Expression of Listeria monocytogenes key virulence genes during growth in liquid medium, on rocket and melon at 4, 10 and 30 °C

Investigating the Effect of Melittin Peptide in Preventing Biofilm Formation, Adhesion and Expression of Virulence Genes in Listeria monocytogenes

Listeria monocytogenes is a notable food-borne pathogen that has the ability to create biofilms on different food processing surfaces, making it more resilient to disinfectants and posing a greater risk to human health. This study assessed melittin peptide's anti-biofilm and anti-pathogenicity effects on L. monocytogenes ATCC 19115. Melittin showed minimum inhibitory concenteration (MIC) of 100 μg/mL against this strain and scanning electron microscopy images confirmed its antimicrobial efficacy. The OD measurement demonstrated that melittin exhibited a strong proficiency in inhibiting biofilms and disrupting pre-formed biofilms at concentrations ranging from 1/8MIC to 2MIC and this amount was 92.59 ± 1.01% to 7.17 ± 0.31% and 100% to 11.50 ± 0.53%, respectively. Peptide also reduced hydrophobicity and self-aggregation of L. monocytogenes by 35.25% and 14.38% at MIC. Melittin also significantly reduced adhesion to HT-29 and Caco-2 cells by 61.33% and 59%, and inhibited invasion of HT-29 and Caco-2 cells by 49.33% and 40.66% for L. monocytogenes at the MIC value. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) revealed melittin's impact on gene expression, notably decreasing inlB (44%) and agrA (45%) gene expression in L. monocytogenes. flaA and hly genes also exhibited reduced expression. Also, significant changes were observed in sigB and prfA gene expression. These results underscore melittin's potential in combating bacterial infections and biofilm-related challenges in the food industry.

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.

Comparison of genetic variations between high- and low-risk Listeria monocytogenes isolates using whole-genome de novo sequencing

In this study, genetic variations and characteristics of Listeria monocytogenes isolates from enoki mushrooms (23), smoked ducks (7), and processed ground meat products (30) were examined with respect to hemolysis, virulence genes, growth patterns, and heat resistance. The isolates that showed the highest pathogenicity and the lowest pathogenicity were analyzed to obtain the whole-genome sequence, and the sequences were further analyzed to identify genetic variations in virulence, low-temperature growth-related, and heat resistance-related factors. All isolates had β-hemolysis and virulence genes (actA, hlyA, inlA, inlB, and plcB). At low temperatures, isolates with high growth (L. monocytogenes strains SMFM 201803 SD 1-1, SMFM 201803 SD 4-2, and SMFM 201804 SD 5-3) and low growth (L. monocytogenes strains SMFM 2019-FV43, SMFM 2019-FV42, and SMFM 2020-BT30) were selected. Among them, L. monocytogenes SMFM 201804 SD 5-3 showed the highest resistance at 60°C and 70°C. The strains SMFM 201804 SD 5-3 (high-risk) and SMFM 2019-FV43 (low-risk) harbored 45 virulence genes; 41 single nucleotide variants (SNVs) were identified between these two isolates. A comparison of 26 genes related to low-temperature growth revealed 18 SNVs between these two isolates; a comparison of the 21 genes related to heat resistance revealed 16 SNVs. These results indicate that the differences in the pathogenicity of L. monocytogenes SMFM 201804 SD 5-3 and L. monocytogenes SMFM 2019-FV43 are associated with the SNVs identified in virulence genes, low-temperature growth-related genes, and heat resistance-related genes.

Influence of temperature on regulation of key virulence and stress response genes in Listeria monocytogenes biofilms

Temperature is a major determinant of Listeria (L.) monocytogenes adherence and biofilm formation on abiotic surfaces. However, its role on gene regulation of L. monocytogenes mature biofilms has not been investigated. In the present study, we aimed to evaluate the impact of temperature up- and down-shift on L. monocytogenes biofilms gene transcription. L. monocytogenes strain EGD-e biofilms were first developed on stainless steel surfaces in Brain Heart Infusion broth at 20 °C for 48 h. Then, nutrient broth was renewed, and mature biofilms were exposed to 10 °C, 20 °C or 37 °C for 24 h. Biofilm cells were harvested and RNA levels of plcA, prfA, hly, mpl, plcB, sigB, bapL, fbpA, fbpB, lmo2178, lmo0880, lmo0160, lmo1115, lmo 2089, lmo2576, lmo0159 and lmo0627 were evaluated by quantitative RT-PCR. The results revealed an over-expression of all genes tested in biofilm cells compared to planktonic cells. When biofilms were further allowed to proliferate at 20 °C for 24 h, the transcription levels of key virulence, stress response and putative binding proteins genes plcA, sigB, fbpA, fbpB, lmo1115, lmo0880 and lmo2089 decreased. A temperature-dependent transcription for sigB, plcA, hly, and lmo2089 genes was observed after biofilm proliferation at 10 °C or 37 °C. Our findings suggest that temperature differentially affects gene regulation of L. monocytogenes mature biofilms, thus modulating attributes such as virulence, stress response and pathogenesis.

Antimicrobial Activity of Ohelo Berry (Vaccinium calycinum) Juice against Listeria monocytogenes and Its Potential for Milk Preservation

Listeria monocytogenes is a foodborne pathogen and causes illnesses with a high mortality rate in susceptible populations. Several dairy-related outbreaks have been attributed to contamination by L. monocytogenes, which requires antimicrobial interventions to enhance the safety of these products. This study aimed to determine the antimicrobial activity of the ohelo berry (Vaccinium calycinum), a Hawaiian wild relative of cranberry, against L. monocytogenes in culture media and milk products. The effect of ohelo berry juice at its sub-inhibitory concentrations on the physicochemical properties, biofilm formation, and gene expression of L. monocytogenes was also investigated. The minimum inhibitory concentration of ohelo berry juice against L. monocytogenes was 12.5%. The sub-inhibitory concentration of ohelo berry juice (6.25%) significantly increased the auto-aggregation and decreased the hydrophobicity, swimming motility, swarming motility, and biofilm formation capability of L. monocytogenes. The relative expression of genes for motility (flaA), biofilm formation and disinfectant resistance (sigB), invasion (iap), listeriolysin (hly), and phospholipase (plcA) was significantly downregulated in L. monocytogenes treated by the 6.25% juice. L. monocytogenes was significantly inhibited in whole and skim milk supplemented with 50% ohelo berry juice, regardless of the fat content. These findings highlight the potential of ohelo berry as a natural preservative and functional food to prevent L. monocytogenes infection.

Control of Listeria monocytogenes growth and virulence in a traditional soft cheese model system based on lactic acid bacteria and a whey protein hydrolysate with antimicrobial activity

"Torta del Casar" is a Spanish soft-ripened cheese made with sheep's raw milk and subjected to a short ripening process, which favors the growth of pathogenic microorganisms including Listeria monocytogenes. The development of strategies to control pathogens and minimize health risks associated with the presence of L. monocytogenes in these products is of great interest. In this regard, the anti-Listeria activity of a whey protein hydrolysate (ProH) alone or combined with six lactic acid bacteria strains isolated from cheese was evaluated in this study as a biocontrol strategy using a "Torta del Casar" cheese-based medium. The most active combinations of lactic acid bacteria assayed induced a reduction higher than two logarithmic units in the growth of L. monocytogenes (serotype 4b) compared to their respective control when they were co-inoculated in "Torta del Casar" cheese-based medium at 7 °C for 7 days. In addition, the observed downregulation of some key virulence genes of L. monocytogenes suggests that the strain Lactiplantibacillus plantarum B2 alone and combined with the strain Lactiplantibacillus spp. B4 are good candidates to be used as biocontrol agents against L. monocytogenes growth in traditional soft cheeses based on raw milk during their storage at refrigeration temperatures.

Lactiplantibacillus plantarum subsp. plantarum and Fructooligosaccharides Combination Inhibits the Growth, Adhesion, Invasion, and Virulence of Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen responsible for many food outbreaks worldwide. This study aimed to investigate the single and combined effect of fructooligosaccharides (FOS) and Lactiplantibacillus plantarum subsp. plantarum CICC 6257 (L. plantarum) on the growth, adhesion, invasion, and virulence of gene expressions of Listeria monocytogenes 19112 serotype 4b (L. monocytogenes). Results showed that L. plantarum combined with 2% and 4% (w/v) FOS significantly (p < 0.05) inhibited the growth of L. monocytogenes (3-3.5 log10 CFU/mL reduction) at the incubation temperature of 10 °C and 25 °C. Under the same combination condition, the invasion rates of L. monocytogenes to Caco-2 and BeWo cells were reduced more than 90% compared to the result of the untreated group. After L. plantarum was combined with the 2% and 4% (w/v) FOS treatment, the gene expression of actin-based motility, sigma factor, internalin A, internalin B, positive regulatory factor A, and listeriolysin O significantly (p < 0.05) were reduced over 91%, 77%, 92%, 89%, 79%, and 79% compared to the result of the untreated group, respectively. The inhibition level of the L. plantarum and FOS combination against L. monocytogenes was higher than that of FOS or L. plantarum alone. Overall, these results indicated that the L. plantarum and FOS combination might be an effective formula against 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.

Growth and Expression of Virulence Genes of Listeria monocytogenes during the Processing of Dry-Cured Fermented “Salchichón” Manufactured with a Selected Lactilactobacillus sakei

Simple Summary

During the ripening process of the dry-cured fermented sausage “salchichón”, Listeria monocytogenes could fail to be eliminated. In addition, the food safety criterion for L. monocytogenes in the European Union sets up a maximum level of 100 units of this microorganism per gram in ready-to-eat products throughout their shelf-life. Thus, since L. monocytogenes could be present in this product, it is necessary to evaluate the impact of the dry-cured fermented processing in the potential virulence of this pathogen, even considering the possible effect of the usual microbiota (lactic-acid bacteria) of “salchichón”. In this work, the effect of the processing of “salchichón”, inoculated with a selected strain of Lactilactobacillus sakei, on the growth of L. monocytogenes and on the expression of its virulence genes, was evaluated. The processing of “salchichón” provoked a relevant reduction in L. monocytogenes, but this pathogen was not completely eliminated. However, a downregulation in the expression of the tested virulence genes was found, which could suppose a reduction in the pathogenic effect of this microorganism. These findings could be of great interest to consider the dry-cured ripening of “salchichón” as a safe process to control the pathogen L. monocytogenes.

Abstract

The effect of the dry-cured fermented processing of “salchichón” inoculated with a selected strain of Lactilactobacillus sakei (205) on the growth and transcriptional response of three virulence genes (plcA, hly, and iap) of Listeria monocytogenes was evaluated. For this, three different batches of “salchichón” were analyzed: batch B (inoculated only with L. sakei), batch L (inoculated only with L. monocytogenes), and batch L + B (inoculated with both microorganisms). Sausages were ripened for 90 days according to a traditional industrial process. The processing of “salchichón” provoked a reduction in L. monocytogenes counts of around 2 log CFU/g. The downregulation of the expression of the three genes was found at the end of ripening when the water activity (a_w) of “salchichón” was <0.85 a_w. The combined effect on the reduction in L. monocytogenes counts together with the downregulation in the expression of the virulence genes throughout the “salchichón” processing could be of great interest to control the hazard caused by the presence of this pathogenic bacterium.

Investigating Transcriptomic Induction of Resistance and/or Virulence in Listeria monocytogenes Cells Surviving Sublethal Antimicrobial Exposure

The potential transcriptomic induction of resistance and/or virulence in two L. monocytogenes strains belonging to the most frequent listeriosis-associated serovars (i.e., 1/2a and 4b), following their sublethal antimicrobial exposure, was studied through qPCR determination of the relative expression of 10 selected related genes (i.e., groEL, hly, iap, inlA, inlB, lisK, mdrD, mdrL, prfA, and sigB). To induce sublethal stress, three common antimicrobials (i.e., benzalkonium chloride, thymol, and ampicillin) were individually applied for 2 h at 37 °C against stationary phase cells of each strain, each at a sublethal concentration. In general, the expression of most of the studied genes remained either stable or was significantly downregulated following the antimicrobial exposure, with some strain-specific differences to be yet recorded. Thymol provoked downregulation of most of the studied genes, significantly limiting the expression of 6/10 and 4/10 genes in the strains of ser. 1/2a and ser. 4b, respectively, including those coding for the master regulators of stress response and virulence (SigB and PrfA, respectively), in both strains. At the same time, the two genes coding for the invasion internalin proteins (InlA and InlB), with crucial role in the onset of L. monocytogenes pathogenesis, were both importantly upregulated in ser. 4b strain. The results obtained increase our knowledge of the stress physiology of L. monocytogenes under certain sublethal antimicrobial conditions that could be encountered within the food chain and in clinical settings, and may assist in better and more effective mitigation strategies.

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.

Characterisation of Listeria monocytogenes food-associated isolates to assess environmental fitness and virulence potential

The ability of Listeria monocytogenes isolates to survive within the food production environment (FPE), as well as virulence, varies greatly between strains. There are specific genetic determinants that have been identified which can strongly influence a strains ability to survive in the FPE and/or within human hosts. In this study, we assessed the FPE fitness and virulence potential, including efficacy of selected hygiene or treatment intervention, against 52 L. monocytogenes strains isolated from various food and food environment sources. Phenotypic tests were performed to determine the minimum inhibitory concentration of cadmium chloride and benzalkonium chloride and the sensitivities to five clinically relevant antibiotics. A genomic analysis was also performed to identify resistance genes correlating to the observed phenotypic resistance profiles, along with genetic determinants of interest which may elude to the FPE fitness and virulence potential. A transposon element containing a novel cadmium resistance gene, cadA7, a Tn916 variant insert in the hypervariable Listeria genomic island 1 region and an LGI2 variant were identified. Resistance to cadmium and disinfectants was prevalent among isolates in this study, although no resistance to clinically important antimicrobials was observed. Potential hypervirulent strains containing full length inlA, LIPI-1 and LIPI-3 were also identified in this study. Cumulatively, the results of this study show a vast array of FPE survival and pathogenicity potential among food production-associated isolates, which may be of concern for food processing operators and clinicians regarding L. monocytogenes strains colonising and persisting within the FPE, and subsequently contaminating food products then causing disease in at risk population groups.

Effect of Zataria multiflora Boiss. essential oil, NaCl, acid, time, and temperature on the growth of Listeria monocytogenes strains in broth and minced rainbow trout

The small outbreaks of listeriosis as one of the leading causes of food poisoning-associated deaths occur more than previously reported. In current study, the growth ability of Listeria monocytogenes strains isolated from different sources of food and human origin was measured under salt stress (0.5%, 2.5%, 5%, 7.5%, and 10%) and acid environments (pH = 6.64 and 5.77) for 96 hr by using a Bioscreen C microbiology reader at 37°C. In further steps of this study, after analysis of constituents of Zataria multiflora Boiss. essential oil (ZMEO), the sensory evaluation of the treated fish meat with ZMEO was performed. Then, the fish isolate of L. monocytogenes was exposed to sensory acceptable and subminimum inhibitory concentrations (subMICs) of ZMEO in fish broth and minced fish meat during incubation at abuse (12°C), room (22°C), and optimum (37°C) temperatures for 48 hr. The MIC of NaCl against four strains of L. monocytogenes was 10% at 37°C. The maximum optical densities (ODs) and under curve areas (AUC) of growth patterns in higher pH value and lower contents of NaCl followed the order of 21C > 6F > 66C > 22C of L. monocytogenes strains, while the lag time was prolonged in the reverse order. The maximum OD, growth, and lag times of samples treated with higher contents of NaCl and lower pH value were affected in a different order. The organoleptic evaluation showed that the fish meat treated with less than 0.5% of ZMEO was sensory acceptable. The population of L. monocytogenes remained relatively constant at the inoculation level of 107 cfu/ml (or g) at 12°C in broth and minced fish mediums. The inhibitory antilisterial activity of essential oil as an extensive-used plant for food and pharmacological applications is negligible due to possible adverse sensory and toxic effects at relevant high doses.

Prior exposure to different combinations of pH and undissociated acetic acid can affect the induced resistance of Salmonella spp. strains in mayonnaise stored under refrigeration and the regulation of acid-resistance related genes

The innate and inducible resistance of six Salmonella strains (4/74, FS8, FS115, P167807, ATCC 13076, WT) in mayonnaise at 5 °C following adaptation to different pH/undissociated acetic acid (UAA) combinations (15mM/pH5.0, 35mM/pH5.5, 45mM/pH6.0) was investigated. The inherent and acid-induced responses were strain-dependent. Two strains (ATCC 13076, WT), albeit not the most resistant innately, exhibited the most prominent adaptive potential. Limited/no adaptability was observed regarding the rest strains, though being more resistant inherently. The individual effect of pH and UAA adaptation in the phenotypic and transcriptomic profiles of ATCC 13076 and WT was further examined. The type (pH, UAA) and magnitude of stress intensity affected their responses. Variations in the type and magnitude of stress intensity also determined the relative gene expression of four genes (adiA, cadB, rpoS, ompR) implicated in Salmonella acid resistance mechanisms. adiA and cadB were overexpressed following adaptation to some treatments; rpoS and ompR were downregulated following adaptation to 15mM/pH5.0 and 35mM/pH5.5, respectively. Nonetheless, the transcriptomic profiles did not always correlate with the corresponding phenotypes. In conclusion, strain variations in Salmonella are extensive. The ability of the strains to adapt and induce resistant phenotypes and acid resistance-related genes is affected by the type and magnitude of the stress applied during adaptation.

Differential Modulation of Listeria monocytogenes Fitness, In Vitro Virulence, and Transcription of Virulence-Associated Genes in Response to the Presence of Different Microorganisms

Interactions between Listeria monocytogenes and food-associated or environmental bacteria are critical not only for the growth but also for a number of key biological processes of the microorganism. In this regard, limited information exists on the impact of other microorganisms on the virulence of L. monocytogenes In this study, the growth of L. monocytogenes was evaluated in a single culture or in coculture with L. innocua, Bacillus subtilis, Lactobacillus plantarum, or Pseudomonas aeruginosa in tryptic soy broth (10°C/10 days and 37°C/24 h). Transcriptional levels of 9 key virulence genes (inlA, inlB, inlC, inlJ, sigB, prfA, hly, plcA, and plcB) and invasion efficiency and intracellular growth in Caco-2 cells were determined for L. monocytogenes following growth in mono- or coculture for 3 days at 10°C or 9 h at 37°C. The growth of L. monocytogenes was negatively affected by the presence of L. innocua and B. subtilis, while the effect of cell-to-cell contact on L. monocytogenes growth was dependent on the competing microorganism. Cocultivation affected the in vitro virulence properties of L. monocytogenes in a microorganism-specific manner, with L. innocua mainly enhancing and B. subtilis reducing the invasion of the pathogen in Caco-2 cells. Assessment of the mRNA levels of L. monocytogenes virulence genes in the presence of the four tested bacteria revealed a complex pattern in which the observed up- or downregulation was only partially correlated with growth or in vitro virulence and mainly suggested that L. monocytogenes may display a microorganism-specific transcriptional response.IMPORTANCE Listeria monocytogenes is the etiological agent of the severe foodborne disease listeriosis. Important insight regarding the physiology and the infection biology of this microorganism has been acquired in the past 20 years. However, despite the fact that L. monocytogenes coexists with various microorganisms throughout its life cycle and during transmission from the environment to foods and then to the host, there is still limited knowledge related to the impact of surrounding microorganisms on L. monocytogenes' biological functions. In this study, we showed that L. monocytogenes modulates specific biological activities (i.e., growth and virulence potential) as a response to coexisting microorganisms and differentially alters the expression of virulence-associated genes when confronted with different bacterial genera and species. Our work suggests that the interaction with different bacteria plays a key role in the survival strategies of L. monocytogenes and supports the need to incorporate biotic factors into the research conducted to identify mechanisms deployed by this organism for establishment in different environments.

Indole signaling decreases biofilm formation and related virulence of Listeria monocytogenes

Bacterial communication system known as quorum sensing (QS) is a pivotal system for bacterial survival, adaptation and pathogenesis. Members in the multicellular community may synthesize or acquire a signaling molecule in order to elicit downstream cellular processes. Roles of indole and derivatives, a new class of quorum-sensing signal molecules, in various bacterial physiologies and virulence have been reported recently. Indole is normally found in mammal gastrointestinal tract as a metabolite of tryptophan metabolism by microbiota. Therefore, interspecies connection via indole signaling among commensal bacteria and enteric pathogens could be anticipated. Effects of indole exposure on the virulence of Listeria monocytogenes were investigated by phenotypic and molecular approaches. Results demonstrated that synthetic indole and indole-rich conditioned medium significantly diminished biofilm formation and related virulence of L. monocytogenes including motility, cell aggregation and exopolysaccharide production. Transcript levels of virulence-associated (pssE, dltA, flaA, fliI, motB, agrA and hly) and regulatory genes (codY, sigB, prfA and gmaR) were substantially downregulated in indole-treated cells. Only mogR gene encoding for a repressor of motility genes was upregulated after indole exposure. Our findings raise the possibility that L. monocytogenes may acquire indole signaling from gut microbiota for resource-effective adaptation upon transition to new environment.

Virulence assessment of Listeria monocytogenes grown in different foods using a Galleria mellonella model

Various produce including cantaloupe, caramel-coated apples, and packaged salads, have been recognized in recent years as vehicles for listeriosis, a human foodborne disease caused by intracellular pathogen Listeria monocytogenes. Our knowledge regarding the role of these foods in L. monocytogenes virulence, however, is limited. Understanding their role in modulating L. monocytogenes virulence can be useful in risk assessments and for developing control measures. In this study, we employed the Galleria mellonella larvae model to evaluate virulence potential of fifteen clinical, environmental and food isolates of L. monocytogenes, related to three major outbreaks, after growth on different foods. The non-human pathogen Listeria innocua was also included in the panel. Strains were inoculated in parallel in 5ml of brain heart infusion (BHI) broth, and on the surfaces of cantaloupe and apple fragments (5g each) at about 10⁵ colony forming units (CFU)/ml/fragment. One set of inoculated broth and food fragments was incubated at 10°C for 5 days while the second set was kept at 25°C for 3 days. L. monocytogenes cells were recovered from the fruits and BHI, washed twice, re-suspended in saline, and used to inoculate G. mellonella larvae at final concentrations of 10⁶ and 10⁵ CFU/larva. The larvae were incubated at 37°C and monitored for mortality (LT₅₀—time taken to kill 50% of the larvae) and phenotypic changes over seven days. L. monocytogenes grown on cantaloupe and apple flesh surfaces resulted in higher virulence than when grown in BHI. L. monocytogenes infection at 10⁶ CFU/larvae resulted in an average LT₅₀ of ≤ 30, 36 and 47 hours on cantaloupe, apples and BHI, respectively. These results represent a 2.5–4-fold increased mortality compared with an LT₅₀ ≥120 hours in larvae infected with the same doses of L. innocua grown in corresponding matrices. Similar trends were also recorded with doses of about 10⁵ CFU /larvae.

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.

Quercetin repressed the stress response factor (sigB) and virulence genes (prfA, actA, inlA, and inlC), lower the adhesion, and biofilm development of L. monocytogenes

The present study explored the effect of quercetin on the expression of virulence genes actA, inlA, inlC, and their regulatory components, sigB and prfA, in L. monocytogenes. Furthermore, the physicochemical changes on the surface, membrane permeability, and biofilm formation of quercetin-treated bacteria were evaluated. An inhibitory dose-dependent effect of quercetin (0.1-0.8 mM) was observed on the cell attachment on stainless steel at 2 and 6 h at 37 °C. Quercetin at 0.8 mM prevented the biofilm formation on stainless steel surfaces after 6 h of incubation at 37 °C, while the untreated bacteria formed biofilms with a cell density of 5.1 Log CFU/cm². The microscopic analysis evidenced that quercetin at 0.2 mM decreased the biovolume and covered area of the attached micro-colonies. Also, sigB, prfA, inlA, inlC, and actA genes were downregulated by 7-29 times lower compared to untreated bacteria. In addition, quercetin decreased the superficial cell charge, increased the membrane permeability, and its surface hydrophobicity. These results demonstrated that quercetin prevented biofilm formation, repressed the genes of stress and virulence of L. monocytogenes and also altered the physicochemical cell properties.

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.

Presence of genes associated with adhesion, invasion, and toxin production in Campylobacter jejuni isolates and effect of temperature on their expression

The aims of this study were to evaluate the presence of genes associated with adhesion (cadF), invasion (ciaB), and cytotoxin production (cdtA, cdtB, and cdtC) among Campylobacter jejuni isolates from a poultry slaughterhouse and to investigate the effect of different temperatures on the expression of these virulence-associated genes. A total of 88 C. jejuni isolates from cecum, liver, chicken carcasses, chilled water, and scalding water were submitted to PCR assay for detection of virulence genes. Representative isolates were selected for gene expression evaluation at 37 and 42 °C, according to their virulence gene profile and genotypic typing. All C. jejuni isolates carried the five virulence-associated genes, which play an important role in the infectious process. Differential gene expression by RT-qPCR was observed among C. jejuni isolates at 37 and 42 °C. The expression levels at 37 °C showed upregulation of the ciaB, cdtA, cdtB, and cdtC genes in five isolates, with the exception of ciaB for isolate 4. At 42 °C, upregulation was observed for ciaB and cdtC, cdtA and cdtB, and cadF in four, three, and two isolates, respectively. The C. jejuni isolates expressed the virulence genes evaluated, and the expression is gene- and isolate-dependent and varied according the temperature.

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.

Campylobacteriosis, Salmonellosis, Yersiniosis, and Listeriosis as Zoonotic Foodborne Diseases: A Review

Zoonoses are diseases transmitted from animals to humans, posing a great threat to the health and life of people all over the world. According to WHO estimations, 600 million cases of diseases caused by contaminated food were noted in 2010, including almost 350 million caused by pathogenic bacteria. Campylobacter, Salmonella, as well as Yersinia enterocolitica and Listeria monocytogenes may dwell in livestock (poultry, cattle, and swine) but are also found in wild animals, pets, fish, and rodents. Animals, often being asymptomatic carriers of pathogens, excrete them with faeces, thus delivering them to the environment. Therefore, pathogens may invade new individuals, as well as reside on vegetables and fruits. Pathogenic bacteria also penetrate food production areas and may remain there in the form of a biofilm covering the surfaces of machines and equipment. A common occurrence of microbes in food products, as well as their improper or careless processing, leads to common poisonings. Symptoms of foodborne infections may be mild, sometimes flu-like, but they also may be accompanied by severe complications, some even fatal. The aim of the paper is to summarize and provide information on campylobacteriosis, salmonellosis, yersiniosis, and listeriosis and the aetiological factors of those diseases, along with the general characteristics of pathogens, virulence factors, and reservoirs.

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.

Effects of prolonged exposure to moderate static magnetic field and its synergistic effects with alkaline pH on Enterococcus faecalis

Static magnetic field (SMF) has been shown to biologically affect various microorganisms, but its effects on Enterococcus faecalis, which is associated with multiple dental infections, have not been reported yet. Besides, Enterococcus faecalis was found to be resistant to the alkaline environment provided by a major dental antimicrobial, calcium hydroxide. Therefore, the antibacterial activity of prolonged exposure to moderate SMF (170 mT) and its possible synergistic activity with alkaline pH (pH = 9) were evaluated in the study. The ability to form a biofilm under these conditions was examined by crystal violet assay. Real-time quantitative PCR was performed to evaluate the relative expression of stress (dnaK and groEL) and virulence (efaA, ace, gelE and fsrC) related genes. As the results indicated, cell proliferation was inhibited after 120 h of SMF exposure. What's more, the combined treatment of SMF and alkaline pH showed significantly improved antimicrobial action when compared to single SMF and alkaline pH treatment for more than 24 h and 72 h respectively. However, the ability to form a biofilm was also enhanced under SMF and alkaline pH treatments. SMF can induce stress response by up-regulating the expression of dnaK and elevate virulence gene expression (efaA and ace). These responses were more significant and more genes were up-regulated including groEL, gelE and fsrC when exposed to SMF and alkaline pH simultaneously. Hence, combination of SMF and alkaline pH could be a promising disinfection strategy in dental area and other areas associated with Enterococcus faecalis infections.

Genetic Characterization of Listeria monocytogenes Isolates from Industrial and Retail Ready-to-Eat Meat-Based Foods and Their Relationship with Clinical Strains from Human Listeriosis in Portugal

Listeria monocytogenes isolates (n = 81) recovered from ready-to-eat meat-based food products (RTEMP) collected in industrial processing plants and retail establishments were genetically characterized for comparison with those from human clinical cases of listeriosis (n = 49). The aim was to assess RTEMP as a possible food source for human infection. L. monocytogenes was detected in 12.5% of the RTEMP samples, and in some cases, counts were above the European food safety criteria. All isolates were assessed by multiplex PCR for serogroup determination and detection of virulence-associated genes inlA, inlB, inlC, inlJ, plcA, hlyA, actA, and iap. Serogroups IIb and IVb dominated in RTEMP and human isolates, and all were positive for the assessed virulence genes. Antibiotic susceptibility testing by the disk diffusion method revealed a low level of resistance among the isolates. Pulsed-field gel electrophoresis (PFGE) of L. monocytogenes isolates, using restriction enzymes ApaI and AscI, revealed genetic variability and differentiated the isolates in five clusters. Although some pulsed-field gel electrophoresis profiles of particular RTEMP and human isolates seemed to be highly related, exhibiting more than 90% similarity, which suggests a possible common source, in most cases the strains were not genetically or temporally matched. The close genetic relatedness of RTEMP and human listeriosis strains stressed the importance of preventive measure implementation throughout the food chain.

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.