Adaptive Response of Listeria monocytogenes to the Stress Factors in the Food Processing Environment

Effect of Temperature, Surface, and Medium Qualities on the Biofilm Formation of Listeria monocytogenes and Their Influencing Effects on the Antibacterial, Biofilm-Inhibitory, and Biofilm-Degrading Activities of Essential Oils

Listeria monocytogenes is a foodborne pathogen with a high tolerance to a wide range of environmental conditions, making its control in the food chain a particular challenge. Essential oils have recently been considered as potential antilisterial agents. In this study, the antilisterial effects of 57 EOs were tested on 13 different L. monocytogenes. Thirty-seven EOs were found to be effective in a strain and temperature-dependent manner. At 37 °C, all EOs were effective against at least one strain of L. monocytogenes. However, at 14 °C and 23 °C, 12 EOs, such as Minth, Nutmeg, Neroli, Pepperminth, etc., became drastically ineffective. The efficacy of the EOs increased at the lowest temperature, as only four EOs, such as Dill seed, Juniper, lemon eucalyptus, and sandalwood, were found to be ineffective at 4 °C. Ajowan and thyme were the only EOs that were antibacterial against each strain at all temperatures tested (4, 14, 23, 37 °C). Biofilm-inhibition tests with 57 EOs, performed on polystyrene plates with different surface qualities and stainless steel, using 0.1% and 0.5% final concentrations, showed the outstanding inhibitory abilities of ajowan, geranium, Lime oil, melissa, palmarosa, rose geranium, sandalwood, and thyme. Fennel, lemon eucalyptus, and chamomile had the potential to inhibit biofilm formation without affecting live bacterial cell counts. Ajowan, geranium, thyme, and palmarosa reduced the biofilm to the optical density of 0.0-0.08, OD: 0.0-0.075, 0.0-0.072, and 0.0-0.04, respectively, compared to the bacterium control 0.085-0.45. The mature antibiofilm eradication ability of the EOs revealed the outstanding features of ajowan, geranium Lime, melissa, palmarosa, rose geranium, and thyme by suppressing the established biofilm to one tenth. The different sensitivities of the isolates and the temperature-dependent antilisterial effect of the tested EOs have to be taken into account if an EO-based food preservation technology is to be implemented, as several L. monocytogenes become resistant to different EOs at medium temperature ranges such as 14 °C and 23 °C.

Characterization of the Listeria monocytogenes PieRS regulon distinguishes the function of the critical secretion chaperone PrsA2 from other regulon members

Listeria monocytogenes (Lm) is a gram-positive pathogen that is widespread throughout the environment and known for its ability to infect mammalian hosts following the ingestion of contaminated food. Lm uses a variety of mechanisms to survive challenging conditions experienced both during life in the outside environment and inside of the infected host. We recently described a novel two-component signaling system known as PieRS that regulates the secretion of the chaperone PrsA2, which is essential for bacterial virulence, as well as its related homolog PrsA1 and a variety of gene products of unknown function. Here, we examine the roles of the less characterized PieRS-regulated gene products and contrast their functions with PrsA2 in terms of bacterial survival under stress conditions and virulence in mice. Characterization of targeted in-frame deletion mutants of PieRS regulon members indicates-in contrast to prsA2 mutants-minimal contributions to stress survival and bacterial virulence. Modest contributions of select regulon members were associated with Lm colonization of the gastrointestinal tract. The PieRS regulon thus consists of gene products that contribute to Lm physiology in ways that are clearly distinct from PrsA2 and the chaperone's essential function for both stress survival and bacterial virulence.

The type of food influences the behaviour of Listeria monocytogenes in a food-gastrointestinal-infection model

Food contaminated with Listeria (L.) monocytogenes is the main source of human listeriosis, but how different food matrices affect the survival and invasion in the gastrointestinal (GI) tract is still unclear. This study examined three ready-to-eat foods - soft-cheese, smoked salmon, and sausage - using a food-GI-infection model. We observed strain-dependent growth rates, but food matrices did not significantly impact growth. However, nutrient sources altered gene expression. Passage through the GI model upregulated 23 stress genes and 29 virulence genes (e.g., clpE, hly, and plcB). L. monocytogenes survival was higher in cheese and fish compared to sausage, due to their lower buffer capacity. Invasion efficiency into Caco-2 cells was highest in fish, potentially linked to its fatty acid composition. Food matrices and GI conditions influenced the transcriptional profiles of stress-associated and virulence genes. This study highlights the significant role of food matrices in L. monocytogenes survival and infection.

Characterizing Environmental Background Microflora and Assessing Their Influence on Listeria Persistence in Dairy Processing Environment

Listeria monocytogenes (Lm) may persist in food processing environments (FPEs) alongside diverse background microflora. While microbial communities in FPEs can influence Lm survival, their role in supporting or suppressing its growth remains unclear. This study aimed to characterize the microflora in floor swabs and air samples collected from a dairy processing facility across three seasons and assess their potential impact on the growth of a Lm test strain previously isolated from a dairy processing environment. A total of 167 environmental isolates, representing 30 bacterial genera, were identified. Pseudomonas was consistently prevalent across all sample types. Seasonal shifts in bacterial genera were observed, with differences in microbial composition and relative abundance between production lines with and without Listeria innocua occurrence. Microflora distribution appeared more influenced by environmental and operational factors than by spatial proximity. Co-culture growth assays revealed no competitive exclusion of the Lm test strain, and no zones of inhibition were observed in antimicrobial assays using cell-free extract and dialyzed cell-free extract from environmental isolates against Lm. These findings suggest that Lm could potentially establish itself within mixed microbial communities in dairy processing environments, emphasizing the complexity of microbial interactions in FPEs and their potential role in Lm persistence.

Unveiling Salmonella Derby Survival: Stress Responses to Prolonged Hyperosmotic Stress

The traditional marination process enhances food flavor and inhibits microbial growth. However, in hyperosmotic environments, microorganisms can activate stress responses to ensure survival, potentially compromising food safety. This study investigated the osmotolerance mechanisms of Salmonella Derby (S. Derby) by comparing a wild-type strain (S. D-WT) and an osmotolerant strain (S. D-OT) under NaCl-induced hyperosmotic stress. Both strains were subjected to 0.85%, 4%, and 16% NaCl for 0, 8, and 16 days, and their growth behavior, membrane integrity, intracellular osmoprotectant content, and transcription of related genes were evaluated. By day 16, both strains showed a growth delay of approximately 3 h. S. D-OT maintained better membrane integrity and exhibited higher intracellular levels of osmoprotectants (K⁺, trehalose, and proline), which aligned with the upregulation of the transcriptional levels of kdpC, kuP, rpoS, and proU. These findings indicated that S. D-OT achieved improved osmotic stress tolerance by regulating osmoprotectant synthesis and maintaining intracellular homeostasis. In contrast, S. D-WT displayed greater resistance to multiple antibiotics (gentamicin, ciprofloxacin, trimethoprim-sulfamethoxazole, and chloramphenicol) under 4% and 16% NaCl conditions, which may pose a higher food safety risk. Overall, this study provides insights for improving microbial control strategies in preserved foods and mitigating foodborne disease risks.

Population structure and gene flux of Listeria monocytogenes ST121 reveal prophages as a candidate driver of adaptation and persistence in food production environments

Listeria monocytogenes is a bacterial pathogen found in an increasing number of food categories, potentially reflecting an expanding niche and food safety risk profile. In the UK, Listeria monocytogenes sequence type (ST) 121 is more frequently isolated from foods and food environments than from cases of clinical listeriosis, consistent with a relatively low pathogenicity. In this study, we determined the evolution associated with the environmental persistence of a Listeria monocytogenes clone by investigating clone-specific genome features in the context of the ST121 population structure from international sources. To enable unambiguous comparative genomic analysis of ST121 strains, we constructed 16 new high-quality genome assemblies from Listeria monocytogenes isolated from foods, food environments and human clinical sources in the UK from 1987 to 2019. Our dataset was supplemented with additional UK and international genomes from databases held by the Institut Pasteur and the UK Health Security Agency. Time-scaled phylogenetic reconstruction revealed that clade-specific microevolution correlated with key characteristics that may confer adaptations important for success in the environmental niche. For example, a prophage designated LP-13-6 unique to a clade is associated with multi-year persistence in a food production setting. This prophage, observed in a strain that persisted for over a decade, may encode mechanisms facilitating environmental persistence, including the exclusion of other bacteriophages. Pangenome analysis provided insights into other candidate genetic elements associated with persistence and biocide tolerance. The comparative genomic dataset compiled in this study includes an international collection of 482 genome sequences that serve as a valuable resource for future studies to explore conserved genes, regulatory regions, mutations and variations associated with particular traits, such as environmental persistence, pathogenicity or biocide tolerance.

Adaptive strategies of Listeria monocytogenes: An in-depth analysis of the virulent strain involved in an outbreak in Italy through quantitative proteomics

Despite the general classification of L. monocytogenes strains as equally virulent by global safety authorities, molecular epidemiology reveals diverse subtypes in food, processing environments, and clinical cases. This study focuses on a highly virulent strain associated with a listeriosis outbreak in Italy in 2022, providing insights through comprehensive foodomics approaches, with a specific emphasis on quantitative proteomics. In particular, the ST155 strain of L. monocytogenes strain was subjected in vitro to growth stress conditions (NaCl 2.4 %, pH 6.2, T 12 °C), mimicking the conditions present in the frankfurter, its original source. Then, the protein expression patterns were compared with those obtained in optimal growth conditions. Through quantitative proteomic analysis and bioinformatic assessment, different proteins associated with virulence during the exponential growth phase were identified. This study unveils unique proteins specific to each environment, providing insights into how L. monocytogenes adapts to conditions that are similar to those encountered in frankfurters. This investigation contributes valuable insights into the adaptive strategies of L. monocytogenes under stressful conditions, with implications for enhancing food safety practices.

The Genetic Determinants of Listeria monocytogenes Resistance to Bacteriocins Produced by Lactic Acid Bacteria

BACKGROUND

Listeria monocytogenes is a Gram-positive bacterium responsible for listeriosis, a serious foodborne disease that can lead to serious health complications. Pregnant women, newborns, the elderly, and patients with weakened immune systems are particularly susceptible to infection. Due to the ability of L. monocytogenes to survive in extreme environmental conditions, such as low temperatures, high salinity, and acidity, this bacterium poses a serious threat to food production plants and is particularly difficult to eliminate from these plants. One of the promising solutions to reduce the presence of this bacterium in food products is bacteriocins as natural control agents. These are substances with antibacterial activity produced by other bacteria, mainly lactic acid bacteria (LAB), which can effectively inhibit the development of pathogens such as L. monocytogenes. The use of bacteriocins in the food industry is beneficial due to their natural origin, specificity of action, and consumer safety. However, the problem of resistance to these substances exists.

RESULTS

This review focuses on the mechanisms of bacteriocin resistance, such as modifications of bacteriocin docking receptors, changes in the structure of the cell wall and membrane, and the occurrence of cross-resistance to different bacteriocins. Genetic factors determining these mechanisms and strategies to cope with the problem of resistance are also presented.

CONCLUSIONS

Research on this issue is crucial for developing effective preventive methods that will enable the safe and long-term use of bacteriocins in food production.

The application of ozone within the food industry, mode of action, current and future applications, and regulatory compliance

The food industry faces numerous challenges today, with the prevention and reduction of microbial contamination being a critical focus. While traditional chemical-based methods are effective and widely used, rising energy costs, the development of microbial tolerances, and growing awareness of the ecological impact of chemical biocides have renewed interest in novel biocides. Ozone, in both its gaseous and aqueous forms, is recognized as a potent disinfectant against bacteria, viruses, and fungi due to its high oxidation potential. Our review highlights several studies on the applications of ozone within the food industry, including its use for surface and aerosol disinfection and its capacity to reduce viable Listeria monocytogenes, a pertinent foodborne pathogen harbouring environmental and biocide stress tolerances and biofilm former. We also explore the use of ozone in food treatment and preservation, specifically on blueberries, apples, carrots, cabbage, and cherry tomatoes. While ozone is an effective disinfectant, it is important to consider material incompatibility, and the risks associated with prolonged human exposure to high concentrations. Nevertheless, for certain applications, ozone proves to be an efficacious and valuable alternative or complementary method for microbial control. Compliance with the biocide products regulation will require ozone device manufacturers to produce proven efficacy and safety data in line with British standards based on European standards (BS EN), and researchers to propose adaptations to account for ozone's unique properties.

Effects of Thermally-Assisted and High-Pressure Processing on Background Microbiota and the Listeria monocytogenes Load of a Minimally Processed Commodity

The current study investigated the impact of treatments with elevated hydrostatic pressure (500 MPa) for inactivation of Listeria monocytogenes on smoked rainbow trout (Oncorhynchus mykiss) at high and low inoculation levels. The temperature values of the trials were set at 4.4 and 60.0 °C, adjusted with a circulating water bath connected to a stainless steel jacket surrounding the pressure processing chamber. Before pressure processing, the counts (selective counts of PALCAM, mean ± SD) of L. monocytogenes were 6.45 ± 0.1 log CFU/g and were reduced (p < 0.05) to 3.72 ± 0.3, and <1.48 ± 0.8 log CFU/g after 10 min of treatment at 4.4 and 60.0 °C, respectively. Treatments of low inoculation level samples were similarly efficacious and resulted in a reduction (p < 0.05) of the pathogen to 1.62 ± 0.3 and <0.82 ± 0.0 log CFU/g for treatments at 4.4 and 60.0 °C, respectively. At 4.4 °C, linear D-value and non-linear kmax1 were 8.68 and 0.50, and 5.81 and 2.41 for high-inoculation and low-inoculation samples, respectively. Application of hydrostatic pressure at 500 MPa at cold and elevated temperatures was efficacious for up to 5.03 log CFU/g reduction of L. monocytogenes, illustrating the potential for further adaptation of this technology.

Formation and recovery of Listeria monocytogenes in viable but nonculturable state under different temperatures combined with low nutrition and high NaCl concentration

The viable but nonculturable (VBNC) state occurs when bacteria lose their ability to grow and multiply on conventional media when stressed by adverse environmental factors, but they remain active and can revive under certain conditions, posing a food safety risk. In this study, the VBNC state of Listeria monocytogenes was induced with different temperatures combined with low nutrient conditions; the VBNC state of L. monocytogenes was confirmed in conjunction with the housekeeping gene abcZ using a molecular biology assay (PMA-qPCR) to calculate the viable bacterial count; The resuscitation conditions for the VBNC state of L. monocytogenes were investigated utilizing various nutrients in the culture medium and pasteurized milk. Four strains of L. monocytogenes reached the VBNC stage after 14, 21, 21, and 35 days at 20°C with 20% (or 30%) NaCl. Resuscitation studies indicate that Trypticase Soy Broth (TSB) combined with Tween 80 and sodium pyruvate is more effective for resuscitation. The Chinese national standard technology GB 4789.30-2016 was used to inoculate lettuce, chicken, and pasteurized milk with L. monocytogenes ATCC 19115 VBNC state. This research has significant implications for commercial food processing, long-term storage, disinfection, disease prevention, and control.

Growth of Listeria monocytogenes in fresh vegetables and vegetable salad products: An update on influencing intrinsic and extrinsic factors

The ability of foodborne pathogens to grow in food products increases the associated food safety risks. Listeria monocytogenes (Lm) is a highly adaptable pathogen that can survive and grow under a wide range of environmental circumstances, including otherwise inhibitory conditions, such as restrictive cold temperatures. It can also survive long periods under adverse environmental conditions. This review examines the experimental evidence available for the survival and growth of Lm on fresh vegetables and ready-to-eat vegetable salads. Published data indicate that, depending on certain intrinsic (e.g., nutrient composition) and extrinsic factors (e.g., storage temperature, packaging atmosphere), Lm can survive on and in a wide variety of vegetables and fresh-cut minimally processed vegetable salads. Studies have shown that temperature, modified atmosphere packaging, relative humidity, pH, water activity, background microbiota of vegetables, microbial strain peculiarities, and nutrient type and availability can significantly impact the fate of Lm in vegetables and vegetable salads. The influence of these factors can either promote its growth or decline. For example, some studies have shown that background microbiota inhibit the growth of Lm in vegetables and minimally processed vegetable salads, but others have reported a promoting, neutral, or insignificant effect on the growth of Lm. A review of relevant literature also indicated that the impact of most influencing factors is related to or interacts with other intrinsic or extrinsic factors. This literature synthesis contributes to the body of knowledge on possible strategies for improving food safety measures to minimize the risk of Lm-associated foodborne outbreaks involving vegetables and vegetable salads.

Microbial Contamination and Disease Outbreaks Associated with Rockmelons (Cucumis melo): Implications for Public Health Protection

Foodborne illnesses caused by consuming contaminated fresh produce not only pose serious public health risks but also lead to huge economic losses. Rockmelons (cantaloupes) have emerged as a recurrent source of disease outbreaks caused by foodborne pathogens, including Listeria monocytogenes, Salmonella, and Escherichia coli. The most common factor of the outbreaks was the microbial contamination of rockmelons at the farm, and subsequently, the pathogenic bacteria were transferred to the flesh during cutting and processing. One of the deadliest outbreaks occurred in the USA due to L. monocytogenes contamination of rockmelons which caused 33 deaths in 2011. Since then, several guidelines and recommendations have been developed for food safety management to reduce the microbial contamination of melons on farms and post-harvest operations. This article explicitly provides an updated overview of microbiological contamination, disease outbreaks, pathogens prevalence, and mitigation strategies to reduce public health risks due to the consumption of rockmelons.

A Comprehensive Methodology for Microbial Strain Typing Using Fourier-Transform Infrared Spectroscopy

Timely and accurate detection and characterization of microbial threats is crucial for effective infection and outbreak management. Additionally, in food production, rapid microbe identification is indispensable for maintaining quality control and hygiene standards. Current methods for typing microbial strains often rely on labor-intensive, time-consuming, and expensive DNA- and sera-serotyping techniques, limiting their applicability in rapid-response scenarios. In this context, the IR Biotyper®, utilizing Fourier-transform infrared (FTIR) spectroscopy, offers a novel approach, providing specific spectra for fast strain typing within 3 h. This methodology article serves as a comprehensive resource for researchers and technicians aiming to utilize FTIR spectroscopy for microbial strain typing. It encompasses detailed guidelines on sample preparation, data acquisition, and analysis techniques, ensuring the generation of reliable and reproducible results. We highlight the IR Biotyper®'s rapid and accurate discrimination capabilities, showcasing its potential for real-time pathogen monitoring and source-tracking to enhance public health and food safety. We propose its integration as an early screening method, followed by more detailed analysis with whole-genome sequencing, to optimize detection accuracy and response efficiency in microbial surveillance systems.

Prevalence and Antimicrobial Resistance of Listeria monocytogenes in Different Raw Food from Reynosa, Tamaulipas, Mexico

Listeria (L.) monocytogenes is an opportunistic foodborne pathogen that causes listeriosis in humans and animals, reaching up to 30% case mortality. There are only a few reports in Mexico about the L. monocytogenes strains found in various foods. The aim of this study was to determine the prevalence of L. monocytogenes, serogroups, virulence genes, and antimicrobial resistance in different foods from Reynosa, Tamaulipas, Mexico. L. monocytogenes strains were characterized by microbiological and molecular methods. Susceptibility to 12 antibiotics was determined according to CLSI and EUCAST. A total of 300 samples of seafood, pasteurized and raw milk, cheese, beef, and chicken were collected from supermarkets and retail markets. The presence of L. monocytogenes was detected in 5.6% of the samples. Most strains belonged to serogroups 4b, 4d, and 4e (68.4%). All strains presented a minimum of four virulence genes; the most common were actA, hly, and plcB (92.1%). A high percentage of antimicrobial susceptibility was observed, with resistance only to STX-TMP (78.9%), STR (26.3%), MEM (21.0%), and E (2.6%). These results show that the foods in Reynosa, Tamaulipas, are a reservoir of L. monocytogenes and represent a potential health risk.

Whole Genome Sequence Analysis of Listeria monocytogenes Isolates Obtained from the Beef Production Chain in Gauteng Province, South Africa

The study used whole-genome sequencing (WGS) and bioinformatics analysis for the genomic characterization of 60 isolates of Listeria monocytogenes obtained from the beef production chain (cattle farms, abattoirs, and retail outlets) in Gauteng province, South Africa. The sequence types (STs), clonal complexes (CCs), and the lineages of the isolates were determined using in silico multilocus sequence typing (MLST). We used BLAST-based analyses to identify virulence and antimicrobial genes, plasmids, proviruses/prophages, and the CRISPR-Cas system. The study investigated any association of the detected genes to the origin in the beef production chain of the L. monocytogenes isolates. Overall, in 60 isolates of Listeria monocytogenes, there were seven STs, six CCs, forty-four putative virulence factors, two resistance genes, one plasmid with AMR genes, and three with conjugative genes, one CRISPR gene, and all 60 isolates were positive for proviruses/prophages. Among the seven STs detected, ST204 (46.7%) and ST2 (21.7%) were the most prominent, with ST frequency varying significantly (p < 0.001). The predominant CC detected were CC2 (21.7%) and CC204 (46.7%) in lineages I and II, respectively. Of the 44 virulence factors detected, 26 (across Listeria Pathogenicity Islands, LIPIs) were present in all the isolates. The difference in the detection frequency varied significantly (p < 0.001). The two AMR genes (fosX and vga(G)) detected were present in all 60 (100%) isolates of L. monocytogenes. The only plasmid, NF033156, was present in three (5%) isolates. A CRISPR-Cas system was detected in six (10%), and all the isolates carried proviruses/prophages. The source and sample type significantly affected the frequencies of STs and virulence factors in the isolates of L. monocytogenes. The presence of fosX and vga(G) genes in all L. monocytogenes isolates obtained from the three industries of the beef production chain can potentially cause therapeutic implications. Our study, which characterized L. monocytogenes recovered from the three levels in the beef production chain, is the first time genomics was performed on this type of data set in the country, and this provides insights into the health implications of Listeria.

Listeria monocytogenes from Food Products and Food Associated Environments: Antimicrobial Resistance, Genetic Clustering and Biofilm Insights

Listeria monocytogenes, a foodborne pathogen, exhibits high adaptability to adverse environmental conditions and is common in the food industry, especially in ready-to-eat foods. L. monocytogenes strains pose food safety challenges due to their ability to form biofilms, increased resistance to disinfectants, and long-term persistence in the environment. The aim of this study was to evaluate the presence and genetic diversity of L. monocytogenes in food and related environmental products collected from 2014 to 2022 and assess antibiotic susceptibility and biofilm formation abilities. L. monocytogenes was identified in 13 out of the 227 (6%) of samples, 7 from food products (meat preparation, cheeses, and raw milk) and 6 from food-processing environments (slaughterhouse-floor and catering establishments). All isolates exhibited high biofilm-forming capacity and antibiotic susceptibility testing showed resistance to several classes of antibiotics, especially trimethoprim-sulfamethoxazole and erythromycin. Genotyping and core-genome clustering identified eight sequence types and a cluster of three very closely related ST3 isolates (all from food), suggesting a common contamination source. Whole-genome sequencing (WGS) analysis revealed resistance genes conferring resistance to fosfomycin (fosX), lincosamides (lin), fluoroquinolones (norB), and tetracycline (tetM). In addition, the qacJ gene was also detected, conferring resistance to disinfecting agents and antiseptics. Virulence gene profiling revealed the presence of 92 associated genes associated with pathogenicity, adherence, and persistence. These findings underscore the presence of L. monocytogenes strains in food products and food-associated environments, demonstrating a high virulence of these strains associated with resistance genes to antibiotics, but also to disinfectants and antiseptics. Moreover, they emphasize the need for continuous surveillance, effective risk assessment, and rigorous control measures to minimize the public health risks associated to severe infections, particularly listeriosis outbreaks. A better understanding of the complex dynamics of pathogens in food products and their associated environments can help improve overall food safety and develop more effective strategies to prevent severe health consequences and economic losses.

Revisiting old friends: updates on the role of two-component signaling systems in Listeria monocytogenes survival and pathogenesis

Listeria monocytogenes is well recognized for both its broad resistance to stress conditions and its ability to transition from a soil bacterium to an intracellular pathogen of mammalian hosts. The bacterium's impressive ability to adapt to changing environments and conditions requires the rapid sensing of environmental cues and the coordinated response of gene products that enable bacterial growth and survival. Two-component signaling systems (TCSs) have been long recognized for their ability to detect environmental stimuli and transmit those signals into transcriptional responses; however, often the precise nature of the stimulus triggering TCS responses can be challenging to define. L. monocytogenes has up to 16 TCSs that have been recognized based on homology and included in this list are several whose functions remain poorly described. This review highlights the current understanding of the breadth and scope of L. monocytogenes TCS as relates to stress resistance and pathogenesis. Precise signals still often remain elusive, but the gene networks associated with TCSs are providing clues into possible functions.

Comparative Analysis of Growth, Survival, and Virulence Characteristics of Listeria monocytogenes Isolated from Imported Meat

Listeria monocytogenes is an important foodborne pathogen with worldwide prevalence. Understanding the variability in the potential pathogenicity among strains of different subtypes is crucial for risk assessment. In this study, the growth, survival, and virulence characteristics of 16 L. monocytogenes strains isolated from imported meat in China (2018-2020) were investigated. The maximum specific growth rate (μmax) and lag phase (λ) were evaluated using the time-to-detection (TTD) method and the Baranyi model at different temperatures (25, 30, and 37 °C). Survival characteristics were determined by D-values and population reduction after exposure to heat (60, 62.5, and 65 °C) and acid (HCl, pH = 2.5, 3.5, and 4.5). The potential virulence was evaluated via adhesion and invasion to Caco-2 cells, motility, and lethality to Galleria mellonella. The potential pathogenicity was compared among strains of different lineages and subtypes. The results indicate that the lineage I strains exhibited a higher growth rate than the lineage II strains at three growth temperatures, particularly serotype 4b within lineage I. At all temperatures tested, serotypes 1/2a and 1/2b consistently demonstrated higher heat resistance than the other subtypes. No significant differences in the log reduction were observed between the lineage I and lineage II strains at pH 2.5, 3.5, and 4.5. However, the serotype 1/2c strains exhibited significantly low acid resistance at pH 2.5. In terms of virulence, the lineage I strains outperformed the lineage II strains. The invasion rate to Caco-2 cells and lethality to G. mellonella exhibited by the serotype 4b strains were higher than those observed in the other serotypes. This study provides meaningful insights into the growth, survival, and virulence of L. monocytogenes, offering valuable information for understanding the correlation between the pathogenicity and subtypes of L. monocytogenes.

Potential public health hazards related to consumption of poultry contaminated with antibiotic resistant Listeria monocytogenes in Egypt

Listeria monocytogenes is an important foodborne pathogen that incorporated into many serious infections in human especially immunocompromised individuals, pregnant women, the elderly, and newborns. The consumption of food contaminated with such bacteria is considered a source of potential risk for consumers. Therefore, a total of 250 poultry purchased in highly popular poultry stores besides 50 swabs from workers hands in the same stores, in Mansoura City had been tested for the L. monocytogenes prevalence, virulence genes, and antibiotic resistance profile illustrating the health hazards from such poultry. The L. monocytogenes were recovered from 9.6% of poultry samples while not detected from workers hand swabs. The antimicrobial susceptibility of 24 L. monocytogenes strains against 24 antibiotics of seven different classes revealed high susceptibility rates to erythromycin (79.17%), streptomycin (66.67%), gentamycin (66.67%), vancomycin (58.33%), chloramphenicol (58.33%) and cefotaxime (41.67%). The majority (79.2%) of L. monocytogenes were classified as multidrug resistant strains with high resistance to tetracyclines and β-lactams antibiotics while 16.7% of the strains were categorized as extensively resistant ones. The iap virulence-specific determination gene had been detected in all recovered L. monocytogenes isolates while 83.33 and 70.83% of the isolates harbored hylA and actA genes. In addition, the study confirmed the capability of most L. monocytogenes isolates for biofilm formation by moderate to strong production and the quantitative risk assessment illustrated the risk of developing listeriosis as the risk value exceeded 100. The current results illustrate that poultry meat can be a source of pathogenic antibiotic resistant strains that may cause infection with limited or no treatment in immunosuppressed consumers via the food chain.

Metabolic reprogramming in the food-borne pathogen Listeria monocytogenes as a critical defence against acid stress

The ability to sense and respond effectively to acidic stress is important for microorganisms to survive and proliferate in fluctuating environments. As specific metabolic activities can serve to buffer the cytoplasmic pH, microorganisms rewire their metabolism to favour these reactions and thereby mitigate acid stress. The orally acquired pathogen Listeria monocytogenes exploits alternative metabolic activities to overcome the acidic stress encountered in the human stomach or food products. In this minireview, we discuss the metabolic processes in L. monocytogenes that mitigate acid stress, with an emphasis on the proton-depleting reactions, including glutamate decarboxylation, arginine/agmatine deimination, and fermentative acetoin production. We also summarize the recent findings on regulatory mechanisms that control the expression of genes that are responsible for these metabolic activities, including the general stress response regulator SigB, arginine repressor ArgR, and the recently discovered RofA-like transcriptional regulatory GadR. We further discuss the importance of this metabolic reprogramming in the context of food products and within the host. Finally, we highlight some outstanding challenges in the field, including an understanding of acid-sensing mechanisms, the role of intraspecies heterogeneity in acid resistance, and how a fundamental understanding of acid stress response can be exploited for food formulation to improve food safety and reduce food waste.

EFSA Panel on Biological Hazards (BIOHAZ), Koutsoumanis K, Allende A, Bolton D, Bover‐Cid S, Chemaly M, De Cesare A, Herman L, Hilbert F, Lindqvist R, Nauta M, Nonno R, Peixe L, Ru G, Simmons M, Skandamis P, Suffredini E, Fox E, Gosling R(B, Gil BM, Møretrø T, Stessl B, da Silva Felício MT, Messens W, Simon AC, Alvarez‐Ordóñez A. Persistence of microbiological hazards in food and feed production and processing environments

Listeria monocytogenes (in the meat, fish and seafood, dairy and fruit and vegetable sectors), Salmonella enterica (in the feed, meat, egg and low moisture food sectors) and Cronobacter sakazakii (in the low moisture food sector) were identified as the bacterial food safety hazards most relevant to public health that are associated with persistence in the food and feed processing environment (FFPE). There is a wide range of subtypes of these hazards involved in persistence in the FFPE. While some specific subtypes are more commonly reported as persistent, it is currently not possible to identify universal markers (i.e. genetic determinants) for this trait. Common risk factors for persistence in the FFPE are inadequate zoning and hygiene barriers; lack of hygienic design of equipment and machines; and inadequate cleaning and disinfection. A well-designed environmental sampling and testing programme is the most effective strategy to identify contamination sources and detect potentially persistent hazards. The establishment of hygienic barriers and measures within the food safety management system, during implementation of hazard analysis and critical control points, is key to prevent and/or control bacterial persistence in the FFPE. Once persistence is suspected in a plant, a 'seek-and-destroy' approach is frequently recommended, including intensified monitoring, the introduction of control measures and the continuation of the intensified monitoring. Successful actions triggered by persistence of L. monocytogenes are described, as well as interventions with direct bactericidal activity. These interventions could be efficient if properly validated, correctly applied and verified under industrial conditions. Perspectives are provided for performing a risk assessment for relevant combinations of hazard and food sector to assess the relative public health risk that can be associated with persistence, based on bottom-up and top-down approaches. Knowledge gaps related to bacterial food safety hazards associated with persistence in the FFPE and priorities for future research are provided.

Impact of High-Pressure Processing (HPP) on Listeria monocytogenes-An Overview of Challenges and Responses

High-pressure processing (HPP) is currently one of the leading methods of non-thermal food preservation as an alternative to traditional methods based on thermal processing. The application of HPP involves the simultaneous action of a combination of several factors-pressure values (100-600 MPa), time of operation (a few-several minutes), and temperature of operation (room temperature or lower)-using a liquid medium responsible for pressure transfer. The combination of these three factors results in the inactivation of microorganisms, thus extending food shelf life and improving the food's microbiological safety. HPP can provide high value for the sensory and quality characteristics of products and reduce the population of pathogenic microorganisms such as L. monocytogenes to the required safety level. Nevertheless, the technology is not without impact on the cellular response of pathogens. L. monocytogenes cells surviving the HPP treatment may have multiple damages, which may impact the activation of mechanisms involved in the repair of cellular damage, increased virulence, or antibiotic resistance, as well as an increased expression of genes encoding pathogenicity and antibiotic resistance. This review has demonstrated that HPP is a technology that can reduce L. monocytogenes cells to below detection levels, thus indicating the potential to provide the desired level of safety. However, problems have been noted related to the possibilities of cell recovery during storage and changes in virulence and antibiotic resistance due to the activation of gene expression mechanisms, and the lack of a sufficient number of studies explaining these changes has been reported.

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

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

Why does Listeria monocytogenes survive in food and food-production environments?

Listeria monocytogenes is one of the most dangerous food-borne pathogens and is responsible for human listeriosis, a severe disease with a high mortality rate, especially among the elderly, pregnant women and newborns. Therefore, this bacterium has an important impact on food safety and public health. It is able to survive and even grow in a temperature range from -0.4°C to 45°C, a broad pH range from 4.6 to 9.5 and at a relatively low water activity (aW < 0.90), and tolerates salt content up to 20%. It is also resistant to ultraviolet light, biocides and heavy metals and forms biofilm structures on a variety of surfaces in food-production environments. These features make it difficult to remove and allow it to persist for a long time, increasing the risk of contamination of food-production facilities and ultimately of food. In the present review, the key mechanisms of the pathogen's survival and stress adaptation have been presented. This information may grant better understanding of bacterial adaptation to food environmental conditions.

Contrasting genes conferring short- and long-term biofilm adaptation in Listeria

Listeria monocytogenes is an opportunistic food-borne bacterium that is capable of infecting humans with high rates of hospitalization and mortality. Natural populations are genotypically and phenotypically variable, with some lineages being responsible for most human infections. The success of L. monocytogenes is linked to its capacity to persist on food and in the environment. Biofilms are an important feature that allow these bacteria to persist and infect humans, so understanding the genetic basis of biofilm formation is key to understanding transmission. We sought to investigate the biofilm-forming ability of L. monocytogenes by identifying genetic variation that underlies biofilm formation in natural populations using genome-wide association studies (GWAS). Changes in gene expression of specific strains during biofilm formation were then investigated using RNA sequencing (RNA-seq). Genetic variation associated with enhanced biofilm formation was identified in 273 genes by GWAS and differential expression in 220 genes by RNA-seq. Statistical analyses show that the number of overlapping genes flagged by either type of experiment is less than expected by random sampling. This novel finding is consistent with an evolutionary scenario where rapid adaptation is driven by variation in gene expression of pioneer genes, and this is followed by slower adaptation driven by nucleotide changes within the core genome.

Comparison of prevalence, characterization, antimicrobial resistance and pathogenicity of foodborne Listeria monocytogenes in recent 5 years in Japan

This study investigated the prevalence, serotype, antimicrobial resistance (AMR), virulence potential, and biofilm formation of Listeria monocytogenes isolated in 2022 in Japan and compared their profiles with those of isolates in 2012 and 2017. A total of 85 chicken samples were randomly collected from different supermarkets in Fukuoka in 2022. L. monocytogenes were isolated by conventional method and characterized by MALDI-TOF MS. Among 85 samples tested in 2022, 9 (10.6%) were positive for L. monocytogenes and 17 strains were isolated from the positive samples. The isolates were serotyped as 1/2b (41.2%), 3a (29.4%), 3b (23.5%) and 1/2a (5.9%). Antimicrobial susceptibility tests of the 2022 isolates showed susceptibility to majority of the antibiotics, except cefoxitin, oxacillin, and fosfomycin. Compared to the previous surveillance results, the prevalence of L. monocytogenes in 2022 (10.6%) was significantly lower (p < 0.05) than those of the isolates in 2017 (24%) and 2012 (52.9%). The distribution of serotypes 1/2a and 1/2b decreased over time, and serotype 4b was not detected in the 2022 isolates. The proportion of multidrug resistant strains in 2022 (16.7%) was significantly lower than those in 2012 (46.7%) and 2017 (82.6%). Moreover, a total of 36 isolates (12 isolates/ year) were used to detect the virulence genes (hlyA, plcA, clpC, and inlA) and biofilm-forming capacity. Most of the isolates from different years harboured four virulence genes. The biofilm formation of the 2022 isolates was significantly weaker (p < 0.05) than those of the 2012 and 2017 isolates. Thus, despite the low rates of contamination in chicken meat and AMR of the isolates, virulent L. monocytogenes contamination in food should still be acknowledged.

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

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

The influence of stress factors on selected phenotypic and genotypic features of Listeria monocytogenes - a pilot study

BACKGROUND

Listeria monocytogenes are Gram-positive rods, widespread in the environment due to their wide tolerance to changing conditions. The apilot study aimed to assess the impact of six various stresses (heat, cold, osmotic, acid, alkali, frozen) on phenotypic features: MIC of antibiotics (penicillin, ampicillin, meropenem, erythromycin, co-trimoxazole; gradient stripes), motility, ability to form a biofilm (crystal violet method) and growth rate (OD and quantitative method), expression level of sigB (stress induced regulator of genes), agrA, agrB (associated with biofilm formation) and lmo2230, lmo0596 (acid and alkali stress) (qPCR) for three strains of L. monocytogenes.

RESULTS

Applied stress conditions contributed to changes in phenotypic features and expression levels of sigB, agrA, agrB, lmo2230 and lmo0596. Stress exposure increased MIC value for penicillin (ATCC 19111 - alkaline stress), ampicillin (472CC - osmotic, acid, alkaline stress), meropenem (strains: 55 C - acid, alkaline, o smotic, frozen stress; 472CC - acid, alkaline stress), erythromycin (strains: 55 C - acid stress; 472CC - acid, alkaline, osmotic stress; ATCC 19111 - osmotic, acid, alkaline, frozen stress), co-trimoxazole (strains: 55 C - acid stress; ATCC 19111 - osmotic, acid, alkaline stress). These changes, however, did not affect antibiotic susceptibility. The strain 472CC (a moderate biofilm former) increased biofilm production after exposure to all stress factors except heat and acid. The ATCC 19111 (a weak producer) formed moderate biofilm under all studied conditions except cold and frozen stress, respectively. The strain 55 C became a strong biofilm producer after exposure to cold and produced a weak biofilm in response to frozen stress. Three tested strains had lower growth rate (compared to the no stress variant) after exposure to heat stress. It has been found that the sigB transcript level increased under alkaline (472CC) stress and the agrB expression increased under cold, osmotic (55 C, 472CC), alkali and frozen (472CC) stress. In contrast, sigB transcript level decreased in response to acid and frozen stress (55 C), lmo2230 transcript level after exposure to acid and alkali stress (ATCC 19111), and lmo0596 transcript level after exposure to acid stress (ATCC 19111).

CONCLUSIONS

Environmental stress changes the ability to form a biofilm and the MIC values of antibiotics and affect the level of expression of selected genes, which may increase the survival and virulence of L. monocytogenes. Further research on a large L. monocytogenes population is needed to assess the molecular mechanism responsible for the correlation of antibiotic resistance, biofilm formation and resistance to stress factors.

The Probiotic Bacillus subtilis MB40 Improves Immunity in a Porcine Model of Listeriosis

Probiotics for humans and direct-fed microbials for livestock are increasingly popular dietary ingredients for supporting immunity. The aim of this study was to determine the effects of dietary supplementation of Bacillus subtilis MB40 (MB40) on immunity in piglets challenged with the foodborne pathogen Listeria monocytogenes (LM). Three-week-old piglets (n = 32) were randomly assigned to four groups: (1) basal diet, (2) basal diet with LM challenge, (3) MB40-supplemented diet, and (4) MB40-supplemented diet with LM challenge. Experimental diets were provided throughout a 14-day (d) period. On d8, piglets in groups 2 and 4 were intraperitoneally inoculated with LM at 108 CFU/mL per piglet. Blood samples were collected at d1, d8, and d15 for biochemical and immune response profiling. Animals were euthanized and necropsied at d15 for liver and spleen bacterial counts and intestinal morphological analysis. At d15, LM challenge was associated with increased spleen weight (p = 0.017), greater circulating populations of neutrophils (p = 0.001) and monocytes (p = 0.008), and reduced ileal villus height to crypt depth ratio (p = 0.009), compared to non-challenged controls. MB40 supplementation reduced LM bacterial counts in the liver and spleen by 67% (p < 0.001) and 49% (p < 0.001), respectively, following the LM challenge, compared to the basal diet. MB40 supplementation was also associated with decreased circulating concentrations of monocytes (p = 0.007). Altogether, these data suggest that MB40 supplementation is a safe and well-tolerated approach to enhance immunity during systemic Listeria infection.

Listeria monocytogenes Strains Persisting in a Meat Processing Plant in Central Italy: Use of Whole Genome Sequencing and In Vitro Adhesion and Invasion Assays to Decipher Their Virulence Potential

In this study, we used both a WGS and an in vitro approach to study the virulence potential of nine Listeria monocytogenes (Lm) strains belonging to genetic clusters persisting in a meat processing plant in Central Italy. The studied clusters belonged to CC1-ST1, CC9-ST9, and CC218-ST2801. All the CC1 and CC218 strains presented the same accessory virulence genes (LIPI-3, gltA, gltB, and aut_IVb). CC1 and CC9 strains presented a gene profile similarity of 22.6% as well as CC9 and CC218 isolates. CC1 and CC218 showed a similarity of 45.2% of the same virulence profile. The hypervirulent strains of lineage I (CC1 and CC218) presented a greater ability to adhere and invade Caco-2 cells than hypovirulent ones (CC9). CC1 strains were significantly more adhesive and invasive compared with CC9 and CC218 strains, although these last CCs presented the same accessory virulence genes. No statistically significant difference was found comparing CC218 with CC9 strains. This study provided for the first time data on the in vitro adhesiveness and invasiveness of CC218-ST2801 and added more data on the virulence characteristics of CC1 and CC9. What we observed confirmed that the ability of Lm to adhere to and invade human cells in vitro is not always decipherable from its virulence gene profile.

Genomic and pathogenicity islands of Listeria monocytogenes-overview of selected aspects

Listeria monocytogenes causes listeriosis, a disease characterized by a high mortality rate (up to 30%). Since the pathogen is highly tolerant to changing conditions (high and low temperature, wide pH range, low availability of nutrients), it is widespread in the environment, e.g., water, soil, or food. L. monocytogenes possess a number of genes that determine its high virulence potential, i.e., genes involved in the intracellular cycle (e.g., prfA, hly, plcA, plcB, inlA, inlB), response to stress conditions (e.g., sigB, gadA, caspD, clpB, lmo1138), biofilm formation (e.g., agr, luxS), or resistance to disinfectants (e.g., emrELm, bcrABC, mdrL). Some genes are organized into genomic and pathogenicity islands. The islands LIPI-1 and LIPI-3 contain genes related to the infectious life cycle and survival in the food processing environment, while LGI-1 and LGI-2 potentially ensure survival and durability in the production environment. Researchers constantly have been searching for new genes determining the virulence of L. monocytogenes. Understanding the virulence potential of L. monocytogenes is an important element of public health protection, as highly pathogenic strains may be associated with outbreaks and the severity of listeriosis. This review summarizes the selected aspects of L. monocytogenes genomic and pathogenicity islands, and the importance of whole genome sequencing for epidemiological purposes.

Antibiotic Resistance in Selected Emerging Bacterial Foodborne Pathogens-An Issue of Concern?

Antibiotic resistance (AR) and multidrug resistance (MDR) have been confirmed for all major foodborne pathogens: Campylobacter spp., Salmonella spp., Escherichia coli and Listeria monocytogenes. Of great concern to scientists and physicians are also reports of antibiotic-resistant emerging food pathogens-microorganisms that have not previously been linked to food contamination or were considered epidemiologically insignificant. Since the properties of foodborne pathogens are not always sufficiently recognized, the consequences of the infections are often not easily predictable, and the control of their activity is difficult. The bacteria most commonly identified as emerging foodborne pathogens include Aliarcobacter spp., Aeromonas spp., Cronobacter spp., Vibrio spp., Clostridioides difficile, Escherichia coli, Mycobacterium paratuberculosis, Salmonella enterica, Streptocccus suis, Campylobacter jejuni, Helicobacter pylori, Listeria monocytogenes and Yersinia enterocolitica. The results of our analysis confirm antibiotic resistance and multidrug resistance among the mentioned species. Among the antibiotics whose effectiveness is steadily declining due to expanding resistance among bacteria isolated from food are β-lactams, sulfonamides, tetracyclines and fluoroquinolones. Continuous and thorough monitoring of strains isolated from food is necessary to characterize the existing mechanisms of resistance. In our opinion, this review shows the scale of the problem of microbes related to health, which should not be underestimated.

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.

Molecular typing and genome sequencing allow the identification of persistent Listeria monocytogenes strains and the tracking of the contamination source in food environments

The presence of Listeria monocytogenes (Lm) in the food processing environment (facilities and products) is a challenging problem in food safety management. Lm is one of the main causes of mortality in foodborne infections, and the trend is continuously increasing. In this study, a collection of 323 Lm strain isolates recovered from food matrices and food industry environments (surfaces and equipment) over four years from 80 food processing facilities was screened using a restriction site-associated tag sequencing (2b-RAD) typing approach developed for Lm. Thirty-six different restriction site-associated DNA (RAD) types (RTs) were identified, most of which correspond to lineage II. RT1, the most represented genotype in our collection and already reported as one of the most prevalent genotypes in the food environment, was significantly associated with meat processing facilities. The sequencing of the genomes of strains belonging to the same RT and isolated in the same facility in different years revealed several clusters of persistence. The definition of the persistent strains (PSs) allowed the identification of the potential source of contamination in the incoming raw meat that is introduced in the facility to be processed. The slaughterhouses, which, according to the European Union (EU) regulation, are not inspected for the presence of Lm could be hotspots for the persistence of Lm PSs.

Deciphering the global roles of Cold shock proteins in Listeria monocytogenes nutrient metabolism and stress tolerance

Listeria monocytogenes (Lm) accounts for serious public health and food safety problems owing to its stress resilience and pathogenicity. Based on their regulatory involvement in global gene expression events, cold-shock domain family proteins (Csps) are crucial in expression of various stress fitness and virulence phenotypes in bacteria. Lm possesses three Csps (CspA, CspB, and CspD) whose regulatory roles in the context of the genetic diversity of this bacterium are not yet fully understood. We examined the impacts of Csps deficiency on Lm nutrient metabolism and stress tolerance using a set of csp deletion mutants generated in different genetic backgrounds. Phenotype microarrays (PM) analysis showed that the absence of Csps in ∆cspABD reduces carbon (C-) source utilization capacity and increases Lm sensitivity to osmotic, pH, various chemical, and antimicrobial stress conditions. Single and double csp deletion mutants in different Lm genetic backgrounds were used to further dissect the roles of individual Csps in these phenotypes. Selected PM-based observations were further corroborated through targeted phenotypic assays, confirming that Csps are crucial in Lm for optimal utilization of various C-sources including rhamnose and glucose as well as tolerance against NaCl, β-phenyethylamine (PEA), and food relevant detergent stress conditions. Strain and genetic lineage background-based differences, division of labour, epistasis, and functional redundancies among the Csps were uncovered with respect to their roles in various processes including C-source utilization, cold, and PEA stress resistance. Finally, targeted transcriptome analysis was performed, revealing the activation of csp gene expression under defined stress conditions and the impact of Csps on expression regulation of selected rhamnose utilization genes. Overall, our study shows that Csps play important roles in nutrient utilization and stress responses in Lm strains, contributing to traits that are central to the public health and food safety impacts of this pathogen.

Comparative Genomics of Listeria Species Recovered from Meat and Food Processing Facilities

Listeria species (spp.) are contaminants that can survive in food, on equipment, and on food processing premises if appropriate hygiene measures are not used. Homologous stress tolerance genes, virulence gene clusters such as the prfA cluster, and clusters of internalin genes that contribute to the pathogenic potential of the strains can be carried by both pathogenic and nonpathogenic Listeria spp. To enhance understanding of the genome evolution of virulence and virulence-associated properties, a comparative genome approach was used to analyze 41 genome sequences belonging to L. innocua and L. welshimeri isolated from food and food processing facilities. Genetic determinants responsible for disinfectant and stress tolerance were identified, including the efflux cassette bcrABC and Tn6188_qac_1 disinfectant resistance determinant, and stress survival islets. These disinfectant-resistant genes were more frequently found in L. innocua (12%) than in L. welshimeri (2%). Several isolates representing the presumed nonpathogenic L. innocua still carried virulence-associated genes, including LGI2, LGI3, LIPI-3, and LIPI-4 which were absent in all L. welshimeri isolates. The mobile genetic elements identified were plasmids (pLGUG1 and J1776) and prophages (PHAGE_Lister_vB_LmoS_188, PHAGE_Lister_LP_030_3, PHAGE_Lister_A118, PHAGE_Lister_B054, and PHAGE_Lister_vB_LmoS_293). The results suggest that the presumed nonpathogenic isolates especially L. innocua can carry genes relevant to the strain's virulence and stress tolerance in the food and food processing facilities. IMPORTANCE This study provides genomic insights into the recently expanded genus in order to gain valuable information about the evolution of the virulence and stress tolerance properties of the genus Listeria and the distribution of these genetic elements pertinent to the pathogenic potential across Listeria spp. and clonal lineages in South Africa (SA).

A subtractive proteomics and immunoinformatics approach towards designing a potential multi-epitope vaccine against pathogenic Listeriamonocytogenes

Listeria monocytogenes is the causative agent of listeriosis, which is dangerous for pregnant women, the elderly or individuals with a weakened immune system. Individuals with leukaemia, cancer, HIV/AIDS, kidney transplant and steroid therapy suffer from immunological damage are menaced. World Health Organization (WHO) reports that human listeriosis has a high mortality rate of 20-30% every year. To date, no vaccine is available to treat listeriosis. Thereby, it is high time to design novel vaccines against L. monocytogenes. Here, we present computational approaches to design an antigenic, stable and safe vaccine against the L. monocytogenes that could help to control the infections associated with the pathogen. Three vital pathogenic proteins of L. monocytogenes, such as Listeriolysin O (LLO), Phosphatidylinositol-specific phospholipase C (PI-PLC), and Actin polymerization protein (ActA), were selected using a subtractive proteomics approach to design the multi-epitope vaccine (MEV). A total of 5 Cytotoxic T-lymphocyte (CTL) and 9 Helper T-lymphocyte (HTL) epitopes were predicted from these selected proteins. To design the multi-epitope vaccine (MEV) from the selected proteins, CTL epitopes were joined with the AAY linker, and HTL epitopes were joined with the GPGPG linker. Additionally, a human β-defensin-3 (hBD-3) adjuvant was added to the N-terminal side of the final MEV construct to increase the immune response to the vaccine. The final MEV was predicted to be antigenic, non-allergen and non-toxic in nature. Physicochemical property analysis suggested that the MEV construct is stable and could be easily purified through the E. coli expression system. This in-silico study showed that MEV has a robust binding interaction with Toll-like receptor 2 (TLR2), a key player in the innate immune system. Current subtractive proteomics and immunoinformatics study provides a background for designing a suitable, safe and effective vaccine against pathogenic L. monocytogenes.

Listeria monocytogenes is a solvent tolerant organism secreting a solvent stable lipase: potential biotechnological applications

Purpose

The emerging biobased economy will require robust, adaptable, organisms for the production and processing of biomaterials as well as for bioremediation. Recently, the search for solvent tolerant organisms and solvent tolerant enzymes has intensified. Resilient organisms secreting solvent stable lipases are of particular interest for biotechnological applications.

Methods

Screening of soil samples for lipase-producing organisms was carried out on Rhodamine B plates. The most productive lipase-producing organisms were further screened for their resistance to solvents commonly used in biotechnological applications.

Results

In the course of screening, one of the isolated organisms that exhibited extracellular lipase activity, was identified as the human pathogen Listeria monocytogenes through 16S rRNA sequencing. Further exploration revealed that this organism was resistant to solvents ranging from log P − 0.81 to 4.0. Moreover, in the presence of these solvents, L. monocytogenes secreted an extracellular, solvent tolerant, lipase activity. This lipase retained approximately 80% activity when incubated in 30% (v/v) methanol for 24 h.

Conclusion

These findings identify L. monocytogenes as a potentially useful organism for biotechnological applications. However, the fact that Listeria is a pathogen is problematic and it will require the use of non-pathogenic or attenuated Listeria strains for practical applications. Nonetheless, the ability to adapt to rapidly changing environmental conditions, to grow at low temperatures, to resist solvents and to secrete an extracellular solvent tolerant lipase are unique and highly useful characteristics. The potential application of L. monocytogenes in wastewater bioremediation and plastics degradation is discussed.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10529-022-03284-5.

Persistence of Listeria monocytogenes ST5 in Ready-to-Eat Food Processing Environment

Most human listeriosis is foodborne, and ready-to-eat (RET) foods contaminated by Listeria monocytogenes during processing are found to be common vehicles. In this study, a total of four L. monocytogens STs (ST5, ST121, ST120, and ST2) have been identified in two RTE food plants from 2019 to 2020 in Shanghai, China. The L. monocytogenes ST5 was predominant in one RTE food processing plant, and it persists in the RTE meat processing plant with continued clone transmission. The genetic features of the four STs isolates were different. ST5 and ST121 had the three genes clpL, mdrL, and lde; however, ST120 and ST2 had two genes except for clpL. SSI-1was present in ST5, ST121, and ST120. Additionally, SSI-2 was present only in the ST121 isolates. ST120 had all six biofilm-forming associated genes (actA, prfA, lmo0673, recO, lmo2504 and luxS). The ST2 isolate had only three biofilm-forming associated genes, which were prfA, lmo0673, and recO. The four ST isolates had different biofilm formation abilities at different stages. The biofilm formation ability of ST120 was significantly higher when grown for one day. However, the biofilm formation ability of ST120 reduced significantly after growing for four days. In contrast, the biofilm formation ability of ST5 and ST121 increased significantly. These results suggested that ST5 and ST121 had stronger ability to adapt to stressful environments. Biofilms formed by all four STs grown over four days can be sanitized entirely by a disinfectant concentration of 500 mg/L. Additionally, only ST5 and ST121 biofilm cells survived in sub-lethal concentrations of chlorine-containing disinfectant. These results suggested that ST5 and ST121 were more resistant to chlorine-containing disinfectants. These results indicated that the biofilm formation ability of L. monocytogenes isolates changed at different stages. Additionally, the persistence in food processing environments might be verified by the biofilm formation, stress resistance, etc. Alternatively, these results underlined that disinfectants should be used at lethal concentrations. More attention should be paid to ST5 and ST121, and stronger surveillance should be taken to prevent and control the clonal spread of L. monocytogenes isolates in food processing plants in Shanghai.

Listeria monocytogenes Pathogenesis: The Role of Stress Adaptation

Adaptive stress tolerance responses are the driving force behind the survival ability of Listeria monocytogenes in different environmental niches, within foods, and ultimately, the ability to cause human infections. Although the bacterial stress adaptive responses are primarily a necessity for survival in foods and the environment, some aspects of the stress responses are linked to bacterial pathogenesis. Food stress-induced adaptive tolerance responses to acid and osmotic stresses can protect the pathogen against similar stresses in the gastrointestinal tract (GIT) and, thus, directly aid its virulence potential. Moreover, once in the GIT, the reprogramming of gene expression from the stress survival-related genes to virulence-related genes allows L. monocytogenes to switch from an avirulent to a virulent state. This transition is controlled by two overlapping and interlinked transcriptional networks for general stress response (regulated by Sigma factor B, (SigB)) and virulence (regulated by the positive regulatory factor A (PrfA)). This review explores the current knowledge on the molecular basis of the connection between stress tolerance responses and the pathogenesis of L. monocytogenes. The review gives a detailed background on the currently known mechanisms of pathogenesis and stress adaptation. Furthermore, the paper looks at the current literature and theories on the overlaps and connections between the regulatory networks for SigB and PrfA.

FepR as a Central Genetic Target in the Adaptation to Quaternary Ammonium Compounds and Cross-Resistance to Ciprofloxacin in Listeria monocytogenes

The foodborne pathogen, Listeria monocytogenes, (Lm), frequently undergoes selection pressure associated with the extensive use of disinfectants, such as quaternary ammonium compounds, which are widely used in food processing plants. The repeated exposure to sub-inhibitory biocide concentrations can induce increased tolerance to these compounds, but can also trigger the development of antibiotic resistance, and both increase the risk of food contamination and persistence in food production environments. Although the acquisition of genes can explain biocide tolerance, the genetic mechanisms underlying the adaptive cross-resistance to antibiotics remain unclear. We previously showed that repeated exposure to benzalkonium chloride (BC) and didecyldimethyl ammonium chloride (DDAC) led to reduced susceptibility to ciprofloxacin in Lm strains from diverse sources. Here, we compared the genomes of 16 biocide-adapted and 10 parental strains to identify the molecular mechanisms of fluoroquinolone cross-resistance. A core genome SNP analysis identified various mutations in the transcriptional regulator fepR (lmo2088) for 94% of the adapted strains and mutations in other effectors at a lower frequency. FepR is a local repressor of the MATE fluoroquinolone efflux pump FepA. The impact of the mutations on the structure and function of the protein was assessed by performing in silico prediction and protein homology modeling. Our results show that 75% of the missense mutations observed in fepR are located in the HTH domain of the protein, within the DNA interaction site. These mutations are predicted to reduce the activity of the regulator, leading to the overexpression of the efflux pump responsible for the ciprofloxacin-enhanced resistance.

Use of Whole Genome Sequencing by the Federal Interagency Collaboration for Genomics for Food and Feed Safety in the United States

ABSTRACT

This multiagency report developed by the Interagency Collaboration for Genomics for Food and Feed Safety provides an overview of the use of and transition to whole genome sequencing (WGS) technology for detection and characterization of pathogens transmitted commonly by food and for identification of their sources. We describe foodborne pathogen analysis, investigation, and harmonization efforts among the following federal agencies: National Institutes of Health; Department of Health and Human Services, Centers for Disease Control and Prevention (CDC) and U.S. Food and Drug Administration (FDA); and the U.S. Department of Agriculture, Food Safety and Inspection Service, Agricultural Research Service, and Animal and Plant Health Inspection Service. We describe single nucleotide polymorphism, core-genome, and whole genome multilocus sequence typing data analysis methods as used in the PulseNet (CDC) and GenomeTrakr (FDA) networks, underscoring the complementary nature of the results for linking genetically related foodborne pathogens during outbreak investigations while allowing flexibility to meet the specific needs of Interagency Collaboration partners. We highlight how we apply WGS to pathogen characterization (virulence and antimicrobial resistance profiles) and source attribution efforts and increase transparency by making the sequences and other data publicly available through the National Center for Biotechnology Information. We also highlight the impact of current trends in the use of culture-independent diagnostic tests for human diagnostic testing on analytical approaches related to food safety and what is next for the use of WGS in the area of food safety.

HIGHLIGHTS

Listeria monocytogenes - How This Pathogen Survives in Food-Production Environments?

The foodborne pathogen Listeria monocytogenes is the causative agent of human listeriosis, a severe disease, especially dangerous for the elderly, pregnant women, and newborns. Although this infection is comparatively rare, it is often associated with a significant mortality rate of 20-30% worldwide. Therefore, this microorganism has an important impact on food safety. L. monocytogenes can adapt, survive and even grow over a wide range of food production environmental stress conditions such as temperatures, low and high pH, high salt concentration, ultraviolet lights, presence of biocides and heavy metals. Furthermore, this bacterium is also able to form biofilm structures on a variety of surfaces in food production environments which makes it difficult to remove and allows it to persist for a long time. This increases the risk of contamination of food production facilities and finally foods. The present review focuses on the key issues related to the molecular mechanisms of the pathogen survival and adaptation to adverse environmental conditions. Knowledge and understanding of the L. monocytogenes adaptation approaches to environmental stress factors will have a significant influence on the development of new, efficient, and cost-effective methods of the pathogen control in the food industry, which is critical to ensure food production safety.

The Viable But Non-Culturable State of Listeria monocytogenes in the One-Health Continuum

Many bacterial species, including several pathogens, can enter a so-called “viable but non-culturable” (VBNC) state when subjected to stress. Bacteria in the VBNC state are metabolically active but have lost their ability to grow on standard culture media, which compromises their detection by conventional techniques based on bacterial division. Under certain conditions, VBNC bacteria can regain their growth capacity and, for pathogens, their virulence potential, through a process called resuscitation. Here, we review the current state of knowledge of the VBNC state of Listeria monocytogenes (Lm), a Gram-positive pathogenic bacterium responsible for listeriosis, one of the most dangerous foodborne zoonosis. After a brief summary of characteristics of VBNC bacteria, we highlight work on VBNC Lm in the environment and in agricultural and food industry settings, with particular emphasis on the impact of antimicrobial treatments. We subsequently discuss recent data suggesting that Lm can enter the VBNC state in the host, raising the possibility that VBNC forms contribute to the asymptomatic carriage of this pathogen in wildlife, livestock and even humans. We also consider the resuscitation and virulence potential of VBNC Lm and the danger posed by these bacteria to at-risk individuals, particularly pregnant women. Overall, we put forth the hypothesis that VBNC forms contribute to adaptation, persistence, and transmission of Lm between different ecological niches in the One-Health continuum, and suggest that screening for healthy carriers, using alternative techniques to culture-based enrichment methods, should better prevent listeriosis risks.

Different Shades of Listeria monocytogenes: Strain, Serotype, and Lineage-Based Variability in Virulence and Stress Tolerance Profiles

Listeria monocytogenes is a public health and food safety challenge due to its virulence and natural stress resistance phenotypes. The variable distribution of L. monocytogenes molecular subtypes with respect to food products and processing environments and among human and animal clinical listeriosis cases is observed. Sixty-two clinical and food-associated L. monocytogenes isolates were examined through phenome and genome analysis. Virulence assessed using a zebrafish infection model revealed serotype and genotype-specific differences in pathogenicity. Strains of genetic lineage I serotype 4b and multilocus sequence type clonal complexes CC1, CC2, CC4, and CC6 grew and survived better and were more virulent than serotype 1/2a and 1/2c lineage II, CC8, and CC9 strains. Hemolysis, phospholipase activity, and lysozyme tolerance profiles were associated with the differences observed in virulence. Osmotic stress resistance evaluation revealed serotype 4b lineage I CC2 and CC4 strains as more osmotolerant, whereas serotype 1/2c lineage II CC9 strains were more osmo-sensitive than others. Variable tolerance to the widely used quaternary ammonium compound benzalkonium chloride (BC) was observed. Some outbreak and sporadic clinical case associated strains demonstrated BC tolerance, which might have contributed to their survival and transition in the food-processing environment facilitating food product contamination and ultimately outbreaks or sporadic listeriosis cases. Genome comparison uncovered various moderate differences in virulence and stress associated genes between the strains indicating that these differences in addition to gene expression regulation variations might largely be responsible for the observed virulence and stress sensitivity phenotypic differences. Overall, our study uncovered strain and genotype-dependent variation in virulence and stress resilience among clinical and food-associated L. monocytogenes isolates with potential public health risk implications. The extensive genome and phenotypic data generated provide a basis for developing improved Listeria control strategies and policies.

Cold Shock Proteins Promote Nisin Tolerance in Listeria monocytogenes Through Modulation of Cell Envelope Modification Responses

Listeria monocytogenes continues to be a food safety challenge owing to its stress tolerance and virulence traits. Several listeriosis outbreaks have been linked to the consumption of contaminated ready-to-eat food products. Numerous interventions, including nisin application, are presently employed to mitigate against L. monocytogenes risk in food products. In response, L. monocytogenes deploys several defense mechanisms, reducing nisin efficacy, that are not yet fully understood. Cold shock proteins (Csps) are small, highly conserved nucleic acid-binding proteins involved in several gene regulatory processes to mediate various stress responses in bacteria. L. monocytogenes possesses three csp gene paralogs; cspA, cspB, and cspD. Using a panel of single, double, and triple csp gene deletion mutants, the role of Csps in L. monocytogenes nisin tolerance was examined, demonstrating their importance in nisin stress responses of this bacterium. Without csp genes, a L. monocytogenes ΔcspABD mutant displayed severely compromised growth under nisin stress. Characterizing single (ΔcspA, ΔcspB, and ΔcspD) and double (ΔcspBD, ΔcspAD, and ΔcspAB) csp gene deletion mutants revealed a hierarchy (cspD > cspB > cspA) of importance in csp gene contributions toward the L. monocytogenes nisin tolerance phenotype. Individual eliminations of either cspA or cspB improved the nisin stress tolerance phenotype, suggesting that their expression has a curbing effect on the expression of nisin resistance functions through CspD. Gene expression analysis revealed that Csp deficiency altered the expression of DltA, MprF, and penicillin-binding protein-encoding genes. Furthermore, the ΔcspABD mutation induced an overall more electronegative cell surface, enhancing sensitivity to nisin and other cationic antimicrobials as well as the quaternary ammonium compound disinfectant benzalkonium chloride. These observations demonstrate that the molecular functions of Csps regulate systems important for enabling the constitution and maintenance of an optimal composed cell envelope that protects against cell-envelope-targeting stressors, including nisin. Overall, our data show an important contribution of Csps for L. monocytogenes stress protection in food environments where antimicrobial peptides are used. Such knowledge can be harnessed in the development of better L. monocytogenes control strategies. Furthermore, the potential that Csps have in inducing cross-protection must be considered when combining hurdle techniques or using them in a series.

Surveillance of Fresh Artisanal Cheeses Revealed High Levels of Listeria monocytogenes Contamination in the Department of Quindío, Colombia

Listeriosis is a foodborne disease caused by Listeria monocytogenes. Because outbreaks of listeriosis are associated with the ingestion of contaminated dairy products, surveillance of artisanal cheeses to detect the presence of this microorganism is necessary. We collected three types of artisanal non-acid fresh cheese (Campesino, Costeño, and Cuajada) from 12 municipalities of the Department of Quindío, Colombia. L. monocytogenes was identified using VIDAS^® and confirmed with API^® Listeria Rapid Kit. L. monocytogenes was detected in 104 (53.6%) of the 194 artisanal fresh-cheese samples analyzed. The highest percentages of contamination were detected in Salento (90.9%), Calracá (65.5%), Armenia (64.9%), and Filandia (50%). A significant association between municipality and contamination with L. monocytogenes was identified. However, no association could be established between the type of cheese and the occurrence of the bacterium. This is the first study on the presence of L. monocytogenes in artisanal fresh cheeses sold in the municipalities of the Department of Quindío, and the findings revealed very high percentages of contaminated samples. The presence of L. monocytogenes in artisanal cheeses remains a public health threat in developing countries, especially Colombia, where existing legislation does not require the surveillance of L. monocytogenes in food.