formation, particularly by affecting the expression of genes involved in the metabolism of carbohydrates (e.g., lmo0096 and lmo0783, encoding PTS components), of amino acids (e.g., lmo1984 and lmo2006, encoding biosynthetic enzymes) and in the Agr quorum sensing-like system (lmo0048-49). Moreover, we show that PNPase affects mRNA levels of the master regulator of virulence PrfA and PrfA-regulated genes, and these results could help to explain the reduced bacterial internalization in human cells of the ΔpnpA mutant. Overall, this work demonstrates that PNPase is an important post-transcriptional regulator for virulence and adaptation to the biofilm lifestyle of Gram-positive bacteria and highlights the expanding role of ribonucleases as critical players in pathogenicity.

Cell-surface anchoring of Listeria adhesion protein on L. monocytogenes is fastened by internalin B for pathogenesis

Listeria adhesion protein (LAP) is a secreted acetaldehyde alcohol dehydrogenase (AdhE) that anchors to an unknown molecule on the Listeria monocytogenes (Lm) surface, which is critical for its intestinal epithelium crossing. In the present work, immunoprecipitation and mass spectrometry identify internalin B (InlB) as the primary ligand of LAP (K_D ∼ 42 nM). InlB-deleted and naturally InlB-deficient Lm strains show reduced LAP-InlB interaction and LAP-mediated pathology in the murine intestine and brain invasion. InlB-overexpressing non-pathogenic Listeria innocua also displays LAP-InlB interplay. In silico predictions reveal that a pocket region in the C-terminal domain of tetrameric LAP is the binding site for InlB. LAP variants containing mutations in negatively charged (E₅₂₃S, E₆₂₁S) amino acids in the C terminus confirm altered binding conformations and weaker affinity for InlB. InlB transforms the housekeeping enzyme, AdhE (LAP), into a moonlighting pathogenic factor by fastening on the cell surface.

Marked inter-strain heterogeneity in the differential expression of some key stress response and virulence-related genes between planktonic and biofilm cells in Listeria monocytogenes

Listeria monocytogenes is a facultatively intracellular pathogenic bacterium that can provoke invasive listeriosis, a severe foodborne infection in humans. Outside the host, this is capable to survive for long periods in soil, and water, as well as on plants, while, like many other microorganisms, this can also attach to abiotic surfaces, such as food contact ones, forming biofilms on them. It has been suggested that inside those sessile communities, L. monocytogenes cells not only display an increased stress tolerance but may also boost their pathogenicity. In this work, the expression of ten key stress response and/or virulence-related genes (i.e., groEL, hly, iap, inlA, inlB, lisK, mdrD, mdrL, prfA, and sigB) was studied in three different L. monocytogenes strains (AAL20066, AAL20107, and PL24), all isolated from foods and each belonging to a different listeriosis-associated serovar (1/2a, 1/2b, and 1/2c, respectively). For this, each strain was initially left to develop a mature biofilm on a model polystyrene surface (Petri dish) by incubating for 144 h (6 days) at 20 °C in tryptone soya broth (with medium renewal every 48 h). Following incubation, both biofilm and the surrounding free-swimming (planktonic) cells were recovered, and their gene expressions were comparatively evaluated through targeted reverse transcription-quantitative polymerase chain reactions (RT-qPCR). Results revealed a strain-dependent differential gene expression between the two cell types. Thus, for instance, in strain AAL20107 (ser. 1/2b) biofilm growth worryingly resulted in a significant overexpression of all the studied genes (P < 0.05), whereas in strain PL24 (ser. 1/2c), the expression of most genes (8/10) did not change upon biofilm growth, with only two of them (groEL and hly) being again significantly upregulated. Such transcriptomic strain variability in stress adaptation and/or virulence induction should be generally considered in the physiological studies of pathogenic biofilms and preferably upon designing and implementing novel and more efficient eradication methods.

Antimicrobial Activity of Sertraline on Listeria monocytogenes

We explored the antimicrobial activity of sertraline on Listeria monocytogenes and further investigated the effects of sertraline on biofilm formation and the virulence gene expression of L. monocytogenes. The minimum inhibitory concentration and minimum bactericidal concentration for sertraline against L. monocytogenes were in the range of 16-32 μg/mL and 64 μg/mL, respectively. Sertraline-dependent damage of the cell membrane and a decrease in intracellular ATP and pH_in in L. monocytogenes were observed. In addition, sertraline reduced the biofilm formation efficiency of the L. monocytogenes strains. Importantly, low concentrations (0.1 μg/mL and 1 μg/mL) of sertraline significantly down-regulated the expression levels of various L. monocytogens virulence genes (prfA, actA, degU, flaA, sigB, ltrC and sufS). These results collectively suggest a role of sertraline for the control of L. monocytogenes in the food industry.

Bactericidal Action of Shrimp Shell Chitooligosaccharide Conjugated with Epigallocatechin Gallate (COS-EGCG) against Listeria monocytogenes

The antibacterial effect of chitooligosaccharide conjugated with five different polyphenols, including catechin (COS-CAT), epigallocatechin gallate (COS-EGCG), gallic acid (COS-GAL), caffeic acid (COS-CAF), and ferulic acid (COS-FER), against Listeria monocytogenes was investigated. Among all the conjugates tested, COS-EGCG showed the highest inhibition toward Listeria monocytogenes, with a minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of 1024 and 1024 µg/mL, respectively. The COS-EGCG conjugate also had a bactericidal effect on the environmental and clinical strains of L. monocytogenes. The low concentration of COS-EGCG conjugate augmented the formation of biofilm and the growth of L. monocytogenes. Nevertheless, the inhibition of biofilm formation and bacterial growth was achieved when treated with the COS-EGCG conjugate at 2 × MIC for 48 h. In addition, the COS-EGCG conjugate at 2 × MIC had the potential to inactivate the pre-biofilm, and it reduced the production of the extracellular polysaccharides of L. monocytogenes. The COS-EGCG conjugate at the MIC/4 effectively impeded the motility (the swimming and swarming) of L. monocytogenes, with an 85.7-94.3% inhibition, while 100% inhibition was achieved with the MIC. Based on scanning electron microscopic (SEM) images, cell wall damage with numerous pores on the cell surface was observed. Such cell distortion resulted in protein leakage. As a result, COS-EGCG could penetrate into the cell and bind with the DNA backbone. Therefore, the COS-EGCG conjugate could be further developed as a natural antimicrobial agent for inhibiting or controlling L. monocytogenes.

Listeria monocytogenes-How This Pathogen Uses Its Virulence Mechanisms to Infect the Hosts

Listeriosis is a serious food-borne illness, especially in susceptible populations, including children, pregnant women, and elderlies. The disease can occur in two forms: non-invasive febrile gastroenteritis and severe invasive listeriosis with septicemia, meningoencephalitis, perinatal infections, and abortion. Expression of each symptom depends on various bacterial virulence factors, immunological status of the infected person, and the number of ingested bacteria. Internalins, mainly InlA and InlB, invasins (invasin A, LAP), and other surface adhesion proteins (InlP1, InlP4) are responsible for epithelial cell binding, whereas internalin C (InlC) and actin assembly-inducing protein (ActA) are involved in cell-to-cell bacterial spread. L. monocytogenes is able to disseminate through the blood and invade diverse host organs. In persons with impaired immunity, the elderly, and pregnant women, the pathogen can also cross the blood-brain and placental barriers, which results in the invasion of the central nervous system and fetus infection, respectively. The aim of this comprehensive review is to summarize the current knowledge on the epidemiology of listeriosis and L. monocytogenes virulence mechanisms that are involved in host infection, with a special focus on their molecular and cellular aspects. We believe that all this information is crucial for a better understanding of the pathogenesis of L. monocytogenes infection.

Low-Cost Nonreversible Electronic-Free Wireless pH Sensor for Spoilage Detection in Packaged Meat Products

Contamination of meat with pathogenic microorganisms can cause severe illnesses and food waste, which has significant negative impacts on both general health and the economy. In many cases, the expiration date is not a good indicator of meat freshness as there is a high risk of contamination during handling throughout the supply chain. Many biomarkers, including color, odor, pH, temperature, and volatile compounds, are used to determine spoilage. Among these, pH presents a simple and effective biomarker directly linked to the overgrowth of bacteria and degradation of the meat tissue. Low-cost methods for wireless pH monitoring are crucial in detecting spoilage on a large commercial scale. Existing technologies are often limited to short-range detection, with the use of batteries and different electronic components that increases both the manufacturing complexity and cost of the final device. To address these shortcomings, we have developed a cost-effective wireless pH sensor, which uses passive resonant frequency (RF) sensing, combined with a pH-responsive polymer that can be placed within packaged meat products and provide a remote assessment of the risk of microbial spoilage throughout the supply chain. The sensor tag consists of a sensing resonator coated with a pH-sensitive material and a passivated reference resonator operating in a differential frequency configuration. Upon exposure to elevated pH levels >6.8, the coating on the sensing resonator dissolves, which in turn results in a distinct change in the resonant frequency with respect to the reference resonator. Systematic theoretical and experimental results at different pH levels demonstrated that a 20% shift in resonant frequency demarcates the point for spoilage detection. As a proof of concept, the performance of the sensor in remotely detecting the risk of food spoilage was validated in packaged poultry over 10 days. The sensor fabrication process takes advantage of recent developments in the scalable manufacturing of flexible, low-cost devices, including selective laser etching of metalized plastic films and doctor-blade coating of stimuli-responsive polymer films. Furthermore, the biocompatibility of all the materials used in the sensor was confirmed with human intestinal cells (HCT-8 cells).

Anti-Biofilm Activity of Cell Free Supernatants of Selected Lactic Acid Bacteria against Listeria monocytogenes Isolated from Avocado and Cucumber Fruits, and from an Avocado Processing Plant

Listeria monocytogenes forms biofilms on food contact surfaces, a niche from where it dislodges to contaminate food products including fresh produce. Probiotics and their derivatives are considered promising alternative strategies to curb the presence of L. monocytogenes in varied food applications. Nonetheless, studies on their anti-biofilm effects against L. monocytogenes from avocados and cucumbers are sparse. This study screened the biofilm formation capabilities of L. monocytogenes strains Avo and Cuc isolated from the avocado and cucumber fruits respectively, and strain 243 isolated from an avocado processing plant; and evaluated the anti-biofilm effects of cell free supernatants (CFS) of Lactobacillus acidophilus La14 150B, Lactiplantibacillus plantarum B411 and Lacticaseibacillus rhamnosus ATCC 53103 against their biofilms formed on polyvinyl chloride (PVC) and stainless steel. All the L. monocytogenes strains formed biofilms (classified either as moderate or strong biofilm formers) on these materials. The presence of CFS reduced the biofilm formation capabilities of these strains and disrupted the integrity of their pre-formed biofilms. Quantitative reverse transcriptase polymerase chain reaction revealed significant reduction of positive regulatory factor A (prfA) gene expression by L. monocytogenes biofilm cells in the presence of CFS (p < 0.05). Thus, these CFS have potential as food grade sanitizers for control of L. monocytogenes biofilms in the avocado and cucumber processing facilities.

Synergistic Effect of Lithocholic Acid with Gentamicin against Gram-Positive Bacteria but Not against Gram-Negative Bacteria

Listeria monocytogenes (L. monocytogenes) is an important Gram-positive food-borne pathogen that severely threatens public health. A checkerboard microdilution method was performed to evaluate the synergistic effect of lithocholic acid (LCA) with Gentamicin (Genta) against L. monocytogenes. BacLight LIVE/DEAD staining, scanning electron microscopy and biofilm inhibition assays were further used to explore the bactericidal effect and antibiofilm effect of this combination on L. monocytogenes. Additionally, the synergistic effects of LCA derivatives with Genta were also evaluated against L. monocytogenes, S.aureus and S. suis. The results indicated that a synergistic bactericidal effect was observed for the combined therapy of LCA at the concentration without affecting bacteria viability, with Genta. Additionally, LCA in combination with Genta had a synergistic effect against Gram-positive bacteria (L. monocytogenes, S. aureus and S. suis) but not against Gram-negative bacteria (E. coli, A. baumannii and Salmonella). BacLight LIVE/DEAD staining and scanning electron microscopy analysis revealed that the combination of LCA with Genta caused L. monocytogenes membrane injury, leading to bacteria death. We found that 8 μg/mL LCA treatment effectively improved the ability of Genta to eradicate L. monocytogenes biofilms. In addition, we found that chenodeoxycholic acid, as a cholic acid derivative, also improved the bactericidal effect of Genta against Gram-positive bacteria. Our results indicate that LCA represents a broad-spectrum adjuvant with Genta for infection caused by L. monocytogenes and other Gram-positive pathogens.

Inactivation of Polymicrobial Biofilms of Foodborne Pathogens Using Epsilon Poly-L-Lysin Conjugated Chitosan Nanoparticles

A mixed culture (polymicrobial) biofilm provides a favorable environment for pathogens to persist in the food processing environment and to contaminate food products. Inactivation and eradication of such biofilms from food processing environments are achieved by using harsh disinfectants, but their toxicity and environmentally hostile characteristics are unsustainable. This study aims to use food-grade natural nanoparticulated antimicrobials to control mixed-culture biofilms. Chitosan, a natural broad-spectrum antimicrobial biopolymer (polysaccharide) from crustaceans, was derivatized to produce chitosan nanoparticles (ChNP) as a carrier for another broad-spectrum antimicrobial agent, ε-poly-L-lysine (PL), to synthesize ChNP-PL conjugate. The antimicrobial activity of ChNP and ChNP-PL was tested against mixed-culture biofilms. ChNP-PL (~100 nm) exhibited a synergistic antimicrobial and anti-biofilm effect against mono or mixed-culture biofilms of five foodborne pathogens, including Listeria monocytogenes, Staphylococcus aureus, Salmonella enterica serovar Enteritidis, Escherichia coli O157:H7, and Pseudomonas aeruginosa. ChNP-PL treatment prevented biofilm formation by mono or mixed cultures of L. monocytogenes, P. aeruginosa, and E. coli O157:H7, and bacterial counts were either below the detection limit or caused 3.5–5 log reduction. ChNP-PL also inactivated preformed biofilms. In monoculture biofilm, ChNP-PL treatment reduced L. monocytogenes counts by 4.5 logs, S. Enteritidis by 2 logs, E. coli by 2 logs, and S. aureus by 0.5 logs, while ChNP-PL had no inhibitory effect on P. aeruginosa. In vitro mammalian cell-based cytotoxicity analysis confirmed ChNP-PL to have no deleterious effect on intestinal HCT-8 cell line. In conclusion, our results show ChNP-PL has strong potential to prevent the formation or inactivation of preformed polymicrobial biofilms of foodborne pathogens.

Landscape of Stress Response and Virulence Genes Among Listeria monocytogenes Strains

The pathogenic microorganism Listeria monocytogenes is ubiquitous and responsible for listeriosis, a disease with a high mortality rate in susceptible people. It can persist in different habitats, including the farm environment, the food production environments, and in foods. This pathogen can grow under challenging conditions, such as low pH, low temperatures, and high salt concentrations. However, L. monocytogenes has a high degree of strain divergence regarding virulence potential, environmental adaption, and stress response. This review seeks to provide the reader with an up-to-date overview of clonal and serotype-specific differences among L. monocytogenes strains. Emphasis on the genes and genomic islands responsible for virulence and resistance to environmental stresses is given to explain the complex adaptation among L. monocytogenes strains. Moreover, we highlight the use of advanced diagnostic technologies, such as whole-genome sequencing, to fine-tune quantitative microbiological risk assessment for better control of listeriosis.

Bacterial Biofilms and Their Implications in Pathogenesis and Food Safety

Biofilm formation is an integral part of the microbial life cycle in nature. In food processing environments, bacterial transmissions occur primarily through raw or undercooked foods and by cross-contamination during unsanitary food preparation practices. Foodborne pathogens form biofilms as a survival strategy in various unfavorable environments, which also become a frequent source of recurrent contamination and outbreaks of foodborne illness. Instead of focusing on bacterial biofilm formation and their pathogenicity individually, this review discusses on a molecular level how these two physiological processes are connected in several common foodborne pathogens such as Listeria monocytogenes, Staphylococcus aureus, Salmonella enterica and Escherichia coli. In addition, biofilm formation by Pseudomonas aeruginosa is discussed because it aids the persistence of many foodborne pathogens forming polymicrobial biofilms on food contact surfaces, thus significantly elevating food safety and public health concerns. Furthermore, in-depth analyses of several bacterial molecules with dual functions in biofilm formation and pathogenicity are highlighted.

Antibody- and nucleic acid-based lateral flow immunoassay for Listeria monocytogenes detection

Listeria monocytogenes is an invasive opportunistic foodborne pathogen and its routine surveillance is critical for protecting the food supply and public health. The traditional detection methods are time-consuming and require trained personnel. Lateral flow immunoassay (LFIA), on the other hand, is an easy-to-perform, rapid point-of-care test and has been widely used as an inexpensive surveillance tool. In recent times, nucleic acid-based lateral flow immunoassays (NALFIA) are also developed to improve sensitivity and specificity. A significant improvement in lateral flow-based assays has been reported in recent years, especially the ligands (antibodies, nucleic acids, aptamers, bacteriophage), labeling molecules, and overall assay configurations to improve detection sensitivity, specificity, and automated interpretation of results. In most commercial applications, LFIA has been used with enriched food/environmental samples to ensure detection of live cells thus prolonging the assay time to 24-48 h; however, with the recent improvement in LFIA sensitivity, results can be obtained in less than 8 h with shortened and improved enrichment practices. Incorporation of surface-enhanced Raman spectroscopy and/or immunomagnetic separation could significantly improve LFIA sensitivity for near-real-time point-of-care detection of L. monocytogenes for food safety and public health applications.

Listeria monocytogenes: review of pathogenesis and virulence determinants-targeted immunological assays

Listeria monocytogenes is one of the most invasive foodborne pathogens and is responsible for numerous outbreaks worldwide. Most of the methods to detect this bacterium in food require selective enrichment using traditional bacterial culture techniques that can be time-consuming and labour-intensive. Moreover, molecular methods are expensive and need specific technical knowledge. In contrast, immunological approaches are faster, simpler, and user-friendly alternatives and have been developed for the detection of L. monocytogenes in food, environmental, and clinical samples. These techniques are dependent on the constitutive expression of L. monocytogenes antigens and the specificity of the antibodies used. Here, updated knowledge on pathogenesis and the key immunogenic virulence determinants of L. monocytogenes that are used for the generation of monoclonal and polyclonal antibodies for the serological assay development are summarised. In addition, immunological approaches based on enzyme-linked immunosorbent assay, immunofluorescence, lateral flow immunochromatographic assays, and immunosensors with relevant improvements are highlighted. Though the sensitivity and specificity of the assays were improved significantly, methods still face many challenges that require further validation before use.