Tolerance of Listeria monocytogenes to biocides used in food processing environments

Antilisterial activity of Thymus vulgaris essential oil: In vitro, in situ, and in silico investigations

Listeria monocytogenes is a major foodborne pathogen that significantly threatens public health and food safety. While Thymus vulgaris essential oil (TV-EO) is widely recognized for its potent antibacterial activity, its specific effects against L. monocytogenes remain unexplored. This study aimed to assess the antilisterial activity of TV-EO using in vitro, in situ, and in silico approaches. The in vitro assessment included disc diffusion method, determination of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC), biofilm inhibition assay, and predictive modeling to assess L. monocytogenes reduction in the presence of TV-EO at 10 °C and 20 °C. In situ approach evaluated the inhibitory effect of TV-EO on L. monocytogenes in minced poultry meat stored at 4 °C. Finally, in silico approach, based on molecular docking, was employed to evaluate the binding affinity of major TV-EO components for β-ketoacyl-ACP synthase II and chorismate synthase, key proteins involved in fatty acid biosynthesis and biofilm formation, respectively. Our finding revealed that TV-EO exhibited strong in vitro antilisterial activity, with inhibitory zones ranging from 51.00 ± 1.00 mm to 55.67 ± 1.15 mm, a MIC value of 0.125 %, and a MBC value of 0.25 %, indicating its bactericidal effect. TV-EO at 0.125 % demonstrated a high capacity to inhibit and eradicate the biofilm, with 100 ± 0.00 % and 91.33 ± 1.23 %, respectively. Predictive modeling, based on the combination of TV-EO and ζ values, revealed that L. monocytogenes inactivation was more pronounced at low temperature. Furthermore, the in-situ approach showed a significant reduction of L. monocytogenes amount, with decreases of 1.068 ± 0.132 log cfu/g, 0.671 ± 0.091 log cfu/g, and 0.317 ± 0.029 log cfu/g at TV-EO concentrations of 1 %, 0.5 %, and 0.25 %, respectively (p < 0.05). In silico analysis indicated that TV-EO components, particularly carvacrol, exhibited high affinity for β-ketoacyl-ACP synthase II and chorismate cynthase, suggesting strong antilisterial and ani-biofilm activity. These findings highlight the antilisterial efficacy of TV-EO, demonstrating its potential as a natural alternative to conventional preservatives for enhancing food preservation and safety.

From Dormancy to Eradication: Strategies for Controlling Bacterial Persisters in Food Settings

Bacterial persistence, a dormant state that enables microorganisms to survive harsh conditions, is a significant concern in food-industry settings, where traditional antimicrobial treatments often fail to eliminate these resilient cells. This article goes beyond conventional review by compiling critical information aimed at providing practical solutions to combat bacterial persisters in food production environments. This review explores the primary mechanisms behind persister cell formation, including toxin-antitoxin systems, the alarmone guanosine tetraphosphate (ppGpp), stochastic processes (in which persistence occurs as a random event), and the SOS response. Given the serious implications for food safety and quality, the authors also report a range of physical, chemical, and biological methods for targeting and eradicating persister cells. The strategies discussed, whether applied individually or in combination, offer varying levels of availability and applicability within the industry and can serve as a guide for implementing microbial contamination control plans. While significant progress has been achieved, further research is crucial to fully understand the complex mechanisms underlying bacterial persistence in food and to develop effective and targeted strategies for its eradication in food-industry settings. Overall, the translation of these insights into practical applications aims to support the food industry in overcoming this persistent challenge, ensuring safer, more sustainable food production.

Effect of neutral electrolyzed water on biofilm formed by meat-related Listeria monocytogenes: Intraspecies variability and influence of the growth surface material

Listeria monocytogenes raises major challenges for the food industry. Due to its capacity to form biofilms, this pathogen can persist in processing environments and contaminate the final products. Neutral electrolyzed water (NEW) may offer a promising and eco-friendly method for controlling L. monocytogenes biofilms, though current in vitro studies on its antibiofilm activity are limited and often focused on reference strains. In this study, we assessed the effect of NEW on biofilms formed by meat-related and reference L. monocytogenes strains on polystyrene and stainless steel. Forty wild-type strains isolated from meat products and processing environments were firstly screened for their biofilm-forming abilities and classified as weak (30 %; 12/40), moderate (55 %; 22/40), and strong (15 %; 6/40) biofilm producers. Twenty-two wild-type and two reference strains were selected for the eradication assays, performed by treating the biofilms with NEW for 9 minutes of total contact time. In silico functional enrichment analysis and the visualization of biofilms by scanning electron microscopy (SEM) were also performed. The NEW treatment resulted in a greater average reduction of viable cells in biofilms formed on polystyrene (4.3 ± 1.0 log10 CFU/cm2) compared to stainless steel (2.9 ± 2.0 log10 CFU/cm2), and a remarkable intraspecies variability was observed. SEM images revealed higher structural damage on biofilms formed on polystyrene. Functional enrichment analysis suggested that clustered regularly interspaced short palindromic repeats (CRISPR)-associated elements could be involved in resistance to the treatments. NEW could be a promising additional tool to mitigate L. monocytogenes biofilms in meat processing environments, although its effect varied with surface material and strain-specific characteristics.

Antimicrobial nanoparticle-containing food packaging films for controlling Listeria spp.: An overview

Bacteria of the genus Listeria are ubiquitous in nature and are found in various food products and food processing facilities. The species Listeria monocytogenes is a food-borne pathogen that causes listeriosis with a high fatality rate. For the prevention and control of listeriosis, the identification of effective antilisterial compounds is desirable. The number of investigations on nanoparticles (NPs) with antimicrobial activity has increased in recent years. In this context, green nanotechnology is a field of science that focuses on the synthesis of NPs through biological pathways using a wide range of microorganisms and plant extracts, which has led to the biofabrication of novel antimicrobial agents that have demonstrated remarkable potential against pathogenic bacteria. In this review, in vitro studies of the inhibitory action of antimicrobial NPs obtained by green biosynthesis, including silver, gold, zinc, zinc oxide, copper, palladium, and selenium NPs, on the growth of Listeria spp. were comprehensively summarized. This review mainly highlights antimicrobial NPs in biopolymer films against L. monocytogenes. Furthermore, studies on NPs in biopolymer-based functional food packaging films against L. monocytogenes are listed. Finally, safety considerations are indicated. This review provides an overview of the antilisterial activity of bio-based antimicrobial NPs and the potential of nanotechnology as an innovative technology for the development of food packaging films containing antimicrobial NPs to control Listeria spp.

Listeria monocytogenes, Salmonella spp., and Staphylococcus aureus: Threats to the Food Industry and Public Health

Foodborne pathogens have always been of public health concern and represent safety issues for food processors. These pathogens develop new ways to overcome antibiotics, survive in different environmental conditions, and the ability to reproduce in many hostile environments configure them as serious health hazards. Considering the huge number of microorganisms, three bacterial representatives were selected to provide a better knowledge about the question of which one is the worst enemy for humans, from the food industry point of view, taking into consideration their multiplication specificity, virulence, and resistance. As we constantly are exposed to these pathogens in our nutritional habits, this overview aims to summarize the most relevant characteristics associated with the pathogenicity, clinical symptoms and most importantly, how deadly Listeria monocytogenes, Salmonella spp., and Staphylococcus aureus can be in the hospital and the food industry, by comparing among them. Overall, the microbiological knowledge clearly suggests that while all three pathogens are dangerous, L. monocytogenes presents the highest risk of death due to their ability to cause severe complications in vulnerable populations as it presents a range of virulence factors that facilitate evasion of the immune system and cytological effects. Additionally, it shows great resistance to standard food processing and preservation techniques, making it one of the most difficult pathogens to control. Understanding the risks and characteristics of these foodborne pathogens is essential for implementing effective control measures to prevent their occurrence in food products and to promote public health.

Recent advances in examining the factors influencing the efficacy of biocides against Listeria monocytogenes biofilms in the food industry: A systematic review

Controlling Listeria monocytogenes and its associated biofilms in the food industry requires various disinfection techniques, including physical, chemical, and biological treatments. Biocides, owing to their ease of use, cost-effectiveness, dissolvability in water, and efficacy against a wide range of microorganisms, are frequently selected options. Nonetheless, concerns have been raised about their efficacy in controlling L. monocytogenes biofilm, as laboratory-based and commercial studies have reported the persistence of this bacterium after cleaning and disinfection. This review systematically examined scientific studies, sourced from the Web of Science, Scopus, and PubMed databases between January 2010 and May 2024, that investigated the effectiveness of the most commonly used biocides in the food industry against L. monocytogenes biofilms. A total of 92 articles which met the screening criteria, were included, with studies utilizing biocides containing sodium hypochlorite, quaternary ammonium compounds, and peroxyacetic acid being predominant. Studies indicated that several key factors may potentially influence biocides' efficacy against L. monocytogenes biofilms. These factors included strain type (persistent, sporadic), serotype, strain origin (clinical, environmental, or food), surface type (biotic or abiotic), surface material (stainless steel, polystyrene, etc.), incubation time (biofilm age) and temperature, presence of organic matter, biocide's active agent, and the co-culture of L. monocytogenes with other bacteria. The induction of the viable but nonculturable (VBNC) state following disinfection is also a critical concern. This review aims to provide a global understanding of how L. monocytogenes biofilms respond to biocides under different treatment conditions, facilitating the development of effective cleaning and disinfection strategies in the food industry.

"Four - in - one" platform based on multifunctional nanozyme for ultra - accurate detection and on - demand disinfection of Listeria monocytogenes

The inability to integrate detection and disinfection hindered building a unified pathogen monitoring platform, risking secondary contamination. Herein, a novel "four - in - one" platform for monitoring foodborne Listeria monocytogenes (L. monocytogenes) was presented. The magnetic daptomycin - functionalized Fe3O4 (Dap/Fe3O4) could selectively bind to L. monocytogenes, enhancing detection accuracy. The separated bacteria were captured by aptamers - functionalized Fe - doped - silica nanoparticles (Apt/Fe@SiNPs) for tri - mode detection. Besides fluorescence, the Apt/Fe@SiNPs converted 3,3',5,5' - tetramethylbenzidine (TMB) to oxidized TMB (oxTMB) via peroxidase activity, allowing colorimetric and subsequent photothermal detection upon irradiation, as low as 2.06 CFU/mL. Magnetic - induced aggregation of Apt/Fe@SiNPs generated toxic hydroxyl radicals around L. monocytogenes, achieving ∼99.6% disinfection. Furthermore, the biofilm of L. monocytogenes was effectively inhibited by the action of hydroxyl radicals. The platform might offer a promising prospect to control L. monocytogenes in food industries.

Listeria monocytogenes in aquatic food products: Spotlight on epidemiological information, bio-based mitigation strategies and predictive approaches

Listeria monocytogenes is the foodborne pathogen responsible for listeriosis in humans. Its ability to grow at refrigeration temperatures, particularly in products that support its growth and have a long-refrigerated shelf-life, poses a significant health risk, especially for vulnerable consumer groups such as pregnant women and immunocompromised individuals. A comprehensive analysis of L. monocytogenes in aquatic food products (AFPs) was conducted, examining the prevalence of the bacterium, the associated outbreaks, and the resulting deaths. Data from 66 studies, comprising a total of 19,373 samples, were analysed from the scientific literature to determine prevalence of the pathogen. The mean pooled prevalence of L. monocytogenes was 11 % (95 % CI: 8-14 %) among different AFPs categories. An overview of worldwide listeriosis outbreaks associated with contaminated AFPs between 1980 and 2023 was provided, totalling 1824 cases, including 41 deaths. Furthermore, a compilation of bio-based mitigation strategies was presented, including the use of lactic acid bacteria (LAB) and bacteriophages as bio-protective cultures to inhibit L. monocytogenes in AFPs. A variety of predictive microbiology models, based on growth prediction and interaction for L. monocytogenes, were reviewed to assess the effectiveness of control strategies in different types of AFPs, offering insights into pathogen behaviour throughout the production chain. The reported growth models describe primarily the impact of storage temperature on pathogen growth parameters, while interaction models, which reflect the inhibitory effect of LAB against L. monocytogenes, were generally defined using the Jameson-effect approach and Lotka-Volterra models' family (i.e., predator-prey models). Both models can be used to describe the simultaneous growth of two bacterial populations and their interactions (i.e., amensalism and antagonisms). Several Quantitative Risk assessment studies have been conducted for AFP, identifying the food category as a relevant contributor to Listeriosis risk, and providing predictive insight critical influence of storage temperature, food microbiota, product shelf-life, and population aging on the risk posed by L. monocytogenes. More importantly, this quantitative approach can serve as a key tool to assess the effectiveness of specific mitigation and intervention strategies to control the pathogen, such as sampling schemes or bio-preservation techniques.

Listeria monocytogenes in Fruits and Vegetables: Antimicrobial Resistance, Biofilm, and Genomic Insights

BACKGROUND/OBJECTIVES

Listeria monocytogenes is a foodborne pathogen that can infect both humans and animals and cause noninvasive gastrointestinal listeriosis or invasive listeriosis. The objectives of this study were to determine the genetic diversity of L. monocytogenes; the genes associated with its resistance to antibiotics, benzalkonium chloride (BC), and cadmium chloride (CdCl2); and its biofilm formation.

METHODS

A total of 132 fresh fruits (44 samples) and vegetables (88 samples) were selected for this study. The genetic diversity of the isolates and the genes associated with their antibiotic resistance were determined using PCR amplification; meanwhile, their levels of susceptibility to antibiotics were determined using the agar diffusion method. Their levels of resistance to BC and CdCl2 were determined using the minimum inhibitory concentration method, and their capacity for biofilm formation was evaluated using the crystal violet staining method.

RESULTS

A total of 17 L. monocytogenes strains were collected: 12.8% (17/132) from fresh fruits and vegetables in this study. The isolates of L. monocytogenes belonged to phylogenetic groups I.1 (29.4% (5/17); serotype 1/2a) and II.2 (70.5% (12/17); serotype 1/2b); strains containing Listeria pathogenicity islands (LIPIs) were also identified at prevalence rates of 100% for LIPI-1 and LIPI-2 (17/17), 29.4% for LIPI-3 (5/17), and 11.7% for LIPI-4 (2/17). The antibiotic susceptibility tests showed that the L. monocytogenes isolates exhibited six different multiresistant patterns, with multiple antibiotic resistance (MAR) index of ≥0.46 (70.5%; 12/17); additionally, the genes Ide, tetM, and msrA, associated with efflux pump Lde, tetracycline, and ciprofloxacin resistance, were detected at 52.9% (9/17), 29.4% (5/17), and 17.6% (3/17), respectively. The phenotypic tests showed that 58.8% (10/17) of cadmium-resistant L. monocytogenes isolates had a co-resistance of 23.5% (4/17) to BC. Finally, all strains of L. monocytogenes exhibited moderate biofilm production.

CONCLUSIONS

The results of this study contribute to our understanding of the persistence and genetic diversity of L. monocytogenes strains isolated from fresh fruits and vegetables; in addition, their resistance to CdCl2, which is correlated with co-resistance to BC disinfectant, is helpful for the food industry.

Potential of enterocin from Enterococcus durans MF5 in controlling Listeria species

This research paper presents the characterization of an enterocin-producing Enterococcus durans MF5 isolate and the determination of the in vitro antilisterial activity of enterocin produced by this isolate, named Ent-MF5. PCR-based screening for bacteriocin biosynthetic genes revealed that E. durans MF5 harbors multiple enterocin-encoding genes (ent A, B, P and X), classified as class II bacteriocins and enterocin-P of Enterococcus faecium (sharing up to 99% similarity at the genetic level). E. durans MF5 is sensitive to eight clinically important antibiotics and does not possess cytolysin activator -cylA, gelatinase -gelE and hyaluronidase -hylA virulence genes. The antilisterial activity of Ent-MF5 was abolished by trypsin, α-chymotrypsin, protease and proteinase-K. Ent-MF5 showed thermal and pH stability. In addition, the activity of Ent-MF5 was unaffected in the presence of various surfactants (1% SDS, Triton X-100, Tween 20, and Tween 80). Ent-MF5 exhibited antimicrobial activity against Listeria monocytogenes, Listeria innocua, Listeria ivanovii and Listeria seeligeri at concentrations as low as 0.13 μg/ml. Ent-MF5 had a bactericidal effect against L. monocytogenes with a significant reduction in surviving cells at concentrations equal to or greater than 0.13 μg/ml. A 75-100% reduction in L. monocytogenes growth and bactericidal effect determined by CFU counts was observed following treatment with Ent-MF5 at 4.47 μg/ml at time points starting at 2 and 4 h, respectively. Ent-MF5 action is associated with Listeria cell membrane damage, as observed by flow cytometry and fluorescence microscopy. Thus, the effective antilisterial activity and stability of Ent-MF5 presents promising perspectives for application as biopreservatives in the food industry.

Sensitive and specific detection of Listeria monocytogenes in food samples using imprinted upconversion fluorescence probe prepared by emulsion polymerization method

Listeria monocytogenes (L. monocytogenes) is a foodborne pathogen with high morbidity and mortality rates, necessitating rapid detection methods. Current techniques, while reliable, are labor-intensive and not amenable to on-site testing. We report the design and synthesis of a novel imprinted upconversion fluorescence probe through Pickering emulsion polymerization for the specific detection of L. monocytogenes. The probe employs trimethylolpropane trimethacrylate and divinylbenzene as cross-linkers, acryloyl-modified chitosan as a functional monomer, and the bacterium itself as the template. The developed probe demonstrated high specificity and sensitivity in detecting L. monocytogenes, with a limit of detection of 72 CFU/mL. It effectively identified the pathogen in contaminated salmon and chicken samples, with minimal background interference. The integration of molecular imprinting and upconversion fluorescence materials presents a potent and reliable approach for the rapid and specific detection of L. monocytogenes, offering considerable potential for on-site food safety testing.

Prevention of foodborne viruses and pathogens in fresh produce and root vegetables

Every year, 1 in 10 people suffers from food poisoning, and in recent years, the highest number of foodborne outbreaks has been attributed to roots/underground vegetables and fresh produce. Major pathogens include as Escherichia coli, Salmonella enterica, Listeria monocytogenes, Human Norovirus, Hepatitis A virus and Cyclospora. The primary sources of contamination for agriculture products stem from uncontrolled exposure to soil, water, and animal waste. Contamination can occur in various ways during food cultivation, harvesting, processing, and distribution. Mechanical washing and disinfection are primarily employed as practices to control biological contaminants such as bacteria, viruses, and parasites. Current practices may encounter challenges such as microbial resistance to disinfectants or antibiotics, and the cleaning effectiveness could be compromised due to the internalization of bacteria and viruses into some plants. High-pressure processing, pulse electric fields, and cold plasma are environmentally friendly technologies, albeit with associated costs. Low-temperature sterilization technologies capable of controlling biological contaminants, such as bacteria and viruses, play a crucial role in preventing food safety issues. Compared to conventional cleaning methods, these technologies are effective in controlling microorganisms that are strongly attached to the food surface or internalized due to damage. Periodic surveillance is essential to ensure the overall microbiological safety of fresh produce and root vegetables.

Biofilm formation by Listeria monocytogenes from the meat processing industry environment and the use of different combinations of detergents, sanitizers, and UV-A radiation to control this microorganism in planktonic and sessile forms

This study aimed to evaluate the ability of biofilm formation by L. monocytogenes from the meat processing industry environment, as well as the use of different combinations of detergents, sanitizers, and UV-A radiation in the control of this microorganism in the planktonic and sessile forms. Four L. monocytogenes isolates were evaluated and showed moderate ability to form biofilm, as well as carried genes related to biofilm production (agrB, agrD, prfA, actA, cheA, cheY, flaA, sigB), and genes related to tolerance to sanitizers (lde and qacH). The biofilm-forming isolates of L. monocytogenes were susceptible to quaternary ammonium compound (QAC) and peracetic acid (PA) in planktonic form, with minimum inhibitory concentrations of 125 and 75 ppm, respectively, for contact times of 10 and 5 min. These concentrations are lower than those recommended by the manufacturers, which are at least 200 and 300 ppm for QAC and PA, respectively. Biofilms of L. monocytogenes formed from a pool of isolates on stainless steel and polyurethane coupons were subjected to 14 treatments involving acid and enzymatic detergents, QAC and PA sanitizers, and UV-A radiation at varying concentrations and contact times. All treatments reduced L. monocytogenes counts in the biofilm, indicating that the tested detergents, sanitizers, and UV-A radiation exhibited antimicrobial activity against biofilms on both surface types. Notably, the biofilm formed on polyurethane showed greater tolerance to the evaluated compounds than the biofilm on stainless steel, likely due to the material's surface facilitating faster microbial colonization and the development of a more complex structure, as observed by scanning electron microscopy. Listeria monocytogenes isolates from the meat processing industry carry genes associated with biofilm production and can form biofilms on both stainless steel and polyurethane surfaces, which may contribute to their persistence within meat processing lines. Despite carrying sanitizer tolerance genes, QAC and PA effectively controlled these microorganisms in their planktonic form. However, combinations of detergent (AC and ENZ) with sanitizers (QAC and PA) at minimum concentrations of 125 ppm and 300 ppm, respectively, were the most effective.

Disinfectant efficacy of glabridin against dried and biofilm cells of Listeria monocytogenes and the impact of residual organic matter

Glabridin is an antimicrobial compound which can be extracted from plants, such as liquorice (Glycyrrhiza glabra) roots. Although its activity against foodborne pathogens and spoilage microorganisms has already been reported, the investigation of potential applications as a surface disinfectant is still largely unexplored. Hence, this study evaluated the disinfectant efficacy of glabridin against Listeria monocytogenes. The activity of glabridin was first tested in vitro in a nutrient-rich medium against eight strains of L. monocytogenes, including food isolates and the model strain EGDe. The tested strains showed similar susceptibility with minimal inhibitory and bactericidal concentrations of 12.5 µg/mL and 25 µg/mL, respectively. Subsequently, L. monocytogenes L6, FBR17 and EGDe were selected to assess the efficacy of glabridin against dried cells (according to the European standard EN 13697:2015 + A1:2019) and biofilm cells on stainless steel surfaces. Moreover, the impact of food residual organic matter was investigated using skim milk, cantaloupe and smoked salmon solution as soiling components. Our results showed that applying 200 µg/mL of glabridin resulted in a substantial reduction (>3 log10) of dried and biofilm cells of L. monocytogenes in standard conditions (i.e. low level of residual organic matter). Cantaloupe soiling components slightly reduced the activity of glabridin, while the efficacy of glabridin when tested with salmon and skim milk residuals was substantially affected. Comparative analysis using standardized protein contents provided evidence that the type of food matrices and type of proteins may impact the activity of glabridin as a disinfectant. Overall, this study showed low strain variability for the activity of glabridin against L. monocytogenes and shed light on the possible application of this natural antimicrobial compound as a surface disinfectant.

Multi-species biofilms of environmental microbiota isolated from fruit packing facilities promoted tolerance of Listeria monocytogenes to benzalkonium chloride

Listeria monocytogenes may survive and persist in food processing environments due to formation of complex multi-species biofilms of environmental microbiota that co-exists in these environments. This study aimed to determine the effect of selected environmental microbiota on biofilm formation and tolerance of L. monocytogenes to benzalkonium chloride in formed biofilms. The studied microbiota included bacterial families previously shown to co-occur with L. monocytogenes in tree fruit packing facilities, including Pseudomonadaceae, Xanthomonadaceae, Microbacteriaceae, and Flavobacteriaceae. Biofilm formation ability and the effect of formed biofilms on the tolerance of L. monocytogenes to benzalkonium chloride was measured in single- and multi-family assemblages. Biofilms were grown statically on polystyrene pegs submerged in a R2A broth. Biofilm formation was quantified using a crystal violet assay, spread-plating, confocal laser scanning microscopy, and its composition was assessed using amplicon sequencing. The concentration of L. monocytogenes in biofilms was determined using the most probable number method. Biofilms were exposed to the sanitizer benzalkonium chloride, and the death kinetics of L. monocytogenes were quantified using a most probable number method. A total of 8, 8, 6, and 3 strains of Pseudomonadaceae, Xanthomonadaceae, Microbacteriaceae, and Flavobacteriaceae, respectively, were isolated from the environmental microbiota of tree fruit packing facilities and were used in this study. Biofilms formed by Pseudomonadaceae, Xanthomonadaceae, and all multi-family assemblages had significantly higher concentration of bacteria, as well as L. monocytogenes, compared to biofilms formed by L. monocytogenes alone. Furthermore, multi-family assemblage biofilms increased the tolerance of L. monocytogenes to benzalkonium chloride compared to L. monocytogenes mono-species biofilms and planktonic multi-family assemblages. These findings suggest that L. monocytogenes control strategies should focus not only on assessing the efficacy of sanitizers against L. monocytogenes, but also against biofilm-forming microorganisms that reside in the food processing built environment, such as Pseudomonadaceae or Xanthomonadaceae.

Linalool Reduces Virulence and Tolerance to Adverse Conditions of Listeria monocytogenes

Listeria monocytogenes, a foodborne pathogen causing listeriosis, poses substantial societal, economic, and public health challenges due to its resistance, persistence, and biofilm formation in the food industry. Exploring subinhibitory concentrations of compounds to target virulence inhibition and increase susceptibility to adverse conditions presents a promising strategy to mitigate its impact of L. monocytogenes and unveils new potential applications. Thus, this study aims to explore the effect of linalool on virulence factors of L. monocytogenes and potential use in the reduction in its tolerance to stressful conditions. This action was analysed considering the use of two sub-inhibitory concentrations of linalool, 0.312 and 0.625 mg/mL. We found that even with the lowest tested concentrations, a 65% inhibition of violacein production by Chromobacterium violaceum, 55% inhibition in biofilm formation by L. monocytogenes and 62% reduction on haemolysis caused by this bacterium were observed. In addition to its impact on virulence factors, linalool diminished the tolerance to osmotic stress (up to 4.3 log reduction after 24 h with 12% NaCl), as well as to high (up to 3.8 log reduction after 15 min at 55 °C) and low temperatures (up to 4.6 log reduction after 84 days with 12% NaCl at 4 °C). Thus, this study paves the way to further investigation into the potential utilization of linalool to mitigate the threat posed by L. monocytogenes in the field of food safety and public health.

Thermal and Chemical Inactivation of Bacillus Phage BM-P1

Bacillus spp. are often used as probiotics; however, they can be infected by phages, leading to significant economic losses. Biocidal and thermal treatments are considered rapid and effective methods for controlling microbial contamination. To prevent viral contamination in industrial dairy production, the impact of temperature and biocides on the viability of Bacillus methylotrophic phage BM-P1 was assessed. The results demonstrated that reconstituted skim milk (RSM) as a medium showed the most effective protective effect on phage BM-P1. Treatment at 90°C for 5 min or 72°C for 10 min inactivated it to nondetectable levels from the initial titer of 7.19 ± 0.11 log, regardless of the culture medium. Sodium hypochlorite exhibited the best inactivating effect, which could reduce the phage titer below the detection level in 4 min at 50 ppm. Additionally, treatment with 75% ethanol for 20 min or 50% isopropanol for 30 min could achieve inactivation to nondetectable levels. The inactivating effect of peracetic acid was limited; even when treated at the highest concentration (0.45%) for 60 min, only a 2.47 ± 0.17 log reduction was observed. This study may provide some theoretical basis and data support for establishing measures against Bacillus spp. phages.

Detection of Listeria Species by Conventional Culture-Dependent and Alternative Rapid Detection Methods in Retail Ready-to-Eat Foods in Turkey

Foodborne pathogens, like Listeria monocytogenes, continue to inflict substantial financial losses on the food industry. Various methods for detecting Listeria in food have been developed and numerous studies have been conducted to compare the different methods. But, in recent years, new Listeria species have been identified, and currently the genus comprises 26 species. Therefore, it would be a more accurate approach to re-evaluate existing detection methods by considering new species. The present investigation involved the analysis of 42 ready-to-eat (RTE) foods, encompassing a variety of food categories, such as mezes, salads, dairy products, and meat products, with the aim of ascertaining the presence of Listeria. Among the traditional culture-dependent reference methods, the ISO 11290 method was preferred. The process of strain identification was conducted with the API Identification System. Furthermore, to ascertain the existence of L. monocytogenes and Listeria spp., the samples underwent additional analysis employing the VIDAS Immunoassay System, ELISA, and RT-PCR methodologies. Thus, four alternative approaches were employed in this study to compare not only the different methods used to determine Listeria while taking into account the newly identified Listeria species, but also to assess the compliance of retail RTE food items with microbiological criteria pertaining to the genus Listeria. Based on the conducted analyses, L. monocytogenes was conclusively determined to be present in one sample. The presence of Listeria spp. was detected in 30.9% of the samples, specifically in Turkish cig kofte, sliced salami, and salads.

An explainable unsupervised risk early warning framework based on the empirical cumulative distribution function: Application to dairy safety

Efficient food safety risk assessment significantly affects food safety supervision. However, food detection data of different types and batches show different feature distributions, resulting in unstable detection results of most risk assessment models, lack of interpretability of risk classification, and insufficient risk traceability. This study aims to explore an efficient food safety risk assessment model that takes into account robustness, interpretability and traceability. Therefore, the Explainable unsupervised risk Warning Framework based on the Empirical cumulative Distribution function (EWFED) was proposed. Firstly, the detection data's underlying distribution is estimated as non-parametric by calculating each testing indicator's empirical cumulative distribution. Next, the tail probabilities of each testing indicator are estimated based on these distributions and summarized to obtain the sample risk value. Finally, the "3σ Rule" is used to achieve explainable risk classification of qualified samples, and the reasons for unqualified samples are tracked according to the risk score of each testing indicator. The experiments of the EWFED model on two types of dairy product detection data in actual application scenarios have verified its effectiveness, achieving interpretable risk division and risk tracing of unqualified samples. Therefore, this study provides a more robust and systematic food safety risk assessment method to promote precise management and control of food safety risks effectively.

Association between the Presence of Resistance Genes and Sanitiser Resistance of Listeria monocytogenes Isolates Recovered from Different Food-Processing Facilities

Sanitisers are widely used in cleaning food-processing facilities, but their continued use may cause an increased resistance of pathogenic bacteria. Several genes have been attributed to the increased sanitiser resistance ability of L. monocytogenes. This study determined the presence of sanitiser resistance genes in Irish-sourced L. monocytogenes isolates and explored the association with phenotypic sanitiser resistance. The presence of three genes associated with sanitiser resistance and a three-gene cassette (mdrL, qacH, emrE, bcrABC) were determined in 150 L. monocytogenes isolates collected from Irish food-processing facilities. A total of 23 isolates contained bcrABC, 42 isolates contained qacH, one isolate contained emrE, and all isolates contained mdrL. Additionally, 47 isolates were selected and grouped according to the number and type of resistance genes, and the minimal inhibitory concentration (MIC) of these isolates for benzalkonium chloride (BAC) was determined experimentally using the broth microdilution method. The BAC resistance of the strain carrying the bcrABC gene cassette was significantly higher than that of strains lacking the gene cassette, and the BAC resistance of the strain carrying the qacH gene was significantly higher than that of strains lacking the qacH gene (p < 0.05). Isolates harbouring both the qacH and bcrABC genes did not show higher BAC resistance. With respect to environmental factors, there was no significant difference in MIC values for isolates recovered from different processing facilities. In summary, this investigation highlights the prevalence of specific sanitiser resistance genes in L. monocytogenes isolates from Irish food-processing settings. While certain genes correlated with increased resistance to benzalkonium chloride, the combination of multiple genes did not necessarily amplify this resistance.

Ag2O-Containing Biocidal Interpolyelectrolyte Complexes on Glass Surfaces-Adhesive Properties of the Coatings

Biocidal coatings are of great interest to the healthcare system. In this work, the biocidal activity of coatings based on a complex biocide containing polymer and inorganic active antibacterial components was studied. Silver oxide was distributed in a matrix of a positively charged interpolyelectrolyte complex (IPEC) of polydiallyldimethylammonium chloride (PDADMAC) and sodium polystyrene sulfonate (PSS) using ultrasonic dispersion, forming nanoparticles with an average size of 5-6 nm. The formed nanoparticles in the matrix are not subject to agglomeration and changes in morphology during storage. It was found that the inclusion of silver oxide in a positively charged IPEC allows a more than 4-fold increase in the effectiveness of the complex biocide against E. coli K12 in comparison with the biocidal effect of PDADMAC and IPEC. Polycation, IPEC, and the IPEC/Ag2O ternary complex form coatings on the glass surface due to electrostatic adsorption. Adhesive and cohesive forces in the resulting coatings were studied with micron-scale coatings using dynamometry. It was found that the stability of the coating is determined primarily by adhesive interactions. At the macro level, it is not possible to reliably identify the role of IPEC formation in adhesion. On the other hand, use of the optical tweezers method makes it possible to analyze macromolecules at the submicron scale and to evaluate the multiple increase in adhesive forces when forming a coating from IPEC compared to coatings from PDADMAC. Thus, the application of ternary IPEC/Ag2O complexes makes it possible to obtain coatings with increased antibacterial action and improved adhesive characteristics.

Characterization of a DsbA family protein reveals its crucial role in oxidative stress tolerance of Listeria monocytogenes

IMPORTANCE

The adaption and tolerance to various environmental stresses are the fundamental factors for the widespread existence of Listeria monocytogenes. Anti-oxidative stress is the critical mechanism for the survival and pathogenesis of L. monocytogenes. The thioredoxin (Trx) and glutaredoxin (Grx) systems are known to contribute to the anti-oxidative stress of L. monocytogenes, but whether the Dsb system has similar roles remains unknown. This study demonstrated that the DsbA family protein Lmo1059 of L. monocytogenes participates in bacterial oxidative stress tolerance, with Cys36 as the key amino acid of its catalytic activity and anti-oxidative stress ability. It is worth noting that Lmo1059 was involved in the invading and cell-to-cell spread of L. monocytogenes. This study lays a foundation for further understanding the specific mechanisms of oxidative cysteine repair and antioxidant stress regulation of L. monocytogenes, which contributes to an in-depth understanding of the environmental adaptation mechanisms for foodborne bacterial pathogens.

Litsea glaucescens Kuth possesses bactericidal activity against Listeria monocytogenes

Background

Litsea glaucencens Kuth is an aromatic plant used for food seasoning food and in Mexican traditional medicine. Among, L. glaucencens leaves properties, it has proven antibacterial activity which can be used against opportunistic pathogens like Listeria monocytogenes, a foodborne bacteria that is the causal agent of listeriosis, a disease that can be fatal in susceptible individuals. The aim of this work was to investigate the antibacterial activity of L. glaucescens Kuth leaf extracts against L. monocytogenes and to identify its bioactive components.

Material and Methods

L. glaucences leaves were macerated with four solvents of different polarity (n-hexane, dichloromethane, ethyl acetate, and methanol). To determine the capacity to inhibit bacterial proliferation in vitro, agar diffusion and microdilution methods were used. Next, we determined the minimal bactericidal concentration (MBC). Finally, we determined the ratio of MBC/MIC. Metabolites present in the active methanolic extract from L. glaucescens Kuth (LgMeOH) were purified by normal-phase open column chromatography. The structure of the antibacterial metabolite was determined using nuclear magnetic resonance (1H, 13C, COSY, HSQC) and by comparison with known compounds.

Results

The LgMeOH extract was used to purify the compound responsible for the observed antimicrobial activity. This compound was identified as 5,7-dihydroxyflavanone (pinocembrin) by analysis of its spectroscopic data and comparison with those described. The MIC and MBC values obtained for pinocembrin were 0.68 mg/mL, and the ratio MBC/MIC for both LgMeOH and pinocembrin was one, which indicates bactericidal activity.

Conclusion

L. glaucences Kuth leaves and its metabolite pinocembrin can be used to treat listeriosis due the bactericidal activity against L. monocytogenes.

Biocide Tolerance and Impact of Sanitizer Concentrations on the Antibiotic Resistance of Bacteria Originating from Cheese

In this study, we determined and identified the bacterial diversity of different types of artisanal and industrially produced cheese. The antibiotic (erythromycin, chloramphenicol, kanamycin, ampicillin, clindamycin, streptomycin, tetracycline, and gentamicin) and biocide (peracetic acid, sodium hypochlorite, and benzalkonium chloride) resistance of clinically relevant bacteria was determined as follows: Staphylococcus aureus, Macrococcus caseolyticus, Bacillus sp., Kocuria varians, Escherichia coli, Enterococcus faecalis, Citrobacter freundii, Citrobacter pasteurii, Klebsiella oxytoca, Klebsiella michiganensis, Enterobacter sp., Enterobacter cloacae, Enterobacter sichuanensis, Raoultella ornithinolytica, Shigella flexneri, and Salmonella enterica. Also, the effect of the sub-inhibitory concentration of three biocides on antibiotic resistance was determined. The microbiota of evaluated dairy products comprise diverse and heterogeneous groups of bacteria with respect to antibiotic and disinfectant tolerance. The results indicated that resistance was common in the case of ampicillin, chloramphenicol, erythromycin, and streptomycin. Bacillus sp. SCSSZT2/3, Enterococcus faecalis SRGT/1, E. coli SAT/1, Raoultella ornithinolytica MTT/5, and S. aureus SIJ/2 showed resistance to most antibiotics. The tested bacteria showed sensitivity to peracetic acid and a different level of tolerance to benzalkonium chloride and sodium hypochlorite. The inhibition zone diameter of antibiotics against Enterococcus faecalis SZT/2, S. aureus JS11, E. coli CSKO2, and Kocuria varians GRT/10 was affected only by the sub-inhibitory concentration of peracetic acid.

Mechanism of acid and alkali electrolyzed water on the elimination of Listeria monocytogenes biofilm based on proteomic analysis

Acidic electrolyzed water is a relatively mature bactericide, which has a certain inhibitory effect on a variety of microorganisms, and is widely used in the field of food processing for cleaning, sterilization and disinfection. This study investigated the deactivation mechanisms of Listeria monocytogenes by Tandem Mass Tags quantitative proteomics analysis. Samples were treated through A1S4 (Alkaline electrolytic water treatment for 1 min and Acid electrolytic water treatment for 4 min), S3A1S1 (Acid electrolyzed water treatment 3 min, Alkaline electrolyzed water treatment 1 min and Acid electrolyzed water treatment 1 min), S5 (Acid electrolytic water treatment for 5 min). Proteomic analysis showed that the mechanism of acid alkaline electrolyzed water treatment to eliminate the inactivation of the biofilm of L. monocytogenes was related to protein transcription and extension, RNA processing and synthesis, gene regulation, sugar and amino acid transport and metabolism, signal transduction and ATP binding. The study on the influence mechanism and action mechanism of the combination of acidic and alkaline electrolyzed water to remove L. monocytogenes biofilm is helpful to understand the development of the process of removing biofilm by electrolyzed water, and provides theoretical support for the treatment of other microbial contamination problems in food processing by electrolyzed water.

Mechanisms of Listeria monocytogenes Disinfection with Benzalkonium Chloride: From Molecular Dynamics to Kinetics of Time-Kill Curves

Unravelling the mechanisms of action of disinfectants is essential to optimise dosing regimes and minimise the emergence of antimicrobial resistance. In this work, we examined the mechanisms of action of a commonly used disinfectant-benzalkonium chloride (BAC)-over a significant pathogen-L. monocytogenes-in the food industry. For that purpose, we used modelling at multiple scales, from the cell membrane to cell population inactivation. Molecular modelling revealed that the integration of the BAC into the membrane requires three phases: (1) the approaching of BAC to the cellular membrane, (2) the absorption of BAC to its surface, and (3) the integration of the compound into the lipid bilayer, where it remains at least for several nanoseconds, probably destabilising the membrane. We hypothesised that the equilibrium of adsorption, although fast, was limiting for sufficiently large BAC concentrations, and a kinetic model was derived to describe time-kill curves of a large population of cells. The model was tested and validated with time series data of free BAC decay and time-kill curves of L. monocytogenes at different inocula and BAC dose concentrations. The knowledge gained from the molecular simulation plus the proposed kinetic model offers the means to design novel disinfection processes rationally.

Rapid separation and detection of Listeria monocytogenes with the combination of phage tail fiber protein and vancomycin-magnetic nanozyme

In this work, a lateral flow assay for Listeria monocytogenes was developed based on phage tail fiber protein (TFP) and triple-functional nanozyme probes with capture-separation-catalytic activity. Inspired by interaction between phage and bacteria, TFP of L. monocytogenes phage was immobilized on test line as capture molecule, which replaced traditional antibody and aptamer. After Gram-positive bacteria was captured and separated from samples by nanozyme probes modified with vancomycin (Van), TFP specifically recognized L. monocytogenes and overcame non-specific binding of Van. Special color reaction between Coomassie Brilliant Blue and bovine serum albumin which was an amplification carrier on probe was simply utilized as control zone to replace traditional control line. Relying on enzyme-like catalytic activity of nanozyme, this biosensor realized improved sensitivity and colorimetric quantitative detection with a detection limit of 10 CFU mL^-1. Analytic performance results suggested this TFP-based biosensor provided a portable, sensitive and specific strategy to detect pathogen.

Cross-contamination of mature Listeria monocytogenes biofilms from stainless steel surfaces to chicken broth before and after the application of chlorinated alkaline and enzymatic detergents

The objectives of this study were, firstly, to compare a conventional (i.e., chlorinated alkaline) versus an alternative (chlorinated alkaline plus enzymatic) treatment effectivity for the elimination of biofilms from different L. monocytogenes strains (CECT 5672, CECT 935, S2-bac and EDG-e). Secondly, to evaluate the cross-contamination to chicken broth from non-treated and treated biofilms formed on stainless steel surfaces. Results showed that all L. monocytogenes strains were able to adhere and develop biofilms at approximately the same growth levels (≈5.82 log CFU/cm²). When non-treated biofilms were put into contact with the model food, obtained an average transference rate of potential global cross-contamination of 20.4%. Biofilms treated with the chlorinated alkaline detergent obtained transference rates similar to non-treated biofilms as a high number of residual cells (i.e., around 4 to 5 Log CFU/cm²) were present on the surface, except for EDG-e strain on which transference rate diminished to 0.45%, which was related to the protective matrix. Contrarily, the alternative treatment was shown to not produce cross-contamination to the chicken broth due to its high effectivity for biofilm control (<0.50% of transference) except for CECT 935 strain that had a different behavior. Therefore, changing to more intense cleaning treatments in the processing environments can reduce risk of cross-contamination.

Biochemical, Antioxidant Properties and Antimicrobial Activity of Epiphytic Leafy Liverwort Frullania dilatata (L.) Dumort

In this study, the biochemical, antioxidant properties, and antimicrobial activity of the epiphytic leafy liverwort Frullania dilatata (L.) Dumort were investigated. Due to the scarcity and difficulty in obtaining liverworts, research on their bioactivity is limited; thus, this study aimed to uncover the potential of F. dilatata. The antimicrobial activity was evaluated against various microorganisms, including food isolates, clinical isolates, multidrug-resistant strains, and standard strains, using the disk diffusion method and determining the minimum inhibitory concentration (MIC) values. This study represents the first antioxidant investigation on F. dilatata and an antimicrobial study using ethanol extract and the disk diffusion method. Notably, susceptibility was observed in Enterococcus faecalis ATCC 29212, Enterococcus faecium FI, Listeria monocytogenes ATCC 7644, Providencia rustigianii MDR, and Staphylococcus aureus ATCC 25923. The antioxidant capacity was assessed using the DPPH method, emphasizing the high scavenging performance. Gas chromatography-mass spectrometry (GC-MS) analysis identified the primary compounds as frullanolide (19.08%), 2,3-Dimethylanisole (15.21%), linoleic acid (11.11%), palmitic acid (9.83%), and valerenic acid (5.3%). The results demonstrated the significant antimicrobial activity of F. dilatata against the tested microorganisms and its potent antioxidant properties. These findings emphasize the potential of F. dilatata as a promising source of natural antimicrobial and antioxidant agents, underscoring the importance of further investigation into its bioactive compounds and elucidating the mechanisms of action in future studies.

Biofilm Formation and Control of Foodborne Pathogenic Bacteria

Biofilms are microbial aggregation membranes that are formed when microorganisms attach to the surfaces of living or nonliving things. Importantly, biofilm properties provide microorganisms with protection against environmental pressures and enhance their resistance to antimicrobial agents, contributing to microbial persistence and toxicity. Thus, bacterial biofilm formation is part of the bacterial survival mechanism. However, if foodborne pathogens form biofilms, the risk of foodborne disease infections can be greatly exacerbated, which can cause major public health risks and lead to adverse economic consequences. Therefore, research on biofilms and their removal strategies are very important in the food industry. Food waste due to spoilage within the food industry remains a global challenge to environmental sustainability and the security of food supplies. This review describes bacterial biofilm formation, elaborates on the problem associated with biofilms in the food industry, enumerates several kinds of common foodborne pathogens in biofilms, summarizes the current strategies used to eliminate or control harmful bacterial biofilm formation, introduces the current and emerging control strategies, and emphasizes future development prospects with respect to bacterial biofilms.

Emergence of Tobramycin Escherichia coli resistance in poultry meat linked to biocides overuse during COVID-19

The effect of excessive use of biocides during the COVID-19, on the resistance of Escherichia coli to Tobramycin in poultry, meat was examined in this observational epidemiological study (Before and after COVID–19). Tobramycin E. coli resistant strains isolated from poultry meat before COVID-19 appearance were compared with those isolated after COVID-19 emergence. Univariable analyses were performed using t-test and chi-squared test. Odds ratios and 95% confidence intervals were used for statistically significant risk factor. Multivariate analysis was done with the binary logistic regression to detect an independent predictor, and with the principal component analysis (PCA), to analyze whether the Tobramycin resistance in E. coli was linked with the COVID-19 outbreak. Statistical significance was set at P<0.05. The frequency of Tobramycin E. coli resistant isolates was more important after COVID-19 emergence (12.5%) than before COVID-19 (2.1%). Graphical representation of PCA qualitative variables shows the interfactor relationship. A significant relationship between Tobramycin E. coli resistance and COVID-19 emergence (P=0.014), and the effect of the emergence of COVID-19 on the Tobramycin E. coli resistance was OR = 6.57 (95% Confidence interval (CI) 1.61-7.94). The probability of Tobramycin E. coli resistance linked with poultry meat bought after COVID-19 was 1.88 times more than before COVID-19 emergence. Poultry meat purchased after COVID-19 found related to Tobramycin resistance in E. coli. It seems possible that the overuse of biocides during COVID-19 increased the risk of Tobramycin E. coli resistance in poultry meat.

Correlation between Perturbation of Redox Homeostasis and Antibiofilm Capacity of Phytochemicals at Non-Lethal Concentrations

Biofilms are the multicellular lifestyle of microorganisms and are present on potentially every type of biotic or abiotic surface. Detrimental biofilms are generally targeted with antimicrobial compounds. Phytochemicals at sub-lethal concentrations seem to be an exciting alternative strategy to control biofilms, as they are less likely to impose selective pressure leading to resistance. This overview gathers the literature on individual phytocompounds rather than on extracts of which the use is difficult to reproduce. To the best of our knowledge, this is the first review to target only individual phytochemicals below inhibitory concentrations against biofilm formation. We explored whether there is an overall mechanism that can explain the effects of individual phytochemicals at sub-lethal concentrations. Interestingly, in all experiments reported here in which oxidative stress was investigated, a modest increase in intracellular reactive oxygen species was reported in treated cells compared to untreated specimens. At sub-lethal concentrations, polyphenolic substances likely act as pro-oxidants by disturbing the healthy redox cycle and causing an accumulation of reactive oxygen species.

Sterilising effect of high power pulse microwave on Listeria monocytogenes

In the present work, Listeria monocytogenes was used as the target strain to investigate the sterilising potential and mechanism of high power pulse microwave (HPPM). Results showed that the inactivation was positively correlated with the pulse frequencies and operating times. The count of Listeria monocytogenes was decreased by 5.09 log CFU/mL under 200 Hz for 9 min, which was used as the optimised condition to further explore the sterilisation mechanism. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that the L. monocytogenes cells of untreated group presented intact surfaces, clear boundary, and its intracellular contents distributed uniformly in the cytoplasm. Following treatment, the cell wall surfaces began to deform in small areas, and cell membranes were severely ruptured, thus resulting in the appearance of electron transmission areas. Extracellular protein and nucleic acid contents, represented by OD260 nm and OD280 nm, increased with the increase in operating time significantly. After treatment, SDS-PAGE profiles of whole-cell proteins displayed that the protein bands became lighter or even disappeared. Na+ K+-ATPase activities and intracellular ATP content decreased by 72.97 and 79.09%, respectively. This was consistent with the cell viability of L. monocytogenes observed by confocal laser scanning microscopy. Overall, the sterilisation mechanism of HPPM on L. monocytogenes may be caused by membrane damage, intracellular component leakage, and energy metabolism hindrance.

Biofilms as a microbial hazard in the food industry: A scoping review

Biofilms pose a serious public health hazard with a significant economic impact on the food industry. The present scoping review is designed to analyse the literature published during 2001-2020 on biofilm formation of microbes, their detection methods, and association with antimicrobial resistance (if any). The peer-reviewed articles retrieved from 04 electronic databases were assessed using PRISMA-ScR guidelines. From the 978 preliminary search results, a total of 88 publications were included in the study. On analysis, the commonly isolated pathogens were Listeria monocytogenes, Staphylococcus aureus, Salmonella spp., Escherichia coli, Bacillus spp., Vibrio spp., Campylobacter jejuni and Clostridium perfringens. The biofilm-forming ability of microbes was found to be influenced by various factors such as attachment surfaces, temperature, presence of other species, nutrient availability etc. A total of 18 studies characterized the biofilm-forming genes, particularly for S. aureus, Salmonella spp., and E. coli. In most studies, polystyrene plate and/or stainless-steel coupons were used for biofilm formation, and the detection was carried out by crystal violet assays and/or by plate counting method. The strain-specific significant differences in biofilm formation were observed in many studies, and few studies carried out analysis of multi-species biofilms. The association between biofilm formation and antimicrobial resistance was not clearly defined. Further, viable but non-culturable form of the foodborne pathogens is posing an unseen (by conventional cultivation techniques) but potent threat to the food safety. The present review recommends the need for carrying out systematic surveys and risk analysis of biofilms in food chain to highlight the evidence-based public health concerns, especially in regions where microbiological food hazards are quite prevalent.

Listeria monocytogenes Outbreaks Related to Commercially Produced Caramel Apples: Developments in Sanitation, Product Formulation, and Packaging: A Review

ABSTRACT

Prior to a deadly 2014 listeriosis outbreak, caramel apples were not thought to be vehicles for the foodborne pathogen Listeria monocytogenes. The purpose of this review article is to summarize what has been learned from research prompted by this outbreak. This overview includes descriptions of the two L. monocytogenes infection outbreaks related to prepackaged caramel apples and a brief discussion of apple sanitation, the production processes used to make caramel apples, and research on ways to prevent future outbreaks associated with caramel apples. A qualitative analysis of the literature and interviews with current caramel apple manufacturers were conducted. Sanitation, packaging, and storage procedures used by manufacturers in the past may not effectively inactivate L. monocytogenes from contaminated product. Novel apple sanitation methods and product formulations to control L. monocytogenes on caramel apples have been developed and, in some cases, implemented in commercial production.

HIGHLIGHTS

Microbial resistance to sanitizers in the food industry: review

Hygiene programs which comprise the cleaning and sanitization steps are part of the Good Hygiene Practices (GHP) and are considered essential to ensure food safety and quality. Inadequate hygiene practices may contribute to the occurrence of foodborne diseases, development of microbial resistance to sanitizers, and economic losses. In general, the sanitizer resistance is classified as intrinsic or acquired. The former is an inherent characteristic, naturally present in some microorganisms, whereas the latter is linked to genetic modifications that can occur at random or after continuous exposure to a nonnormal condition. The resistance mechanisms can involve changes in membrane permeability or in the efflux pump, and enzymatic activity. The efflux pump mechanism is the most elucidated in relation to the resistance caused by the use of different types of sanitizers. In addition, microbial resistance to sanitizers can also be favored in the presence of biofilms due to the protection given by the glycocalyx matrix and genetic changes. Therefore, this review aimed to show the main microbial resistance mechanisms to sanitizers, including genetic modifications, biofilm formation, and permeability barrier.

Nonsynonymous Mutations in fepR Are Associated with Adaptation of Listeria monocytogenes and Other Listeria spp. to Low Concentrations of Benzalkonium Chloride but Do Not Increase Survival of L. monocytogenes and Other Listeria spp. after Exposure to Benzalkonium Chloride Concentrations Recommended for Use in Food Processing Environments

Selection for Listeria monocytogenes strains that are tolerant to quaternary ammonium compounds (such as benzalkonium chloride [BC]) is a concern across the food industry, including in fresh produce processing environments. This study evaluated the ability of 67 strains of produce-associated L. monocytogenes and other Listeria spp. (“parent strains”) to show enhanced BC tolerance after serial passaging in increasing BC concentrations and to maintain this tolerance after substreaking in the absence of BC. After serial passaging in BC, 62/67 “BC passaged cultures” showed higher MICs (4 to 20 mg/L) than parent strains (2 to 6 mg/L). After the substreaking of two isolates from BC passaged cultures for each parent strain, 105/134 “adapted isolates” maintained MICs (4 to 6 mg/L) higher than parent strain MICs. These results suggested that adapted isolates acquired heritable adaptations that confer BC tolerance. Whole-genome sequencing and Sanger sequencing of fepR, a local repressor of the MATE family efflux pump FepA, identified nonsynonymous fepR mutations in 48/67 adapted isolates. The mean inactivation of adapted isolates after exposure to use-level concentrations of BC (300 mg/L) was 4.48 log, which was not significantly different from inactivation observed in parent strains. Serial passaging of cocultures of L. monocytogenes strains containing bcrABC or qacH did not yield adapted isolates that showed enhanced BC tolerance in comparison to that of monocultures. These results suggest that horizontal gene transfer either did not occur or did not yield isolates with enhanced BC tolerance. Overall, this study provides new insights into selection of BC tolerance among L. monocytogenes and other Listeria spp.

IMPORTANCEListeria monocytogenes tolerance to quaternary ammonium compounds has been raised as a concern with regard to L. monocytogenes persistence in food processing environments, including in fresh produce packing and processing environments. Persistence of L. monocytogenes can increase the risk of product contamination, food recalls, and foodborne illness outbreaks. Our study shows that strains of L. monocytogenes and other Listeria spp. can acquire heritable adaptations that confer enhanced tolerance to low concentrations of benzalkonium chloride, but these adaptations do not increase survival of L. monocytogenes and other Listeria spp. when exposed to concentrations of benzalkonium chloride used for food contact surface sanitation (300 mg/L). Overall, these findings suggest that the emergence of benzalkonium chloride-tolerant Listeria strains in food processing environments is of limited concern, as even strains adapted to gain higher MICs in vitro maintain full sensitivity to the concentrations of benzalkonium chloride used for food contact surface sanitation.

Bacterial Resistance Toward Antimicrobial Ionic Liquids Mediated by Multidrug Efflux Pumps

The effective elimination of foodborne pathogens through cleaning and disinfection measures is of great importance to the food processing industry. As food producers rely heavily on disinfectants to control pathogenic bacteria in their facilities, the increasing spread of tolerant, often even multidrug resistant, strains is of particular concern. In addition to efforts to prevent or at least reduce development and spread of strains resistant to disinfectants and sanitizers, there is an urgent need for new and effective antimicrobials. One new class of promising antimicrobials is ionic liquids (ILs), which have been reported to be effective against resistant strains as they interact with bacterial cells in multiple ways, but investigations of their effectivity against MDR bacteria or specific defense mechanisms are still limited. This study investigates the role of multidrug efflux pumps of the Resistance Nodulation-Division family (RND) on the resistance of bacterial pathogens Escherichia coli and Salmonella enterica serovar Typhimurium toward 10 antimicrobial active ILs. Results reveal that, while known structure–activity relationships (SARs), such as the side-chain effect, were found for all strains, antimicrobial ILs with one elongated alkyl side chain were significantly affected by the RND efflux pump, highlighting the importance of efflux pumps for future IL toxicity studies. In case of antimicrobial ILs with multiple side chains and different cationic head groups, two ILs were identified that were highly active against all investigated strains with little to no effect of the efflux pump. The results obtained in this study for RND efflux pumps can serve as a starting point for identifying and designing antimicrobial ILs as effective biocides against MDR bacteria.

An Exploration of Listeria monocytogenes, Its Influence on the UK Food Industry and Future Public Health Strategies

Listeria monocytogenes is a Gram-positive intracellular pathogen that can cause listeriosis, an invasive disease affecting pregnant women, neonates, the elderly, and immunocompromised individuals. Principally foodborne, the pathogen is transmitted typically through contaminated foods. As a result, food manufacturers exert considerable efforts to eliminate L. monocytogenes from foodstuffs and the environment through food processing and disinfection. However, L. monocytogenes demonstrates a range of environmental stress tolerances, resulting in persistent colonies that act as reservoirs for the reintroduction of L. monocytogenes to food contact surfaces and food. Novel technologies for the rapid detection of L. monocytogenes and disinfection of food manufacturing industries have been developed to overcome these obstacles to minimise the risk of outbreaks and sporadic cases of listeriosis. This review is aimed at exploring L. monocytogenes in the UK, providing a summary of outbreaks, current routine microbiological testing and the increasing awareness of biocide tolerances. Recommendations for future research in the UK are made, pertaining to expanding the understanding of L. monocytogenes dissemination in the UK food industry and the continuation of novel technological developments for disinfection of food and the food manufacturing environment.

Listeria monocytogenes exposed to antimicrobial peptides displays differential regulation of lipids and proteins associated to stress response

With the onset of Listeria monocytogenes resistance to the bacteriocin nisin, the search for alternative antimicrobial treatments is of fundamental importance. In this work, we set out to investigate proteins and lipids involved in the resistance mechanisms of L. monocytogenes against the antimicrobial peptides (AMPs) nisin and fengycin. The effect of sub-lethal concentrations of nisin and lipopeptide fengycin secreted by Bacillus velezensis P34 on L. monocytogenes was investigated by mass spectrometry-based lipidomics and proteomics. Both AMPs caused a differential regulation of biofilm formation, confirming the promotion of cell attachment and biofilm assembling after treatment with nisin, whereas growth inhibition was observed after fengycin treatment. Anteiso branched-chain fatty acids were detected in higher amounts in fengycin-treated samples (46.6%) as compared to nisin-treated and control samples (39.4% and 43.4%, respectively). In addition, a higher relative abundance of 30:0, 31:0 and 32:0 phosphatidylglycerol species was detected in fengycin-treated samples. The lipidomics data suggest the inhibition of biofilm formation by the fengycin treatment, while the proteomics data revealed downregulation of important cell wall proteins involved in the building of biofilms, such as the lipoteichoic acid backbone synthesis (Lmo0927) and the flagella-related (Lmo0718) proteins among others. Together, these results provide new insights into the modification of lipid and protein profiles and biofilm formation in L. monocytogenes upon exposure to antimicrobial peptides.

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.

Genetic Diversity and Relationships of Listeria monocytogenes Serogroup IIa Isolated in Poland

In the present study, 100 L. monocytogenes isolates of serogroup IIa from food and food production environments in Poland were characterized towards the presence of virulence, resistance, and stress response genes using whole-genome sequencing (WGS). The strains were also molecularly typed and compared with multi-locus sequence typing (MLST) and core genome MLST analyses. The present isolates were grouped into 6 sublineages (SLs), with the most prevalent SL155 (33 isolates), SL121 (32 isolates), and SL8 (28 isolates) and classified into six clonal complexes, with the most prevalent CC155 (33 strains), CC121 (32 isolates), and CC8 (28 strains). Furthermore, the strains were grouped to eight sequence types, with the most prevalent ST155 (33 strains), ST121 (30 isolates), and ST8 (28; strains) followed by 60 cgMLST types (CTs). WGS data showed the presence of several virulence genes or putative molecular markers playing a role in pathogenesis of listeriosis and involved in survival of L. monocytogenes in adverse environmental conditions. Some of the present strains were molecularly closely related to L. monocytogenes previously isolated in Poland. The results of the study showed that food and food production environments may be a source of L. monocytogenes of serogroup IIa with pathogenic potential.

Probing antimicrobial resistance and sanitizer tolerance themes and their implications for the food industry through the Listeria monocytogenes lens

The development of antibiotic resistance is a serious public health crisis, reducing our ability to effectively combat infectious bacterial diseases. The parallel study of reduced susceptibility to sanitizers is growing, particularly for environmental foodborne pathogens, such as Listeria monocytogenes. As regulations demand a seek-and-destroy approach for L. monocytogenes, understanding sanitizer efficacy and its uses are critical for the food industry. Studies have reported the ability of L. monocytogenes to survive in sanitizer concentrations 10-1000 times lower than the manufacturer-recommended concentration (MRC). Notably, data show that at MRC and when applied according to the label instructions, sanitizers remain largely effective. Studies also report that variables such as the presence of organic material, application time/temperature, and bacterial attachment to surfaces can impact sanitizer effectiveness. Due to the lack of standardization in the methodology and definitions of sanitizer resistance, tolerance, and susceptibility, different messages are conveyed in different studies. In this review, we examine the diversity of definitions, terminology, and methodologies used in studies examining L. monocytogenes resistance and susceptibility to antimicrobials. Research available to date fails to demonstrate "resistance" of L. monocytogenes to recommended sanitizer treatments as prescribed by the label. As such, sanitizer tolerance would be a more accurate description of L. monocytogenes response to low sanitizer concentrations (i.e., sub-MRC). Conservative use of word "resistance" will reduce confusion and allow for concise messaging as sanitizer research findings are communicated to industry and regulators.