Impact of exposure to cold and cold-osmotic stresses on virulence-associated characteristics of Listeria monocytogenes strains

One Health approach for prioritization of potential foodborne pathogens: Risk-ranking, Delphi survey, and criteria evaluation pre- and post-COVID-19 pandemic

Frequent foodborne illnesses with unknown causative agents highlight the need to explore zoonotic potential foodborne pathogens (PFPs). An effective PFP prioritization tool is indispensable, especially after experiencing the recent pandemic caused by zoonotic SARS-CoV-2. Risk information on pathogens (excluding 30 known foodborne pathogens) provided by governmental and international organizations was reviewed to generate a list of PFPs. Risk-ranking of PFPs was conducted based on a literature review of food poisoning or detection cases, and the ranks were determined with a decision tree. PFPs were prioritized by infectious disease (ID), veterinary medicine (VET), and food safety (FS) experts through a pre- and postpandemic Delphi survey, and key criteria in their decisions were illuminated. Among 339 PFPs, 32 rank-1 PFPs were involved in the foodborne outbreak(s). Discrepancies in opinions on prioritization between experts in different fields deepened after the pandemic. Only VET and FS experts valued the plausibility of foodborne transmission in evaluating bacteria and viruses, and a significant correlation between their selection of PFPs was found (p < .05). The impact of the pandemic induced all fields to focus more on human transmission and severity/fatality in prioritizing viruses, and only FS experts emphasized the plausibility of foodborne transmission after the pandemic. In contrast to prioritizing bacteria or viruses, ID and VET experts are unusually focused on foodborne transmission when prioritizing parasites. Criteria of consensus deduced by interdisciplinary experts with different interests and the criteria directly related to foodborne transmission should be acknowledged for adequate PFP prioritization.

Stress Adaptation Responses of a Listeria monocytogenes 1/2a Strain via Proteome Profiling

Listeria monocytogenes is a foodborne pathogen that is ubiquitous and largely distributed in food manufacturing environments. It is responsible for listeriosis, a disease that can lead to significant morbidity and fatality in immunocompromised patients, pregnant women, and newborns. Few reports have been published about proteome adaptation when L. monocytogenes is cultivated in stress conditions. In this study, we applied one-dimensional electrophoresis and 2D-PAGE combined with tandem mass spectrometry to evaluate proteome profiling in the following conditions: mild acid, low temperature, and high NaCl concentration. The total proteome was analyzed, also considering the case of normal growth-supporting conditions. A total of 1,160 proteins were identified and those related to pathogenesis and stress response pathways were analyzed. The proteins involved in the expression of virulent pathways when L. monocytogenes ST7 strain was grown under different stress conditions were described. Certain proteins, particularly those involved in the pathogenesis pathway, such as Listeriolysin regulatory protein and Internalin A, were only found when the strain was grown under specific stress conditions. Studying how L. monocytogenes adapts to stress can help to control its growth in food, reducing the risk for consumers.

From chicken to salad: Cooking salt as a potential vehicle of Salmonella spp. and Listeria monocytogenes cross-contamination

Epidemiological studies show that improper food handling practices at home account for a significant portion of foodborne illness cases. Mishandling of raw meat during meal preparation is one of the most frequent hazardous behaviours reported in observational research studies that potentially contributes to illness occurrence, particularly through the transfer of microbial pathogens from the raw meat to ready-to-eat (RTE) foods. This study evaluated the transfer of two major foodborne pathogens, Salmonella enterica and Listeria monocytogenes, from artificially contaminated chicken meat to lettuce via cooking salt (used for seasoning) during simulated domestic handling practices. Pieces of chicken breast fillets were spiked with five different loads (from ca. 1 to 5 Log CFU/g) of a multi-strain cocktail of either S. enterica or L. monocytogenes. Hands of volunteers (gloved) contaminated by handling the chicken, stirred the cooking salt that was further used to season lettuce leaves. A total of 15 events of cross-contamination (three volunteers and five bacterial loads) were tested for each pathogen. Immediately after the events, S. enterica was isolated from all the cooking salt samples (n = 15) and from 12 samples of seasoned lettuce; whereas L. monocytogenes was isolated from 13 salt samples and from all the seasoned lettuce samples (n = 15). In addition, S. enterica and L. monocytogenes were able to survive in artificially contaminated salt (with a water activity of 0.49) for, at least, 146 days and 126 days, respectively. The ability of these foodborne pathogens to survive for a long time in cooking salt, make it a good vehicle for transmission and cross-contamination if consumers do not adopt good hygiene practices when preparing meals.

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Pathogens may contaminate cooking salt via cross-contamination from unwashed hands.

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Salmonella enterica and Listeria monocytogenes survive on salt for several weeks.

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Pathogens are transmitted from contaminated chicken to hands, and then to salt.

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L. monocytogenes and S. enterica present in lettuce seasoned with contaminated salt.

Evaluation of VITEK® 2 and MALDI-TOF/MS automated methodologies in the identification of atypical Listeria spp. isolated from food in different regions of Brazil

Listeria monocytogenes is a pathogen responsible for listeriosis, a foodborne disease with high mortality rates (20-30%). It mainly affects the elderly, pregnant women, and immunocompromised people. Although not pathogenic, the isolation and identification of Listeria innocua are critical since they can indicate L. monocytogenes' presence as they are closely related and widely distributed in the environment and food processing plants. The objective of this study was to evaluate the effectiveness of the automated methods VITEK® 2 and MALDI-TOF/MS in identifying 94 strains of the genus Listeria with atypical identification profile. The resulting identification by Polymerase Chain Reaction (PCR), using specific primers for the most common species of Listeria, was considered the correct identification and presented a total of 31 strains identified as Listeria innocua (LI), 54 as L. monocytogenes (LM), 8 as Listeria welshimeri (LW) and 1 as Listeria grayi (LG). The VITEK® 2 automated system correctly identified, on average, 79% of the LI strains, 16% of the LM strains, and 88.0% of the LW strains. In the analysis by MALDI-TOF/MS, on average, 73% of LM strains were correctly identified, few LW strains were correctly identified, and all LI strains were incorrectly identified. Both VITEK® 2 and MALDI-TOF/MS correctly identified the LG strain in both analyzes. The results demonstrate that automated methodologies could not discriminate atypical strains of the Listeria genus and point to the need for the use of complementary tests, such as PCR and chromogenic media, for the correct identification of these strains.

Pathogenicity and virulence of Listeria monocytogenes: A trip from environmental to medical microbiology

Listeria monocytogenes is a saprophytic gram-positive bacterium, and an opportunistic foodborne pathogen that can produce listeriosis in humans and animals. It has evolved an exceptional ability to adapt to stress conditions encountered in different environments, resulting in a ubiquitous distribution. Because some food preservation methods and disinfection protocols in food-processing environments cannot efficiently prevent contaminations, L. monocytogenes constitutes a threat to human health and a challenge to food safety. In the host, Listeria colonizes the gastrointestinal tract, crosses the intestinal barrier, and disseminates through the blood to target organs. In immunocompromised individuals, the elderly, and pregnant women, the pathogen can cross the blood-brain and placental barriers, leading to neurolisteriosis and materno-fetal listeriosis. Molecular and cell biology studies of infection have proven L. monocytogenes to be a versatile pathogen that deploys unique strategies to invade different cell types, survive and move inside the eukaryotic host cell, and spread from cell to cell. Here, we present the multifaceted Listeria life cycle from a comprehensive perspective. We discuss genetic features of pathogenic Listeria species, analyze factors involved in food contamination, and review bacterial strategies to tolerate stresses encountered both during food processing and along the host's gastrointestinal tract. Then we dissect host-pathogen interactions underlying listerial pathogenesis in mammals from a cell biology and systemic point of view. Finally, we summarize the epidemiology, pathophysiology, and clinical features of listeriosis in humans and animals. This work aims to gather information from different fields crucial for a comprehensive understanding of the pathogenesis of L. monocytogenes.

Listeria monocytogenes 10403S Alternative Sigma-54 Factor σL Has a Negative Role on Survival Ability Under Bile Exposure

Listeria monocytogenes is a Gram-positive bacterium causing listeriosis in animals and humans. To initiate a foodborne infection, L. monocytogenes has to pass through the host gastrointestinal tract (GIT). In this study, we evaluated survival abilities of L. monocytogenes 10403S wild type (WT) and its isogenic mutants in alternative sigma (σ) factor genes (i.e., sigB, sigC, sigH, and sigL) under simulated gastric, duodenal, and bile fluids. Within 10min of exposures, only bile fluid was able to significantly reduce survival ability of L. monocytogenes WT by 2 logs CFU/ml. Loss of sigL showed the greatest bile resistance among 16 strains tested, p<0.0001, (i.e., WT, four single alternative σ factor mutants, six double mutants, four triple mutants, and one quadruple mutant). To further investigate the role of σ^L in bile response, RNA-seq was conducted to compare the transcriptional profiles among L. monocytogenes 10403S ΔBCH triple mutant (lacking sigB, sigC, and sigH genes; expressing housekeeping σ^A and σ^L) and ΔBCHL quadruple mutant (lacking all alternative sigma factor genes; expressing only σ^A) strains under BHI and 1% bile conditions. A total of 216 and 176 differentially expressed genes (DEGs) were identified in BHI and bile, respectively. We confirmed that mpt operon was shown to be strongly activated by σ^L. Interestingly, more than 80% of DEGs were found to be negatively regulated in the presence of σ^L. This includes PrfA regulon and its mediated genes (i.e., hly, hpt, inlB, clpP, clpE, groL, and inlC) which were downregulated in response to bile in the presence of σ^L. This result suggests the potential negative role of σ^L on bile survival, and the roles of σ^L and σ^B might be in a seesaw model prior to host cell invasion.

In vitro virulence potential, surface attachment and transcriptional response of sublethally injured Listeria monocytogenes following exposure to peracetic acid

The disinfectant Peracetic acid (PAA) can cause high levels of sublethal injury to L. monocytogenes. This study aims to evaluate phenotypic and transcriptional characteristics concerning surface attachment and virulence potential of sublethally injured L. monocytogenes ScottA and EGDe after exposure to 0.75 ppm PAA for 90 min at 4°C and subsequent incubation in TSBY at 4°C. Results showed that injured L. monocytogenes cells (99% of total population) were able to attach (after 2 and 24h) on stainless steel coupons at 4°C and 20°C. In vitro virulence assays using human intestinal epithelial Caco-2 cells showed that injured L. monocytogenes could invade host cells but could not proliferate intracellularly. In vitro virulence response was strain-dependent; injured ScottA was more invasive than EGDe. Assessment of PAA-injury at the transcriptional level showed upregulation of genes (motB, flaA) involved in flagellum motility and surface attachment. The transcriptional response of L. monocytogenes EGDe and ScottA was different; only injured ScottA demonstrated upregulation of the virulence genes inlA and plcA. Downregulation of the stress-related genes fri and kat, and upregulation of lmo0669 was observed in injured ScottA. The obtained results indicate that sublethally-injured L. monocytogenes cells may retain part of their virulence properties as well as their ability to adhere on food processing surfaces. Transmission to food products and introduction of these cells in the food chain is therefore a plausible scenario that is worth taking into consideration in terms of risk assessment. Importance L. monocytogenes is the causative agent of listeriosis a serious food-borne illness. Antimicrobial practices, such as disinfectants used for the elimination of this pathogen in food industry can produce a sublethally injured population fraction. Injured cells of this pathogen, that may survive an antimicrobial treatment, may pose a food safety-risk. Nevertheless, knowledge regarding how sublethal injury may impact important cellular traits and phenotypic responses of this pathogen is limited. This work suggests that sublethally injured L. monocytogenes cells maintain the virulence and surface attachment potential and highlights the importance of the occurrence of sublethally injured cells regarding food safety.

Innovative hurdle system towards Listeria monocytogenes inactivation in a fermented meat sausage model - high pressure processing assisted by bacteriophage P100 and bacteriocinogenic Pediococcus acidilactici

Consumers' quest for healthier, locally produced foods, renders the demand for these products increasingly prominent. The purpose of the present work was to evaluate the impact of a non-thermal multi-hurdle approach, which associated mild high hydrostatic pressure (HHP, 300 MPa), the bacteriophage Listex™ P100, and the pediocin PA-1 producing Pediococcus acidilactici HA 6111-2, as a novel minimal processing towards Listeria monocytogenes eradication in Alheira (a traditional fermented meat sausage from Northern Portugal). The combination of the three hurdles achieved the USDA-FSIS 5 log reduction (in accordance with the standard guidelines for ready-to-eat foods), being the only treatment to elicit the absence of L. monocytogenes immediately following processing (p < 0.05). The pair association of HHP with Listex™ P100 was unable to eliminate L. monocytogenes, whilst in the HHP-pediocin PA-1 producing P. acidilactici treated samples the eradication was delayed when compared to the three hurdles combination. In addition to the listericidal effect of the HHP-phage-lactic acid bacterium treatment, no significant differences (p > 0.05) in the pH values were observed, and the semi-quantification of the in situ biosynthesized pediocin PA-1 was documented for the first time in a fermented meat sausage model.

Transcriptomic Response of L. monocytogenes to Co-Culture with S. cerevisiae

The aim of the present study was to assess the transcriptomic response of L. monocytogenes during co-culture with three S. cerevisiae strains. For this purpose, BHI broth was inoculated with 7 log CFU·mL−1 L. monocytogenes serotype 4b strain LQC 15257, isolated from a strawberry sample and 4 log CFU·mL−1 S. cerevisiae strains Y32, Y34 and Y37, isolated from spontaneous olive fermentation. Sampling took place after 24 and 48 h incubation at 5 and 20 °C. RNA was extracted, stabilized and the transcription of virulence associated genes prfA, sigB, hly, plcA, plcB, inlA, inlB, inlC and inlJ, was assessed by RT-qPCR. Co-culture with the yeast strains mostly affected the transcription of sigB and inlJ, the upregulation of which during growth at 5 °C for 24 h, reached 10.13 and 9.76 log2(fold change), respectively. Similarly, the effect that incubation time had on the relative transcription of the genes under study was dependent on the co-cultivating yeast strain. On the other hand, the effect of the yeast strain was less pronounced when the relative transcription of the genes under study was assessed between 20 °C and 5 °C. In that case, incubation temperature seemed to have an important effect since, in the 79.2% of the samples analyzed, upregulation was evident, irrespective of yeast strain presence. These results highlight the complex trophic relationships that take place during co-existence between L. monocytogenes and S. cerevisiae.

Listeria monocytogenes Cold Shock Proteins: Small Proteins with A Huge Impact

Listeria monocytogenes has evolved an extensive array of mechanisms for coping with stress and adapting to changing environmental conditions, ensuring its virulence phenotype expression. For this reason, L. monocytogenes has been identified as a significant food safety and public health concern. Among these adaptation systems are cold shock proteins (Csps), which facilitate rapid response to stress exposure. L. monocytogenes has three highly conserved csp genes, namely, cspA, cspB, and cspD. Using a series of csp deletion mutants, it has been shown that L. monocytogenes Csps are important for biofilm formation, motility, cold, osmotic, desiccation, and oxidative stress tolerance. Moreover, they are involved in overall virulence by impacting the expression of virulence-associated phenotypes, such as hemolysis and cell invasion. It is postulated that during stress exposure, Csps function to counteract harmful effects of stress, thereby preserving cell functions, such as DNA replication, transcription and translation, ensuring survival and growth of the cell. Interestingly, it seems that Csps might suppress tolerance to some stresses as their removal resulted in increased tolerance to stresses, such as desiccation for some strains. Differences in csp roles among strains from different genetic backgrounds are apparent for desiccation tolerance and biofilm production. Additionally, hierarchical trends for the different Csps and functional redundancies were observed on their influences on stress tolerance and virulence. Overall current data suggest that Csps have a wider role in bacteria physiology than previously assumed.

Advances of microfluidic intestine-on-a-chip for analyzing anti-inflammation of food

Microfluidic intestine-on-a-chip enables novel means of emulating human intestinal pathophysiology in vitro, which can potentially reduce animal testing and substitute simple 2D culture system. Though a great deal of work has been done in the development of microfluidic platforms for intestinal disease modeling and drug screening, potential investigation of the effect of bioactive food compounds on intestinal inflammation remains largely unexplored. In this review, different biomaterials and chip designs have been explored in the fabrication of intestine-on-a-chip. Other key parameters must be carefully controlled and selected, including shear stress, cell type and cell co-culture spatial configuration, etc. Appropriate techniques to quantify the barrier integrity including trans-epithelial electric resistance, specific tight junction markers and permeability measurements should be standardized and compared with in vivo data. Integration of the gut microbiome and the provision of intestinal-specific environment are the key parameters to realize the in vivo intestinal model simulation and accelerate the screening efficiency of bioactive food compounds.

Potassium Lactate as a Strategy for Sodium Content Reduction without Compromising Salt-Associated Antimicrobial Activity in Salami

Reformulating recipes of ready-to-eat meat products such as salami to reduce salt content can mitigate the negative health impacts of a high salt diet. We evaluated the potential of potassium lactate (KL) as a sodium chloride (NaCl) replacer during salami production. NaCl and KL stress tolerance comparisons showed that four food-derived Listeria innocua isolates were suitable as biologically safe Listeria monocytogenes surrogates. Effects of the high salt (4% NaCl) concentration applied in standard salami recipes and a low salt (2.8% NaCl) plus KL (1.6%) combination on product characteristics and growth of contaminating Listeria and starter culture were compared. Simulated salami-ripening conditions applied in meat simulation broth and beef showed that the low salt plus KL combination retained similar to superior anti-Listeria activity compared to the high salt concentration treatment. Salami challenge tests showed that the low NaCl plus KL combination had comparable anti-Listeria activity as the high NaCl concentration during ripening and storage. No significant differences were detected in starter culture growth profiles and product characteristics between the high NaCl and low NaCl plus KL combination treated salami. In conclusion, KL replacement enabled a 30% NaCl reduction without compromising the product quality and antimicrobial benefits of high NaCl concentration inclusion.

New insights into thermo-acidophilic properties of Alicyclobacillus acidoterrestris after acid adaptation

Alicyclobacillus acidoterrestris has unique thermo-acidophilic properties and is the main cause of fruit juice deterioration. Given the acidic environment and thermal treatment during juice processing, the effects of acid adaptation (pH 3.5, 3.2, and 3.0) on the resistance of A. acidoterrestris to heat (65 °C, 5 min) and acid (pH = 2.2, 1 h) stresses were investigated for the first time. The results showed that acid adaptation induced cross-protection against heat stress of A. acidoterrestris and acid tolerance response, and the extent of induced tolerance was increased with the decrease of adaptive pH values. Acid adaptation treatments did not disrupt the membrane potential stability and intracellular pH homeostasis, but reduced intracellular ATP concentration, increased cyclic fatty acids content, and changed the acquired Fourier transform infrared spectra. Transcription levels of stress-inducible (dnaK, grpE, clpP, ctsR) genes and genes related to spore formation (spo0A, ctoX) were up-regulated after acid adaptation, and spore formation was observed by scanning electron microscopy. This study revealed that the intracellular microenvironment homeostasis, expression of chaperones and proteases, and spore formation played a coordinated role in acid stress adaptive responses, with implications for applications in fruit juice processing.

β-Phenylethylamine as a Natural Food Additive Shows Antimicrobial Activity against Listeria monocytogenes on Ready-to-Eat Foods

Listeria monocytogenes is an important foodborne pathogen and a major cause of death associated with bacterial foodborne infections. Control of L. monocytogenes on most ready-to-eat (RTE) foods remains a challenge. The potential use of β-phenylethylamine (PEA) as an organic antimicrobial against L. monocytogenes was evaluated in an effort to develop a new intervention for its control. Using a collection of 62 clinical and food-related isolates we determined the minimum inhibitory concentration (MIC) of PEA against L. monocytogenes in different broth and agar media. Bologna type sausage (lyoner) and smoked salmon were used as food model systems to validate the in vitro findings. PEA had a growth inhibitory and bactericidal effect against L. monocytogenes both in in vitro experiments as well as on lyoner and smoked salmon. The MIC’s ranged from 8 to 12.5 mg/mL. Furthermore, PEA also inhibited L. monocytogenes biofilm formation. Based on good manufacturing practices as a prerequisite, the application of PEA to RTE products might be an additional hurdle to limit L. monocytogenes growth thereby increasing food safety.