EXPRESSION OF VIRULENCE GENES OF LISTERIA MONOCYTOGENES IN FOOD

Druggability Analysis of Protein Targets for Drug Discovery to Combat Listeria monocytogenes

Extensive research has been conducted to identify key proteins governing stress responses, virulence, and antimicrobial resistance, as well as to elucidate their interactions within Listeria monocytogenes. While these proteins hold promise as potential targets for novel strategies to control L. monocytogenes, given their critical roles in regulating the pathogen's metabolism, additional analysis is needed to further assess their druggability-the chance of being effectively bound by small-molecule inhibitors. In this work, 535 binding pockets of 46 protein targets for known drugs (mainly antimicrobials) were first analyzed to extract 13 structural features (e.g., hydrophobicity) in a ligand-protein docking platform called Molsoft ICM Pro. The extracted features were used as inputs to develop a logistic regression model to assess the druggability of protein binding pockets, with a value of one if ligands can bind to the protein pocket. The developed druggability model was then used to evaluate 23 key proteins from L. monocytogenes that have been identified in the literature. The following proteins are predicted to be high-potential druggable targets: GroEL, FliH/FliI complex, FliG, FlhB, FlgL, FlgK, InlA, MogR, and PrfA. These findings serve as an initial point for future research to identify specific compounds that can inhibit druggable target proteins and to design experimental work to confirm their effectiveness as drug targets.

The ARIMA model approach for the biofilm-forming capacity prediction of Listeria monocytogenes recovered from carcasses

The present study aimed to predict the biofilm-formation ability of L. monocytogenes isolates obtained from cattle carcasses via the ARIMA model at different temperature parameters. The identification of L. monocytogenes obtained from carcass samples collected from slaughterhouses was determined by PCR. The biofilm-forming abilities of isolates were phenotypically determined by calculating the OD value and categorizing the ability via the microplate test. The presence of some virulence genes related to biofilm was revealed by QPCR to support the biofilm profile genotypically. Biofilm-formation of the isolates was evaluated at different temperature parameters (37 °C, 22 °C, 4 °C and - 20 °C). Estimated OD values were obtained with the ARIMA model by dividing them into eight different estimation groups. The prediction performance was determined by performance measurement metrics (ME, MAE, MSE, RMSE, MPE and MAPE). One week of incubation showed all isolates strongly formed biofilm at all controlled temperatures except - 20 °C. In terms of the metrics examined, the 3 days to 7 days forecast group has a reasonable prediction accuracy based on OD values occurring at 37 °C, 22 °C, and 4 °C. It was concluded that measurements at 22 °C had lower prediction accuracy compared to predictions from other temperatures. Overall, the best OD prediction accuracy belonged to the data obtained from biofilm formation at -20 °C. For all temperatures studied, especially after the 3 days to 7 days forecast group, there was a significant decrease in the error metrics and the forecast accuracy increased. When evaluating the best prediction group, the lowest RMSE at 37 °C (0.055), 22 °C (0.027) and 4 °C (0.024) belonged to the 15 days to 21 days group. For the OD predictions obtained at -20 °C, the 15 days to 21 days prediction group had also good performance (0.011) and the lowest RMSE belongs to the 7 days to 15 days group (0.007). In conclusion, this study will guide in using indicator parameters to evaluate biofilm forming ability to predict optimum temperature-time. The ARIMA models integrated with this study can be useful tools for industrial application and risk assessment studies using different parameters such as pH, NaCl concentration, and especially temperature applied during food processing and storage on the biofilm-formation ability of L. monocytogenes.

Investigation on the inactivation effect and mechanism of Listeria monocytogenes in fresh-cut cucumber during storage by ultrasound combined with sodium hypochlorite

Fresh agricultural products are frequently contaminated with Listeria monocytogenes (L. monocytogenes), which threatens consumer health. The mechanism of the inhibitory effect of ultrasound and sodium hypochlorite (US-NaClO) on L. monocytogenes on fresh-cut cucumber remains poorly understood. Therefore, the bactericidal ability and mechanism of US-NaClO treatment on L. monocytogenes were studied on fresh-cut cucumber during storage using various approaches such as determination of intracellular material leakage, scanning electron microscopy, flow cytometry, and expression analysis of virulence genes. The results showed that the number of L. monocytogenes on fresh-cut cucumber was significantly reduced after ultrasound treatment for 5 min in combined with 75 ppm sodium hypochlorite treatment(P < 0.05). The US-NaClO treatment affected cell morphology, impaired cell membrane integrity, increased cell membrane permeability, and reduced the concentration of K+, inorganic phosphate, ATP, proteins, and DNA in bacterial cells, leading to the inactivation of microorganisms. In addition, the US-NaClO treatment downregulated expression of the virulence genes actA, hly, inlA, mpl, pclA, and plcB, thus decreasing the pathogenicity of bacteria. It can avoid contamination by pathogenic bacteria during the production of fresh-cut cucumber, while providing safety assurance for production.

Spatially localised expression of the glutamate decarboxylase gadB in Escherichia coli O157:H7 microcolonies in hydrogel matrices

Functional diversity within isogenic spatially organised bacterial populations has been shown to trigger emergent community properties such as stress tolerance. Considering gadB gene encoding a key glutamate decarboxylase involved in E. coli tolerance to acidic conditions, we investigated its expression in hydrogels mimicking the texture of some structured food matrices (such as minced meat or soft cheese). Taking advantage of confocal laser scanning microscopy combined with a genetically-engineered dual fluorescent reporter system, it was possible to visualise the spatial patterns of bacterial gene expression from in-gel microcolonies. In E. coli O157:H7 microcolonies, gadB showed radically different expression patterns between neutral (pH 7) or acidic (pH 5) hydrogels. Differential spatial expression was determined in acidic hydrogels with a strong expression of gadB at the microcolony periphery. Strikingly, very similar spatial patterns of gadB expression were further observed for E. coli O157:H7 grown in the presence of L. lactis. Considering the ingestion of contaminated foodstuff, survival of E. coli O157:H7 to acidic stomachal stress (pH 2) was significantly increased for bacterial cells grown in microcolonies in acidic hydrogels compared to planktonic cells. These findings have significant implications for risk assessment and public health as they highlight inherent differences in bacterial physiology and virulence between liquid and structured food products. The contrasting characteristics observed underscore the need to consider the distinct challenges posed by these food types, thereby emphasising the importance of tailored risk mitigation strategies.

Genomic Insights into Listeria monocytogenes: Organic Acid Interventions for Biofilm Prevention and Control

Listeria monocytogenes is an important pathogen that has been implicated in foodborne illness. The aim of the present study was to investigate the diversity of virulence factors associated with the mechanisms of pathogenicity, persistence, and formation of biofilm L. monocytogenes by tandem analysis of whole-genome sequencing. The lineages that presented L. monocytogenes (LmAV-2, LmAV-3, and LmAV-6) from Hass avocados were lineages I and II. Listeria pathogenicity island 1 (LIPI-1) and LIPI-2 were found in the isolates, while LIPI-3 and Listeria genomic island (LGI-2) only was in IIb. Stress survival island (SSI-1) was identified in lineage I and II. In the in silico analysis, resistance genes belonging to several groups of antibiotics were detected, but the bcrABC and transposon Tn6188 related to resistance to quaternary ammonium salts (QACs) were not detected in L. monocytogenes. Subsequently, the anti-L. monocytogenes planktonic cell effect showed for QACs (MIC = 6.25 ppm/MBC = 100 ppm), lactic acid (MBC = 1 mg/mL), citric acid (MBC = 0.5 mg/mL) and gallic acid (MBC = 2 mg/mL). The anti-biofilm effect with organic acids (22 °C) caused a reduction of 4-5 log10 cfu/cm2 after 10 min against control biofilm L. monocytogenes formed on PP than SS. This study is an important contribution to understanding the genomic diversity and epidemiology of L. monocytogenes to establish a control measure to reduce the impact on the environment and the consumer.

Protein Interaction Network Analysis to Investigate Stress Response, Virulence, and Antibiotic Resistance Mechanisms in Listeria monocytogenes

Listeria monocytogenes is a deadly and costly foodborne pathogen that has a high fatality rate in the elderly, pregnant women, and people with weakened immunity. It can survive under various stress conditions and is a significant concern for the food industry. In this work, a data analysis approach was developed with existing tools and databases and used to create individual and combined protein interaction networks to study stress response, virulence, and antimicrobial resistance and their interaction with L. monocytogenes. The networks were analyzed, and 28 key proteins were identified that may serve as potential targets for new strategies to combat L. monocytogenes. Five of the twenty-eight proteins (i.e., sigB, flaA, cheA, cheY, and lmo0693) represent the most promising targets because they are highly interconnected within the combined network. The results of this study provide a new set of targets for future work to identify new strategies to improve food preservation methods and treatments for L. monocytogenes.

Effect of sublethal concentrations of bactericidal antibiotics on mutation frequency and stress response of Listeria monocytogenes

The purpose of this study was to investigate sublethal concentrations (SLC) of bactericidal antibiotics (ampicillin, gentamicin, kanamycin, and vancomycin) on the mutation frequency and stress response of antibiotic-induced-mutated (AIM) Listeria monocytogenes. Three L. monocytogenes strains (reference, clinical, and food isolate strains) were used in this study. SLC of bactericidal antibiotics significantly increased the mutation frequency in L. monocytogenes. It was found that AIM L. monocytogenes had a superior biofilm-forming ability than nontreated L. monocytogenes. This result correlated with the amounts of EPS produced (polysaccharide and protein) in the early stage of biofilm formation. AIM L. monocytogenes showed strong viability under food-associated stress (thermal, osmotic, and acidic) compared to nontreated L. monocytogenes. In addition, expression levels of motility (flaA) and virulence genes (hlyA, actA, and prfA) of AIM L. monocytogenes were significantly downregulated in the reference strain but significantly upregulated or similar to the expression levels in the clinical and food isolate strains compared to nontreated L. monocytogenes. Based on our results, SLC of bactericidal antibiotics increased the mutation frequency in L. monocytogenes, facilitated the adaptation of the bacterium to food-associated stress, and led to an increase in its pathogenicity.

Effect of Enterocins A and B on the Viability and Virulence Gene Expression of Listeria monocytogenes in Sliced Dry-Cured Ham

Dry-cured ham can be contaminated with Listeria monocytogenes during its industrial processing. The use of bacteriocins could ensure the safety of such meat products, but their effect on pathogen physiology is unknown. Therefore, the impact of enterocins A and B on the L. monocytogenes population, and the expression patterns of five genes (inlA, inlB, clpC, fbpA and prfA) related to adhesion/invasion and virulence regulation have been monitored in sliced dry-cured ham during 30 d of storage in refrigeration (4 °C) and temperature-abuse conditions (20 °C). L. monocytogenes strains S2 (serotype 1/2a) and S7-2 (serotype 4b) counts were reduced by 0.5 and 0.6 log units immediately after the application of enterocins A and B, a decrease lower than previously reported. Differences in gene expression were found between the two strains. For strain S2, expression tended to increase for almost all genes up to day seven of storage, whereas this increase was observed immediately after application for strain S7-2; however, overall gene expression was repressed from day one onwards, mainly under temperature-abuse conditions. L. monocytogenes strains investigated in the present work exhibited a mild sensitivity to enterocins A and B in sliced dry-cured ham. Bacteriocins caused changes in the expression patterns of virulence genes associated with adhesion and invasion, although the potential virulence of surviving cells was not enhanced.

The Effect of Incubation Temperature, Substrate and Initial pH Value on Plantaricin Activity and the Relative Transcription of pln Genes of Six Sourdough Derived Lactiplantibacillus plantarum Strains

The aim of the present study was to assess the effect of sourdough related parameters on the growth and plantaricin activity of six Lactiplantibacillus plantarum strains against a mixture of 5 Listeria monocytogenes strains and to analyze the transcriptomic response of their pln genes. Parameters included 3 substrates (MRS broth, mMRS broth, WFE), 3 temperatures (20, 30, 37 °C), 2 initial pH values (5.0, 6.0), 2 NaCl concentrations (0.0, 1.8%) and 12 time points (ranging from 0 to 33 h). The transcriptomic response of the plantaricin genes to the aforementioned parameters was assessed after 21 h of growth. In general, plantaricin activity was strain dependent with that of Lp. plantarum strains LQC 2422, 2441, 2485 and 2516, harboring four pln genes, namely, pln423 (plαA), plαΒ, plαC and plαD, reaching 2560 AU/mL. However, strains LQC 2320 and 2520, in which 18 pln genes were detected, namely, plNC8a, plNC8b, plNC8c, plnL, plnR, plnJ, plnK, plnE, plnF, plnH, plnS, plnY, plNC8-IF, plNC8-HK, plnD, plnI, plnM and plnG, exhibited plantaricin activity barely reaching 160 AU/mL. Substrate, temperature, initial pH value and strains significantly affected plantaricin activity, while NaCl had only a marginal effect. Similarly, growth substrate and temperature had a more pronounced effect than initial pH value on gene transcription. A strong correlation between the transcription of the genes belonging to the same locus was observed; however, only a weak correlation, if any, was observed between plantaricin activity and the transcription of the genes assessed.

Prevalence and molecular characterization of Listeria monocytogenes isolated from wastewater of cattle slaughterhouses in Turkey

AIM

The study aimed to investigate the role of cattle slaughterhouse wastewater as a possible source for the environmental distribution of Listeria monocytogenes.

METHODS AND RESULTS

Listeria spp. isolation was performed by collecting 117 wastewater samples from four different cattle slaughterhouses in Turkey. Species-specific identification was performed biochemically, and L. monocytogenes isolates were confirmed with polymerase chain reaction (PCR). In all, 71 (62.2%) of the wastewater samples were found to be positive for Listeria spp., and 17 (14.9%) of these samples were contaminated with L. monocytogenes. Pulsed field gel electrophoresis (PFGE) analysis revealed that all L. monocytogenes isolates were of different pulsotypes and isolates belonged to seven different phylogenetic clusters. Multiplex PCR analysis for genoserotypes and lineage determination showed that the isolates were divided into genoserotypes IVb and IIc in Lineages I and II. Also, it has been investigated with SYBR-Green Real-time PCR whether the L. monocytogenes isolates harboured virulence genes (hly, sigB, plcA, plcB, inlA, inlB, inlC and inlJ), and it was found that all isolates were substantially positive. Antibiotic resistance profiles and MIC values of the isolates were determined, and all L. monocytogenes isolates were found susceptible to ampicillin. In contrast, two isolates were resistant to meropenem and erythromycin, and three isolates were resistant to trimethoprim/sulfamethoxazole.

CONCLUSION

L. monocytogenes, which pose a threat to public health and resists to antibiotics effectively used in treatments, can environmentally spread via wastewater of cattle slaughterhouses. The wastewater of the food industry, which has rich microbiota, should be treated carefully, and possible environmental contamination should be prevented.

SIGNIFICANCE AND IMPACT OF STUDY

This is the first study that investigates the molecular characterization of L. monocytogenes isolated from cattle slaughterhouse wastewater and the findings represent the importance of cattle wastewater in the epidemiology of L. monocytogenes in Turkey.

Effect of E-beam treatment on expression of virulence and stress-response genes of Listeria monocytogenes in dry-cured ham

Various adverse conditions can trigger defensive mechanisms in Listeria monocytogenes that can increase the virulence of surviving cells. The objective of this study was to evaluate the expression of one stress-response (sigB) and three virulence (plcA, hly, and iap) genes in L. monocytogenes exposed to a sub lethal dose of E-beam irradiation in dry-cured ham. To accomplish this, dry-cured ham slices (10 g) were immersed in a 10⁹ CFU/mL suspension of L. monocytogenes strain S4-2 and subsequently irradiated with 1, 2, or 3 kGy. After irradiation, samples were stored at 7 °C or 15 °C for 30 days. Absolute gene expression levels were determined by RT-qPCR, and numbers of surviving Listeria cells were assessed by microbial counts after different storage times (0, 7, 15, and 30 days). At 7 °C, after E-beam treatment at doses of 2 or 3 kGy, Listeria gene expression significantly increased (p ≤ 0.05) up to day 15. Listeria counts decreased with increasing dosage. The relationship between absolute gene expression and the number of surviving Listeria cells could indicate that sublethal doses of E-beam irradiation can increase expression of the genes studied. We observed no significant influence of storage time or temperature on gene expression (p > 0.05). Listeria that survives E-beam treatment may display increased virulence, constituting a significant potential public health risk.

Impact of Water Activity on the Inactivation and Gene Expression of Listeria monocytogenes during Refrigerated Storage of Pressurized Dry-Cured Ham

Listeria monocytogenes population and the expression patterns of three virulence (plcA, hly, and iap) and one stress-related (sigB) genes in dry-cured ham with different water activity (aw) values (0.92, 0.88, and 0.84) and treated with high pressure processing (HPP, 450 MPa/10 min and 600 MPa/5 min) were monitored throughout 30 days (d) at 4 °C. The antimicrobial effect of HPP at 600 MPa against L. monocytogenes S4-2 (serotype 1/2b) and S12-1 (serotype 1/2c) was greater in dry-cured ham with aw values of 0.92, with reductions of 2.5 and 2.8 log units, respectively. The efficacy of HPP treatments decreased at lower aw values. Regarding gene expression, L. monocytogenes strains responded differently to HPP. For strain S4-2, the four target genes were generally overexpressed in dry-cured ham immediately after HPP treatments at the three aw values investigated, although the extent of this induction was lower in the samples pressurized at 600 MPa and with aw values of 0.84. For strain S12-1, the expression of all target genes was repressed at the three aw values investigated. The antimicrobial efficacy of HPP against L. monocytogenes could be compromised by low aw values in food products. However, no growth of HPP-survival cells was observed during refrigerated storage in low-aw dry-cured ham, and the overexpression of virulence and stress-related genes decreased.

In Vitro Gene Transcription of Listeria monocytogenes after Exposure to Human Gastric and Duodenal Aspirates

The aim of the present study was to assess, for the first time to our knowledge, Listeria monocytogenes CFU changes, as well as to determine the transcription of key virulence genes, namely, sigB, prfA, hly, plcA, plcB, inlA, inlB, inlC, inlJ, inlP, and lmo2672 after in vitro exposure to human gastric and duodenal aspirates. Furthermore, investigations of the potential correlation between CFU changes and gene regulation with factors influencing gastric (proton pump inhibitor intake and presence of gastric atrophy) and duodenal pH were the secondary study aims. Gastric and duodenal fluids that were collected from 25 individuals undergoing upper gastrointestinal endoscopy were inoculated with L. monocytogenes serotype 4b strain LQC 15257 at 9 log CFU·mL^-1 and incubated at 37°C for 100 min and 2 h, respectively, with the time corresponding to the actual exposure time to gastric and duodenal fluids in the human gastrointestinal tract. Sampling was performed upon gastric fluid inoculation, after incubation of the inoculated gastric fluids, upon pathogen resuspension in duodenal fluids and after incubation of the inoculated duodenal fluids. L. monocytogenes CFU changes were assessed by colony counting, as well as reverse transcription quantitative PCR by using inlB as a target. Gene transcription was assessed by reverse transcription quantitative PCR. In 56% of the cases, reduction of the pathogen CFU occurred immediately after exposure to gastric aspirate. Upregulation of hly and inlC was observed in 52 and 58% of the cases, respectively. On the contrary, no upregulation or downregulation was noticed regarding sigB, prfA, plcA, plcB, inlA, inlB, inlJ, inlP, and lmo2672. In addition, sigB and plcA transcription was positively and negatively associated, respectively, with an increase of the pH value, and inlA transcription was negatively associated with the presence of gastric atrophy. Finally, a positive correlation between the transcriptomic responses of plcB, inlA, inlB, inlC, inlJ, inlP, and lmo2672 was detected. This study revealed that the CFU of the pathogen was negatively affected after exposure to human gastroduodenal aspirates, as well as significant correlations between the characteristics of the aspirates with the virulence potential of the pathogen.

Dual Transcriptional Profile of Aspergillus flavus during Co-Culture with Listeria monocytogenes and Aflatoxin B1 Production: A Pathogen-Pathogen Interaction

The objective of this study was to investigate the effect of growth temperature and co-culture of Aspergillus flavus with Listeria monocytogenes on the production of Aflatoxin B1 (AFB1) and the transcriptional profile of associated regulatory and biosynthetic genes. The transcription of virulence- and homeostasis-associated genes of L. monocytogenes was also assessed. For this purpose, mono- and co-cultures of L. monocytogenes strain LQC 15257 and A. flavus strain 18.4 were inoculated into Malt Extract broth and allowed to grow for seven days at 25 °C and 30 °C. AFB1 quantification was performed by HPLC analysis and gene expression assessment by RT-qPCR. AFB1 production was lower at 30 °C compared to 25 °C during monoculture and also lower during co-cultures at both temperatures. This was accompanied by downregulation of aflM, aflR, aflP, and aflS during monoculture and aflM and aflS during co-culture at 30 °C. On the other hand, transcription of prfA, plcA, plcB, inlA, inlB, inlJ, murE, accA, acpP, as well as fapR, was not affected. sigB gene was downregulated after co-culture with the fungus at 25 °C and hly was downregulated after monoculture at 30 °C compared to 25 °C. In this work, the molecular interactions between A. flavus and L. monocytogenes were studied for the first time, offering a novel insight into their co-occurrence. Monitoring of their toxigenic and virulence potential at the molecular level revealed a complex dynamic in natural ecosystems.

Evaluation of Plantaricin Genes Expression During Fermentation of Raphanus sativus Roots with a Plantaricin-Producing Lactobacillus plantarum Starter

The aim of the present study was to assess the transcription of the plnE/F, plnN, plnG, plnD and plnI genes during lactic acid fermentation of radish (Raphanus sativus) roots by Lactobacillus plantarum strain LQC 740 at 20 and 30 °C. At both temperatures, this strain dominated the fermentation process, as indicated by (GTG)₅ analysis. A total of five pln genes were detected in the genome of this strain, namely plnE/F, plnN, plnG, plnD and plnI. Regarding plantaricin genes expression, no regulation was observed in the majority of the samples at both temperatures, therefore, the transcription of the pln genes was not affected by the experimental conditions, i.e. radish fermentation vs. growth in MRS broth. Although transcription of the pln genes was similar between the two conditions, bacteriocin activity was different. The maximum plantaricin activity was 87.5 AU/mL during radish fermentation and 700 AU/mL during growth in MRS broth. Thus, no apparent correlation between bacteriocin activity and transcription level of the five pln genes could be established.

Role and regulation of the stress activated sigma factor sigma B (σB) in the saprophytic and host-associated life stages of Listeria monocytogenes

The stress activated sigma factor sigma B (σ^B) plays a pivotal role in allowing the food-borne bacterial pathogen Listeria monocytogenes to modulate its transcriptional landscape in order to survive in a variety of harsh environments both outside and within the host. While we have a comparatively good understanding of the systems under the control of this sigma factor much less is known about how the activity of σ^B is controlled. In this review, we present a current model describing how this sigma factor is thought to be controlled including an overview of what is known about stress sensing and the early signal transduction events that trigger its activation. We discuss the known regulatory overlaps between σ^B and other protein and RNA regulators in the cell. Finally, we describe the role of σ^B in surviving both saprophytic and host-associated stresses. The complexity of the regulation of this sigma factor reflects the significant role that it plays in the persistence of this important pathogen in the natural environment, the food chain as well as within the host during the early stages of an infection. Understanding its regulation will be a critical step in helping to develop rational strategies to prevent its growth and survival in the food destined for human consumption and in the prevention of listeriosis.

Use of Two-Dimensional Electrophoresis to Explore Foodborne Bacterial Pathogen Responses to Gastrointestinal Stress

Proteomics was applied here to study Listeria monocytogenes response to gastrointestinal stress. It separated extracted proteins by their isoelectric point (pI) in the first dimension followed by separation by molecular weight in the second dimension on a polyacrylamide gel. L. monocytogenes was grown in an appropriate culture medium after which it was transferred to a simulated cheese medium for 2 h. Bacteria were exposed to gastric stress using artificial saliva and gastric fluid for 5 min and 2 h, respectively. After each step samples were taken for protein extraction and a two-dimensional electrophoresis approach. Proteins were separated on 18 cm Immobiline DryStrip gels with a pH range of 4-7 and the protein pattern analyzed.

Global transcriptomic response of Listeria monocytogenes during growth on cantaloupe slices

Understanding a pathogen's response to food environments is imperative to develop effective control strategies as well as to elucidate the impact of foods on virulence potential. The purpose of this study was to assess transcriptional response of Listeria monocytogenes after growth in cantaloupe, as well as its impact on survival in synthetic gastric fluid (SGF). The transcriptional profiles of L. monocytogenes grown in cantaloupe or Brain Heart Infusion (BHI) under refrigeration were compared by a custom-designed microarray. A total of 286 and 175 genes were significantly up- and down-regulated, respectively, in L. monocytogenes grown in cantaloupe as compared to BHI (fold change ≥ 2.5 and adj. P < 0.05). The majority of upregulated genes belonged to functions related to amino acid and nucleotide metabolism, flagellar biosynthesis, and iron acquisition, while most downregulated genes belonged to carbohydrate metabolism. Notably, the branched chain amino acid (BCAA: leucine, isoleucine, valine) biosynthesis operon was shown to be highly upregulated as well as the purine and pyrimidine biosynthesis pathways. Transcript levels of several stress- and virulence-related genes were significantly altered, implying an impact of growth in cantaloupe on the virulence potential of L. monocytogenes. Enhanced survival of L. monocytogenes in SGF following growth in cantaloupe further demonstrated the impact of cantaloupe-associated growth on the pathogen's subsequent response to a host relevant stress.

Genetic Analysis of the Listeria Pathogenicity Island 1 of Listeria monocytogenes 1/2a and 4b Isolates

The aim of the present study was to apply descriptive, phylogenetic, recombination, and selection analyses on alignments of the Listeria Pathogenicity Island 1 (LIPI-1) of 1/2a and 4b Listeria monocytogenes isolates of different origin in order to gain insights into the evolution of this virulence gene cluster. For that purpose, a total of 19 L. monocytogenes isolates (9 meat isolates, serotype 1/2a; 5 meat isolates, serotype 4b; 5 strawberry isolates, serotype 4b) that have been previously separated at strain level were subjected to sequencing of their LIPI-1. Descriptive analysis revealed extensive nucleotide diversity mostly in the intragenic regions. The actA gene of 1/2a and 4b meat isolates and the hly gene of the 4b strawberry isolates exhibited the higher diversity; limited diversity was observed in prfA and plcA genes of the 4b isolates and mpl gene of the 1/2a isolates. Phylogenetic analysis of the complete island resulted in two major clusters that were consistent with serotype assignment of the isolates. Moreover, effective discrimination between serotypes was obtained by plcA, plcB, mpl, actA and the intergenic regions plcA-prfA and plcA-hly. In all cases but plcB and plcA-prfA 4b isolates were also differentiated according to their source of isolation as well. Selection analysis revealed that the island consisted of randomly evolving DNA with the exception of prfA gene of 1/2a isolates and actA gene of 4b meat isolates for which purifying selection or population expansion was indicated. Finally, no statistically significant evidence for recombination has been observed.

Listeria monocytogenes contamination of ready-to-eat foods and the risk for human health in the EU

Food safety criteria for Listeria monocytogenes in ready-to-eat (RTE) foods have been applied from 2006 onwards (Commission Regulation (EC) 2073/2005). Still, human invasive listeriosis was reported to increase over the period 2009-2013 in the European Union and European Economic Area (EU/EEA). Time series analysis for the 2008-2015 period in the EU/EEA indicated an increasing trend of the monthly notified incidence rate of confirmed human invasive listeriosis of the over 75 age groups and female age group between 25 and 44 years old (probably related to pregnancies). A conceptual model was used to identify factors in the food chain as potential drivers for L. monocytogenes contamination of RTE foods and listeriosis. Factors were related to the host (i. population size of the elderly and/or susceptible people; ii. underlying condition rate), the food (iii. L. monocytogenes prevalence in RTE food at retail; iv. L. monocytogenes concentration in RTE food at retail; v. storage conditions after retail; vi. consumption), the national surveillance systems (vii. improved surveillance), and/or the bacterium (viii. virulence). Factors considered likely to be responsible for the increasing trend in cases are the increased population size of the elderly and susceptible population except for the 25-44 female age group. For the increased incidence rates and cases, the likely factor is the increased proportion of susceptible persons in the age groups over 45 years old for both genders. Quantitative modelling suggests that more than 90% of invasive listeriosis is caused by ingestion of RTE food containing > 2,000 colony forming units (CFU)/g, and that one-third of cases are due to growth in the consumer phase. Awareness should be increased among stakeholders, especially in relation to susceptible risk groups. Innovative methodologies including whole genome sequencing (WGS) for strain identification and monitoring of trends are recommended.

The Role of Stress and Stress Adaptations in Determining the Fate of the Bacterial Pathogen Listeria monocytogenes in the Food Chain

The foodborne pathogen Listeria monocytogenes is a highly adaptable organism that can persist in a wide range of environmental and food-related niches. The consumption of contaminated ready-to-eat foods can cause infections, termed listeriosis, in vulnerable humans, particularly those with weakened immune systems. Although these infections are comparatively rare they are associated with high mortality rates and therefore this pathogen has a significant impact on food safety. L. monocytogenes can adapt to and survive a wide range of stress conditions including low pH, low water activity, and low temperature, which makes it problematic for food producers who rely on these stresses for preservation. Stress tolerance in L. monocytogenes can be explained partially by the presence of the general stress response (GSR), a transcriptional response under the control of the alternative sigma factor sigma B (σB) that reconfigures gene transcription to provide homeostatic and protective functions to cope with the stress. Within the host σB also plays a key role in surviving the harsh conditions found in the gastrointestinal tract. As the infection progresses beyond the GI tract L. monocytogenes uses an intracellular infectious cycle to propagate, spread and remain protected from the host's humoral immunity. Many of the virulence genes that facilitate this infectious cycle are under the control of a master transcriptional regulator called PrfA. In this review we consider the environmental reservoirs that enable L. monocytogenes to gain access to the food chain and discuss the stresses that the pathogen must overcome to survive and grow in these environments. The overlap that exists between stress tolerance and virulence is described. We review the principal measures that are used to control the pathogen and point to exciting new approaches that might provide improved means of control in the future.