A Systematic Review of Listeria Species and Listeria monocytogenes Prevalence, Persistence, and Diversity throughout the Fresh Produce Supply Chain

Investigation of the presence and persistence of bacteria in seawater and oysters from an aquaculture farm in Rehoboth Bay, Delaware

The filter-feeding nature of oysters, anthropogenic activities, and increasing agriculture in Delaware compromise the microbial safety of Eastern oysters from local aquaculture farms. From July to October 2023, we evaluated the presence and persistence of eight bacteria in seawater and oysters produced from off-bottom and bottom cultures at Sally Cove, an aquaculture farm within Rehoboth Bay in Delaware. A control site within Sally Cove, which was without oyster cultures, was also included in the study. Seawater temperature, salinity, pH, and dissolved oxygen were measured in situ during sampling. Molecular confirmation with PCR and qPCR showed that Vibrio parahaemolyticus, Shiga-toxin-producing Escherichia coli, Salmonella enterica, Staphylococcus aureus, Pseudomonas aeruginosa, and Clostridium spp. were present and persisted in seawater and oyster samples from both cultures at Sally Cove and in off-bottom and bottom seawater samples from the control site throughout the study. Shigella spp. and Listeria monocytogenes were consistently found in seawater and oyster samples from July to September. However, Shigella spp. was only detected in samples from the bottom cultures, whereas L. monocytogenes was undetectable in all samples from both cultures in October. The observed temperature, salinity, pH, and dissolved oxygen levels across the study period were in the range of 15.30-29.67°C, 29.33-31.87 ppt, 7.25-7.95, and 3.79-8.10 mg/L, respectively, and comparable with the conditions suitable for the growth and survival of these bacteria. These findings suggest that consuming raw oysters from Sally Cove poses contamination risks from several bacteria, especially in the summer months.IMPORTANCEAlthough studies have evaluated bacterial contamination in seawater and oysters within the Delaware Inland Bays and nearby areas, the focus has primarily been on Vibrio species. However, other bacteria have been found in seawater and seafood at various locations and could potentially occur in oysters produced from aquaculture farms within the Delaware Inland Bays. Sally Cove is an oyster aquaculture farm that produces Eastern oysters (Crassostrea virginica) for consumption in Delaware using both off-bottom and bottom culturing methods. The risk of bacterial contamination from consuming raw oysters from this farm is unknown. This paper shows the presence and persistence of several bacteria, including those associated with waste, in seawater and oysters at the farm. The findings can inform consumers about the contamination risks from consuming raw oysters produced at the farm.

A Critical Review of Parameters Relevant for Shiga Toxin-producing Escherichia coli and Listeria monocytogenes Risk Assessments of Leafy Greens

In the past decade, several quantitative models and risk assessments for Shiga toxin-producing Escherichia coli (STEC) and Listeria monocytogenes have been developed to guide the management of these pathogens in fresh produce. However, there is a need to collect and critically review the parameters used to guide their potential reuse in future risk assessments. This review (i) identifies 11 and 7 recently published quantitative models and risk assessments for STEC and L. monocytogenes, respectively, in leafy greens, (ii) summarizes parameters, and (iii) reviews the underlying data sources or mathematical formulas used. A total of 70 unique key parameters (55 and 25 for STEC and L. monocytogenes, respectively, 10 shared) were extracted from the reviewed models across five supply chain stages, including: Preharvest, Harvest, Processing, Presentation to Consumer (Retail or Foodservice Locations), and Consumer Handling. Primary growth, secondary growth, and dose-response equations and parameters for STEC and L. monocytogenes were also extracted. Additional literature reviews were performed if (i) certain key parameters were based on a single or a few data sources or (ii) key parameters were identified in a supply chain stage for one pathogen, but missing from the other. The critical summaries of parameters presented here (i) provide a resource for future risk assessments, (ii) help define future data collection needs, and (iii) represent a starting point for similar reviews focusing on other produce commodities and pathogens.

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.

Integrating Bacteriocins and Biofilm-Degrading Enzymes to Eliminate L. monocytogenes Persistence

Listeria monocytogenes is a Gram-positive bacterium causing listeriosis, a severe infection responsible for significant morbidity and mortality globally. Its persistence on food processing surfaces via biofilm formation presents a major challenge, as conventional sanitizers and antimicrobials exhibit limited efficacy against biofilm-embedded cells. This study investigates a novel approach combining an engineered polysaccharide-degrading enzyme (CAase) with a bacteriocin (thermophilin 110) produced by Streptococcus thermophilus. Laboratory assays evaluated the effectiveness of this combination in disrupting biofilms and inactivating L. monocytogenes on various surfaces. The results demonstrated that CAase effectively disrupts biofilm structures, while thermophilin 110 significantly reduces bacterial growth and viability. The preliminary trials indicate a dual-action approach offers a potential alternative to conventional treatments, enhancing food safety by effectively controlling Listeria biofilms in food processing environments.

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.

Fortification of Orange and Apple Juices with Ferulic Acid: Implications for Food Safety and Quality

In recent years, the awareness of healthier lifestyles among consumers has driven to an increased interest in more natural, nutritious, and low-processed foods. Ferulic acid, one of the most abundant phenolic acids in plants, has demonstrated a wide spectrum of antimicrobial activities and a range of biomedical effects, including antioxidant, antiallergic, hepatoprotective, anticarcinogenic, anti-inflammatory, and antithrombotic, among others. The objective of this work was to study the antilisterial effect of ferulic acid (FA, 1500 mg/L) on fresh, ready-to-eat orange (FOJ) and apple (FAJ) juices and evaluate its effect on product quality. The results showed that FA reduced the Listeria monocytogenes population after 9 days of storage at 4 °C, but no effect on the indigenous microbiota was observed. The titratable acidity and color significantly changed. The antioxidant capacity and total phenolic content significantly increased with the addition of FA, being at least two times greater in fortified juices. FAJ and FOJ containing FA were scored lower (6.8 and 5.7 on a 9-point hedonic scale, respectively) than their respective controls. Overall, our results demonstrated that FA treatment could be a useful strategy to maintain the safety of fresh apple and orange juices and increase the antioxidant activity and phenolic content. The potential industrial applications and health benefits of the fortification of fruit juices with FA should be further explored.

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.

Developing an Agent-Based Model that Predicts Listeria spp. Transmission to Assess Listeria Control Strategies in Retail Stores

Contamination of fresh produce with Listeria monocytogenes can occur throughout the supply chain, including at retail, where Listeria spp., including L. monocytogenes, may be introduced and spread via various routes. However, limited tools are available for retailers to assess practices that can enhance control of Listeria transmission to fresh produce. Therefore, we developed an agent-based model that can simulate Listeria transmission in retail produce sections to optimize environmental sampling programs and evaluate control strategies. A single retail store was used as a model environment, in which various routes of Listeria introduction into and transmission between environmental surfaces were modeled. Model prediction (i.e., Listeria prevalence) was validated using a published longitudinal study for all surfaces that were included in both the model and the validation data. Sensitivity analysis using the Partial Rank Correlation Coefficient showed that (i) initial Listeria concentration from incoming produce, (ii) transfer coefficient from produce to employee's hands, and (iii) transfer coefficient from consumer to produce were the top three parameters that were significantly (p < 0.0018) associated with the mean Listeria prevalence across all agents, suggesting that the accuracy of these parameters are important for prediction of overall Listeria prevalence at retail. Cluster analysis grouped agents with similar contamination patterns into six unique clusters; this information can be used to optimize the sampling plans for retail environments. Scenario analysis suggested that (i) more stringent supplier control as well as (ii) practices reducing Listeria transmission via consumer's hands may have the largest impact on reducing finished product contamination. Overall, we show that an agent-based model can serve as a foundational tool to help with decision-making on Listeria control strategies at retail.

Ultra violet-C pretreatment enhances the antimicrobial efficacy of unpeeled carrots against subsequent contamination with Listeria monocytogenes

To our knowledge, this study is the first to elucidate the bactericidal efficacy of unpeeled carrots (hereafter referred to as carrots) pretreated with Ultra Violet-C (UV-C) against subsequent contamination with Listeria monocytogenes. Carrots pretreated with UV-C (240 mJ/cm2) exhibited a significant antilisterial effect within 2 h. In fact, the population of UV-C-pretreated carrots decreased from 7.94 log CFU/cm2 to levels below the limit of detection (LOD; <1.65 log CFU/cm2) within 24 h. For carrots that were not pretreated with UV-C, 3-4 log reductions were found after 24 h. Carrots pretreated with UV-C exhibited antimicrobial activity against another gram-positive pathogen, Staphylococcus aureus, but not against the gram-negative pathogens, E. coli O157:H7 and Salmonella enterica. Pretreatment with UV-C created a lasting antimicrobial effect as introducing L. monocytogenes on carrots, 72 h post-UV-C treatment, still maintained the antilisterial effect. Notably, all UV-C doses in the range of 48-240 mJ/cm2 induced a lasting antilisterial effect. The bactericidal effects against L. monocytogenes were confirmed in three varieties of washed and unwashed carrots (Danvers, Nantes, and Chantenay). Fluorescence microscopy confirmed the bactericidal effect of UV-C-pretreated carrots on the survival of L. monocytogenes. Conclusively, pretreating carrots with UV-C can reduce the population of L. monocytogenes to levels below the LOD and may further prevent pathogen growth during cold storage. Additional studies are necessary to discern the mechanism underlying the bactericidal efficacy of UV-C-pretreated carrots.

Application of Environmental Monitoring Programs and Root Cause Analysis to Identify and Implement Interventions to Reduce or Eliminate Listeria Populations in Apple Packinghouses

Controlling Listeria in produce packinghouses can be challenging due to the large number of potential contamination routes. For example, repeated isolation of the same Listeria subtype in a packinghouse could indicate persistence in the packinghouse or reintroduction of the same Listeria from an upstream source. To improve understanding of Listeria transmission patterns in packinghouses, we performed a longitudinal study in four apple packinghouses, including testing of 1,339 environmental sponges and whole genome sequencing (WGS)-based characterization of 280 isolates. Root cause analysis and subsequent intervention implementation were also performed and assessed for effectiveness. Listeria prevalence among environmental sponges collected from the four packinghouses was 20% (range of 5-31% for individual packinghouses). Sites that showed high Listeria prevalence included drains, forklift tires and forks, forklift stops, and waxing area equipment frames. A total of 240/280 WGS-characterized isolates were represented in 41 clusters, each containing two or more isolates that differed by ≤50 high-quality single nucleotide polymorphisms (hqSNPs); 21 clusters were isolated from one packinghouse over ≥2 samplings (suggesting persistence or possibly reintroduction), while 11 clusters included isolates from >2 packinghouses, suggesting common upstream sources. Some interventions successfully (i) reduced Listeria detection on forklift tires and forks (across packinghouses) and (ii) mitigated packinghouse-specific Listeria issues (e.g., in catch pans). However, interventions that lacked enhanced equipment disassembly when persistence was suspected typically appeared to be unsuccessful. Overall, while our data suggest a combination of intensive environmental sampling with subtyping and root cause analysis can help identify effective interventions, implementation of effective interventions continues to be a challenge in packinghouses.

Chemical mutagenesis of Listeria monocytogenes for increased tolerance to benzalkonium chloride shows independent genetic underpinnings and off-target antibiotic resistance

Listeria monocytogenes, a potentially fatal foodborne pathogen commonly found in food processing facilities, creates a significant economic burden that totals more than $2 billion annually in the United States due to outbreaks. Quaternary ammonium compounds (QACs), including benzalkonium chloride (BAC), are among the most widely used sanitizers to inhibit the growth and spread of L. monocytogenes from food processing facilities. However, resistance to QACs has been increasing in L. monocytogenes and different genetic mechanisms conferring resistance have been discovered. Here, we used ethyl methanesulfonate (EMS) to chemically mutagenize the BAC-susceptible strain, L. monocytogenes FSL-N1-304. We isolated two mutants with increased tolerance to BAC compared to the parental strain. Next, we assessed the off-target effect of increased tolerance to BAC by measuring the minimum inhibitory concentrations (MICs) of a diverse set of antibiotics, revealing that mut-1 and mut-2 displayed significantly increased resistance to fluoroquinolone antibiotics compared to the parental strain. A hemolysis assay was then used to investigate a potential correlation between BAC tolerance and virulence. Interestingly, mut-1 and mut-2 both exhibited significantly higher hemolysis percentage than the parental strain. We then sequenced the genomes of the parental strain and both mutants to identify mutations that may be involved in the increased resistance to BAC. We identified 3 and 29 mutations in mut-1 and mut-2, respectively. mut-1 contained nonsynonymous mutations in dagK (a diacylglycerol kinase), lmo2768 (a permease-encoding gene), and lmo0186 (resuscitation promoting factor). mut-2 contained a nonsense mutation in the nucleotide excision repair enzyme UvrABC system protein B encoding gene, uvrB, which likely accounts for the higher number of mutations observed. Transcriptome analysis in the presence of BAC revealed that genes related to the phosphotransferase system and internalins were up-regulated in both mutants, suggesting their significance in the BAC stress response. These two mutants provide insights into alternative mechanisms for increased BAC tolerance and could further our understanding of how L. monocytogenes persists in the food processing environment.

Electrochemical DNA Sensor Designed Using the Pencil Graphite Electrode to Detect Listeria monocytogenes

In the present work, a novel electrochemical DNA sensor was designed to detect L. monocytogenes. Two different gene fragments were selected for the sensor design. One is a 702 bp long fragment of the hlyA gene, encoding the synthesis of listeriolysin O toxin, which is unique only to pathogenic strains of L. monocytogenes and is essential for pathogenicity. The other is a 209 bp long fragment of the 16 S RNA gene found in all species of the Listeria genus. As the working electrode, the pencil graphite electrode was modified in various ways (activated or covered with polypyrrole), and six different combinations were constituted using three types of the modified working electrode and two different gene fragments. The developed system is based on differential pulse voltammetric transduction of guanine oxidation after hybridization between the selected gene fragment (38 µg/mL) and the selected fragment-specific inosine-modified probe (1.8 µmol/L) immobilized on a pencil graphite electrode surface. The comparison of all combinations demonstrates that the best results are obtained with the combination formed from a polypyrrole-coated pencil graphite electrode (prepared at 2 scans) and 702 bp fragment of the hlyA gene. The analysis time is less than 1 hour, and the necessary DNA concentrations for the analysis have been determined as 8.2 × 10-11 M DNA and 2.7 × 10-10 M DNA respectively, for the hlyA gene and 16 S RNA gene.

Investigation of the Efficacy of a Listeria monocytogenes Biosensor Using Chicken Broth Samples

Foodborne pathogens are microbes present in food that cause serious illness when the contaminated food is consumed. Among these pathogens, Listeria monocytogenes is one of the most serious bacterial pathogens, and causes severe illness. The techniques currently used for L. monocytogenes detection are based on common molecular biology tools that are not easy to implement for field use in food production and distribution facilities. This work focuses on the efficacy of an electrochemical biosensor in detecting L. monocytogenes in chicken broth. The sensor is based on a nanostructured electrode modified with a bacteriophage as a bioreceptor which selectively detects L. monocytogenes using electrochemical impedance spectroscopy. The biosensing platform was able to reach a limit of detection of 55 CFU/mL in 1× PBS buffer and 10 CFU/mL in 1% diluted chicken broth. The biosensor demonstrated 83-98% recovery rates in buffer and 87-96% in chicken broth.

A comprehensive systematic review and meta-analysis of Listeria monocytogenes prevalence in food products in South Korea

Listeria monocytogenes is a foodborne pathogen that can cause deadly severe listeriosis. While systematic review and meta-analysis are powerful tools for comprehensive analysis by pooling every related study, these approaches to L.monocytogenes contamination food have yet to be studied in South Korea. We aimed to identify high-risk L.monocytogenes foods in South Korea through a prevalence survey of retail food products for the first time. A total of 13,684 samples of 59 articles were used for meta-analysis through the systematic review, and the results were synthesized using a random-effects model considering the heterogeneity. The overall pooled prevalence was 2.26 % (95 % CI: 1.44-3.52 %). Among nine food categories, meat exhibited the highest prevalence at 8.32 % (95 % CI: 4.42-12.14 %) after sample size restriction. Specifically, a post-hoc sensitivity analysis was conducted to identify the prevalence difference among subgroups and the source of heterogeneity. Intriguingly, the analysis revealed chicken as the primary contributor to the elevated prevalence of L.monocytogenes, a key factor deriving the observed heterogeneity. This study carries significant implications for public health and food safety in Korea. Furthermore, knowledge of differences in prevalence levels in various foods will be able to be used as a predictive guideline for foodborne outbreaks.

Efficacy of cleaning and sanitizing procedures to reduce Listeria monocytogenes on food contact surfaces commonly found in fresh produce operations

Cleaning and/or sanitizing methods were evaluated to reduce Listeria monocytogenes (Lm) on coupons of porous (PS; polyester-nylon with coating conveyor belt [PNCB], plywood [PW]) and non-porous (NPS; high density polyethylene, stainless steel) surfaces. Coupons (2.5 cm diameter) were inoculated with six-strain cocktail with cabbage juice, inverted and incubated on tryptic soy agar with yeast extract (TSAYE; 37 °C, 24 h). Coupons (n = 4; ∼9 log CFU/coupon) were rinsed only (RO), multi-step cleaned (MSC), sanitized only (SO; peroxyacetic acid [PAA], bleach, quaternary ammonium compounds [QAC]), or cleaned and sanitized (MSC + S), transferred to Dey/Engley broth (DEB) with glass beads (1 g), vortexed and enumerated on Harlequin-TSAYE. Half of the coupons were dried prior to transfer to DEB. MSC resulted in ave. 2 log CFU/coupon reductions on NPS, and 0.6-1.1 log on PS. MSC + S led to >5-log reduction on NPS (81%; n = 48). On PS, MSC + S-PAA resulted in 1.8 and 1.9 log reductions on PW and PNCB, respectively. RO and SO reduced Lm on all surfaces, although less effectively than MSC and MSC + S. On PS, MSC + S-PAA was most effective, followed by bleach and QAC (p < 0.05). Drying after sanitizing increased Lm reduction by ∼0.4 log (p < 0.05). Partial cleaning and/or sanitizing leads to minimal reduction of Lm, while multi-step cleaning with sanitizing is highly effective.

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.

Taxonomy, ecology, and relevance to food safety of the genus Listeria with a particular consideration of new Listeria species described between 2010 and 2022

Since 2010, the genus Listeria has had the addition of 22 new species that more than tripled the number of species identified until 2010. Sixteen of these 22 new species are distantly related to the type species, Listeria monocytogenes, and several of these present phenotypes that distinguish them from classical Listeria species (L. monocytogenes, Listeria innocua, Listeria ivanovii, Listeria seeligeri, Listeria welshimeri, and Listeria grayi). These 22 newly described species also show that Listeria is more genetically diverse than previously estimated. While future studies and surveys are needed to clarify the distribution of these species, at least some of these species may not be widely spread, while other species may be frequently found spread to human-related settings (e.g., farms and processing facilities), and others may be adapted to specific environmental habitats. Here, we review the taxonomic, phylogenetic, and ecological characteristics of these new Listeria species identified since 2010 and re-iterate the suggestion of re-classification of some species into three new genera: Murraya, Mesolisteria, and Paenilisteria. We also provide a review of current detection issues and the relevance to food safety related to the identification of these new species. For example, several new non-pathogenic species could be misidentified as the pathogen L. monocytogenes, based on methods that do not target L. monocytogenes-specific virulence genes/factors, leading to unnecessary product recalls. Moreover, eight species in the proposed new genus Mesolisteria are not good indicators of environmental conditions that could allow L. monocytogenes to grow since Mesolisteria species are unable to grow at low temperatures.

Role of Whole Genome Sequencing in Assessing Resident and Transient Listeria monocytogenes in a Produce Packinghouse

Whole genome sequencing (WGS) is a powerful tool that may be used to assist in identifying Listeria contamination sources and movement within environments, and to assess persistence. This study investigated sites in a produce packinghouse where Listeria had been historically isolated; and aimed to characterize dispersal patterns and identify cases of transient and resident Listeria. Environmental swab samples (n = 402) were collected from 67 sites at two time-points on three separate visits. Each sample was tested for Listeria, and Listeria isolates were characterized by partial sigB sequencing to determine species and allelic type (AT). Representative isolates from the three most common L. monocytogenes ATs (n = 79) were further characterized by WGS. Of the 144 Listeria species positive samples (35.8%), L. monocytogenes was the most prevalent species. L. monocytogenes was often coisolated with another species of Listeria. WGS identified cases of sporadic and continued reintroduction of L. monocytogenes from the cold storages into the packinghouse and demonstrated cases of L. monocytogenes persistence over 2 years in cold storages, drains, and on a forklift. Nine distinct clusters were found in this study. Two clusters showed evidence of persistence. Isolates in these two clusters (N = 11, with one historical isolate) were obtained predominantly and over multiple samplings from cold storages, with sporadic movement to sites in the packing area, suggesting residence in cold storages with opportunistic dispersal within the packinghouse. The other seven clusters demonstrated evidence of transient Listeria, as isolation was sporadic over time and space during the packing season. Our data provide important insights into likely L. monocytogenes harborage points and transfer in a packinghouse, which is key to root cause analysis. While results support Listeria spp. as a suitable indicator organism for environmental monitoring surveys, findings were unable to establish a specific species as an index organism for L. monocytogenes. Findings also suggest long-term persistence with substantial SNP diversification, which may assist in identifying potential contamination sources and implementing control measures.

Prevalence of Campylobacter spp., Salmonella spp., and Listeria monocytogenes, and Population Levels of Food Safety Indicator Microorganisms in Retail Raw Chicken Meat and Ready-To-Eat Fresh Leafy Greens Salads Sold in Greece

The presence of microbial pathogens in foods compromises their safety resulting in foodborne illnesses, public health disorders, product recalls, and economic losses. In this work, 60 samples of chilled raw chicken meat and 40 samples of packaged ready-to-eat (RTE) fresh leafy greens salads, sold in Greek retail stores (butchers and supermarkets), were analyzed for the presence of three important foodborne pathogenic bacteria, i.e., Campylobacter spp., Salmonella spp., and Listeria monocytogenes, following the detection protocols of the International Organization for Standardization (ISO). In parallel, the total aerobic plate count (APC), Enterobacteriaceae, total coliforms, Escherichia coli, and staphylococci were also enumerated as hygiene (safety) indicator organisms. When present, representative typical colonies for each pathogen were biochemically verified, following the ISO guidelines. At the same time, all the Campylobacter isolates from chicken (n = 120) were identified to the species level and further phylogenetically discriminated through multiplex and repetitive sequence-based (rep) polymerase chain reaction (PCR) methods, respectively. Concerning raw chicken, Campylobacter spp. were recovered from 54 samples (90.0%) and Salmonella spp. were recovered from 9 samples (15.0%), while L. monocytogenes was present in 35 samples (58.3%). No Campylobacter was recovered from salads, and Salmonella was present in only one sample (2.5%), while three salads were found to be contaminated with L. monocytogenes (7.5%). The 65% of the Campylobacter chicken isolates belonged to C. jejuni, whereas the rest, 35%, belonged to C. coli. Alarmingly, APC was equal to or above 106 CFU/g in 53.3% and 95.0% of chicken and salad samples, respectively, while the populations of some of the other safety indicators were in some cases also high. In sum, this study unravels high occurrence percentages for some pathogenic and food safety indicator microorganisms in raw chicken meat and RTE fresh leafy greens salads sold in Greek retail, highlighting the need for more extensive microbiological control throughout the food production chain (from the farm/field to the market).

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.

Population dynamics of Listeria spp., Salmonella spp., and Escherichia coli on fresh produce: A scoping review

Collation of the current scope of literature related to population dynamics (i.e., growth, die-off, survival) of foodborne pathogens on fresh produce can aid in informing future research directions and help stakeholders identify relevant research literature. A scoping review was conducted to gather and synthesize literature that investigates population dynamics of pathogenic and non-pathogenic Listeria spp., Salmonella spp., and Escherichia coli on whole unprocessed fresh produce (defined as produce not having undergone chopping, cutting, homogenization, irradiation, or pasteurization). Literature sources were identified using an exhaustive search of research and industry reports published prior to September 23, 2021, followed by screening for relevance based on strict, a priori eligibility criteria. A total of 277 studies that met all eligibility criteria were subjected to an in-depth qualitative review of various factors (e.g., produce commodities, study settings, inoculation methodologies) that affect population dynamics. Included studies represent investigations of population dynamics on produce before (i.e., pre-harvest; n = 143) and after (i.e., post-harvest; n = 144) harvest. Several knowledge gaps were identified, including the limited representation of (i) pre-harvest studies that investigated population dynamics of Listeria spp. on produce (n = 13, 9% of pre-harvest studies), (ii) pre-harvest studies that were carried out on non-sprouts produce types grown using hydroponic cultivation practices (n = 7, 5% of pre-harvest studies), and (iii) post-harvest studies that reported the relative humidity conditions under which experiments were carried out (n = 56, 39% of post-harvest studies). These and other knowledge gaps summarized in this scoping review represent areas of research that can be investigated in future studies.

Optimization of the Use of a Commercial Phage-Based Product as a Control Strategy of Listeria monocytogenes in the Fresh-Cut Industry

A commercial phage biocontrol for reducing Listeria monocytogenes has been described as an effective tool for improving fresh produce safety. Critical challenges in the phage application must be overcome for the industrial application. The validation studies were performed in two processing lines of two industry collaborators in Spain and Denmark, using shredded iceberg lettuce as the ready-to-eat (RTE), high process volume product. The biocontrol treatment optimized in lab-scale trials for the application of PhageGuard ListexTM was confirmed in industrial settings by four tests, two in Spain and two in Denmark. Results showed that the method of application that included the device and the processing operation step was appropriate for the proper application. The proper dose of Phage Guard ListexTM was reached in shredded iceberg lettuce and the surface was adequately covered for the successful application of phages. There was no impact on the headspace gas composition (CO2 and O2 levels), nor on the color when untreated and treated samples were compared. The post-process treatment with PhageGuard ListexTM did not cause any detrimental impact on the sensory quality, including flavor, texture, browning, spoilage, and visual appearance over the shelf-life as the phage solution was applied as a fine, mist solution.

16S microbiome analysis of microbial communities in distribution centers handling fresh produce

Little is known about the microbial communities found in distribution centers (DCs), especially in those storing and handling food. As many foodborne bacteria are known to establish residence in food facilities, it is reasonable to assume that DCs handling foods are also susceptible to pathogen colonization. To investigate the microbial communities within DCs, 16S amplicon sequencing was completed on 317 environmental surface sponge swabs collected in DCs (n = 18) across the United States. An additional 317 swabs were collected in parallel to determine if any viable Listeria species were also present at each sampling site. There were significant differences in median diversity measures (observed, Shannon, and Chao1) across individual DCs, and top genera across all reads were Carnobacterium_A, Psychrobacter, Pseudomonas_E, Leaf454, and Staphylococcus based on taxonomic classifications using the Genome Taxonomy Database. Of the 39 16S samples containing Listeria ASVs, four of these samples had corresponding Listeria positive microbiological samples. Data indicated a predominance of ASVs identified as cold-tolerant bacteria in environmental samples collected in DCs. Differential abundance analysis identified Carnobacterium_A, Psychrobacter, and Pseudomonas_E present at a significantly greater abundance in Listeria positive microbiological compared to those negative for Listeria. Additionally, microbiome composition varied significantly across groupings within variables (e.g., DC, season, general sampling location).

Human Listeriosis

Listeria monocytogenes is a Gram-positive facultative intracellular pathogen that can cause severe invasive infections upon ingestion with contaminated food. Clinically, listerial disease, or listeriosis, most often presents as bacteremia, meningitis or meningoencephalitis, and pregnancy-associated infections manifesting as miscarriage or neonatal sepsis. Invasive listeriosis is life-threatening and a main cause of foodborne illness leading to hospital admissions in Western countries. Sources of contamination can be identified through international surveillance systems for foodborne bacteria and strains' genetic data sharing. Large-scale whole genome studies have increased our knowledge on the diversity and evolution of L. monocytogenes, while recent pathophysiological investigations have improved our mechanistic understanding of listeriosis. In this article, we present an overview of human listeriosis with particular focus on relevant features of the causative bacterium, epidemiology, risk groups, pathogenesis, clinical manifestations, and treatment and prevention.

Characterization and Antibiotic Resistance of Listeria monocytogenes Strains Isolated from Greek Myzithra Soft Whey Cheese and Related Food Processing Surfaces over Two-and-a-Half Years of Safety Monitoring in a Cheese Processing Facility

Listeriosis is a serious infectious disease with one of the highest case fatality rates (ca. 20%) among the diseases manifested from bacterial foodborne pathogens in humans, while dairy products are often implicated as sources of human infection with Listeria monocytogenes. In this study, we characterized phenotypically and genetically by whole-genome sequencing (WGS) 54 L. monocytogenes strains isolated from Myzithra, a traditional Greek soft whey cheese (48 isolates), and swabs collected from surfaces of a cheese processing plant (six isolates) in the Epirus region of Greece. All but one strain of L. monocytogenes belonged to the polymerase chain reaction (PCR) serogroups IIa (16.7%) and IIb (81.5%), corresponding to serotypes 1/2a, 3a and 1/2b, 3b, 7, respectively. The latter was identified as a PCR-serogroup IVb strain (1.8%) of serotypes 4b, 4d, 4e. Bioinformatics analysis revealed the presence of five sequence types (STs) and clonal complexes (CCs); ST1, ST3, ST121, ST 155, ST398 and CC1, CC3, CC121, CC155, CC398 were thus detected in 1.9, 83.3, 11.0, 1.9, and 1.9% of the L. monocytogenes isolates, respectively. Antibiograms of the pathogen against a panel of seven selected antibiotics (erythromycin, tetracycline, benzylpenicillin, trimethoprim-sulfamethoxazole, ampicillin, ciprofloxacin, and meropenem) showed that 50 strains (92.6%), the six surface isolates also included, were intermediately resistant to ciprofloxacin and susceptible to the rest of the six antimicrobial agents tested, whereas strong resistance against the use of a single from three implicated antibiotics was recorded to four strains (7.4%) of the pathogen isolated from Myzithra cheese samples. Thence, the minimum inhibitory concentrations (MICs) were determined for erythromycin (MIC = 0.19 μg/mL), ciprofloxacin (MIC ≥ 0.19 μg/mL), and meropenem (MIC = 0.64 μg/mL), and finally, just one strain was deemed resistant to the latter antibiotic. The phylogenetic positions of the L. monocytogenes strains and their genetic variability were determined through WGS, whilst also stress response and virulence gene analysis for the isolates was conducted. Findings of this work should be useful as they could be utilized for epidemiological investigations of L. monocytogenes in the food processing environment, revealing possible contamination scenarios, and acquired antimicrobial resistance along the food production chain.

Optimizing the effects of nisin and nacl to thermal inactivate listeria monocytogenes in ground beef with chipotle sauce during sous-vide processing

Mild cooking thermal treatments, like sous-vide, can compromise ground meat entrees such as meatballs with chipotle sauce, especially when salt levels are reduced during its preparation. Listeria monocytogenes is a thermoresistant pathogen that can be in ready-to-eat food. On the other hand, nisin, due to its thermal stability, can be a good alternative to aid on the thermal inactivation of L. monocytogenes and ensure meat safety. The objective was to optimize the amount of nisin and salt concentrations to thermally inactivate L. monocytogenes during the sous-vide cooking of ground beef marinated in chipotle sauce, and to generate a predictive model. A four-strain cocktail was prepared and inoculated in ground beef in combination (3:2) with chipotle sauce added with nisin (0-150 IU) and salt (0-2%). After that, meat samples were sous-vide cooked at different temperatures, nisin, and salt concentrations, established by a central composite design. Depending on the levels of these factors, D-values ranged from 49.71 to 0.27 min. A predictive model (p < 0.05) was obtained by response surface, which described that D-values variation was explained by the linear effects of the three factors, the interaction between nisin and temperature, and the quadratic effects of salt and temperature. It was also observed that nisin presented a bactericidal effect while salt presented a protective effect during the thermal inactivation of L. monocytogenes. Adding 120 IU of nisin and 0.4% of salt to the meat product at 63°C temperature can help to ensure food safety by making L. monocytogenes cells more sensitive to the lethal effect of heat. The model developed in this study can be used by food processors for planning and designing effective levels of salt and nisin to thermally inactivate L. monocytogenes in ground beef products marinated with chipotle sauce to ensure their safety.

Hurdle Technology Approach to Control Listeria monocytogenes Using Rhamnolipid Biosurfactant

This study evaluates the combination of mild heat with a natural surfactant for the inactivation of L. monocytogenes Scott A in low-water-activity (a_w) model systems. Glycerol or NaCl was used to reduce the a_w to 0.92, and different concentrations of rhamnolipid (RL) biosurfactant were added before heat treatment (60 °C, 5 min). Using glycerol, RL treatment (50-250 µg/mL) reduced bacterial population by less than 0.2 log and heat treatment up to 1.5 log, while the combination of both hurdles reached around 5.0 log reduction. In the NaCl medium, RL treatment displayed higher inactivation than in the glycerol medium at the same a_w level and a larger synergistic lethal effect when combined with heat, achieving ≥ 6.0 log reduction at 10-250 µg/mL RL concentrations. The growth inhibition activity of RL was enhanced by the presence of the monovalent salts NaCl and KCl, reducing MIC values from >2500 µg/mL (without salt) to 39 µg/mL (with 7.5% salt). The enhanced antimicrobial activity of RL promoted by the presence of salts was shown to be pH-dependent and more effective under neutral conditions. Overall, results demonstrate that RL can be exploited to design novel strategies based on hurdle approaches aiming to control L. monocytogenes.

Examining Patterns of Persistent Listeria Contamination in Packinghouses Using Agent-Based Models

ABSTRACT

Persistent Listeria monocytogenes contamination may occur in a packinghouse if the pathogen successfully infiltrates the facility and reaches a harborage site, where it may be difficult to remove and may contaminate produce within the facility. There is a need for simulation-based decision support tools that can predict which equipment sites are more likely to undergo persistent contamination and simulate potential corrective actions to prevent this contamination. Thus, we adapted for longer term simulation two existing applications of an agent-based model of Listeria spp. hourly contamination dynamics in produce packinghouses. Next, we developed a novel approach to identify and analyze persistent and transient Listeria contamination patterns on simulated agents representing equipment sites and employees. Testing of corrective actions showed that methods that involved targeted, facility-specific, risk-based sanitation were the most effective in reducing both the likelihood and duration of persistent contamination. Generic approaches to controlling Listeria (e.g., more concentrated sanitizers) are unlikely to be successful and suggest that use of sanitation schedules produced through facility-specific root cause analysis and hygienic design are key in reducing persistence. Hourly Listeria contamination patterns also suggest that transient contamination may be mistaken for persistent contamination, depending on the frequency of environmental sampling. Likewise, as concentrations of Listeria on most contaminated agents were predicted to be very low, there is also a possibility to mistake persistence for transient contamination of sites, or even miss it outright, due to false-negative environmental Listeria monitoring results. These findings support that agent-based models may be valuable decision support tools, aiding in the identification of contamination patterns within packinghouses and assessing the viability of specific corrective actions.

HIGHLIGHTS

Integrative Assessment of Reduced Listeria monocytogenes Susceptibility to Benzalkonium Chloride in Produce Processing Environments

For decades, quaternary ammonium compounds (QAC)-based sanitizers have been broadly used in food processing environments to control foodborne pathogens such as Listeria monocytogenes. Still, there is a lack of consensus on the likelihood and implication of reduced Listeria susceptibility to benzalkonium chloride (BC) that may emerge due to sublethal exposure to the sanitizers in food processing environments. With a focus on fresh produce processing, we attempted to fill multiple data and evidence gaps surrounding the debate. We determined a strong correlation between tolerance phenotypes and known genetic determinants of BC tolerance with an extensive set of fresh produce isolates. We assessed BC selection on L. monocytogenes through a large-scale and source-structured genomic survey of 25,083 publicly available L. monocytogenes genomes from diverse sources in the United States. With the consideration of processing environment constraints, we monitored the temporal onset and duration of adaptive BC tolerance in both tolerant and sensitive isolates. Finally, we examined residual BC concentrations throughout a fresh produce processing facility at different time points during daily operation. While genomic evidence supports elevated BC selection and the recommendation for sanitizer rotation in the general context of food processing environments, it also suggests a marked variation in the occurrence and potential impact of the selection among different commodities and sectors. For the processing of fresh fruits and vegetables, we conclude that properly sanitized and cleaned facilities are less affected by BC selection and unlikely to provide conditions that are conducive for the emergence of adaptive BC tolerance in L. monocytogenes. IMPORTANCE Our study demonstrates an integrative approach to improve food safety assessment and control strategies in food processing environments through the collective leveraging of genomic surveys, laboratory assays, and processing facility sampling. In the example of assessing reduced Listeria susceptibility to a widely used sanitizer, this approach yielded multifaceted evidence that incorporates population genetic signals, experimental findings, and real-world constraints to help address a lasting debate of policy and practical importance.

Cultivation Conditions of Spinach and Rocket Influence Epiphytic Growth of Listeria monocytogenes

Leafy vegetables are associated with Listeriosis outbreaks due to contamination with Listeria monocytogenes. To date, contradictory findings were reported on spinach, rocket, and kale, where some studies reported growth of L. monocytogenes, while others did not. Thus, the current study investigated the reason for conflicting findings by producing leafy vegetables, where cultivation factors were known for growth potential studies. Of all polytunnel produce, kale Nero di Toscana demonstrated the highest growth potential (2.56 log cfu g−1), followed by spinach F1 Cello (1.84 log cfu g−1), rocket Buzz (1.41 log cfu g−1), spinach F1 Trumpet (1.37 log cfu g−1), and finally rocket Esmee (1.23 log cfu g−1). Thus, plant species and variety influenced L. monocytogenes growth potentials. Moreover, significantly lower growth potentials of 0.3 log cfu g−1 were identified when rocket Buzz was cultivated in open fields (1.11 log cfu g−1) instead of a polytunnel. The opposite effect was observed for spinach F1 Trumpet, where growth potentials increased significantly by 0.84 log cfu g−1 when cultivated in open fields (2.21 log cfu g−1). Furthermore, a significant seasonality effect between batches was found (p < 0.05). This study revealed that spinach and rocket cultivation conditions are at least co-factors in the reporting of differing growth potentials of L. monocytogenes across literature and should be considered when conducting future growth potential studies.

Anti-Biofilm Activity of Cell Free Supernatants of Selected Lactic Acid Bacteria against Listeria monocytogenes Isolated from Avocado and Cucumber Fruits, and from an Avocado Processing Plant

Listeria monocytogenes forms biofilms on food contact surfaces, a niche from where it dislodges to contaminate food products including fresh produce. Probiotics and their derivatives are considered promising alternative strategies to curb the presence of L. monocytogenes in varied food applications. Nonetheless, studies on their anti-biofilm effects against L. monocytogenes from avocados and cucumbers are sparse. This study screened the biofilm formation capabilities of L. monocytogenes strains Avo and Cuc isolated from the avocado and cucumber fruits respectively, and strain 243 isolated from an avocado processing plant; and evaluated the anti-biofilm effects of cell free supernatants (CFS) of Lactobacillus acidophilus La14 150B, Lactiplantibacillus plantarum B411 and Lacticaseibacillus rhamnosus ATCC 53103 against their biofilms formed on polyvinyl chloride (PVC) and stainless steel. All the L. monocytogenes strains formed biofilms (classified either as moderate or strong biofilm formers) on these materials. The presence of CFS reduced the biofilm formation capabilities of these strains and disrupted the integrity of their pre-formed biofilms. Quantitative reverse transcriptase polymerase chain reaction revealed significant reduction of positive regulatory factor A (prfA) gene expression by L. monocytogenes biofilm cells in the presence of CFS (p < 0.05). Thus, these CFS have potential as food grade sanitizers for control of L. monocytogenes biofilms in the avocado and cucumber processing facilities.

Genetic diversity of Listeria monocytogenes isolates from farm products in Shanghai, China

Farm products are considered important vehicles for the transmission of Listeria monocytogenes. The typing of L. monocytogenes from farm products contributes to the surveillance and source tracing of the pathogen. In this study, 77 L. monocytogenes strains from seven farm product categories in Shanghai were analyzed by serological typing, multilocus sequence typing (MLST), pulsed-field gel electrophoresis (PFGE) and whole-genome nucleotide polymorphism (wgSNP). The results showed that the 77 isolates were classified into four serovars (1/2a, 1/2c, 1/2b and 4b), and of these, 1/2a (n=47) was the most prevalent. Seventeen sequence types (STs) were generated by MLST with two novel STs (ST1402 and ST1403), and 20 of 77 L. monocytogenes isolates had high genetic identity with previously documented outbreak isolates according to the minimum spanning tree from the MLST results. Moreover, 34 PFGE patterns (PF1-PF34) were differentiated, and based on a similarity value higher than 80% by the UPGMA dendrogram, the discriminatory index was relatively low (equal to 0.775). Furthermore, 14 isolates were chosen and further analyzed by wgSNP based on the previous typing results, which demonstrated that wgSNP and MLST yielded mostly consistent typing results but higher resolution than PFGE. In conclusion, 77 L. monocytogenes isolates from farm products collected in nine districts in Shanghai were highly genetically diverse, and 20 of these isolates had high relatedness with previously documented outbreak strains worldwide. The results indicate a possible cross-contamination risk of L. monocytogenes and a potential public health concern resulting from farm products during the supply chain in Shanghai, China.

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.

Ecology of Listeria monocytogenes and Listeria species in India: the occurrence, resistance to biocides, genomic landscape and biocontrol

Listeria monocytogenes, the causative agent of listeriosis, has been implicated in increasing foodborne outbreaks worldwide. The disease is manifested in various forms ranging from severe sepsis in immune-compromised individuals, febrile gastroenteritis, still birth, abortions and meningoencephalitis. In India, data from studies on the detection and molecular epidemiological analysis of L. monocytogenes are only recently emerging. The presence of Listeria in different ecological niches has been recorded from India, including foods, soil, vegetables, mangrove swamps, seafood, freshwater fishes, clinical cases, and also insects. The organism has also been isolated from women with spontaneous abortions, miscarriage or recurrent obstetric history, aborted foetuses, animal clinical cases and wildlife samples. A novel species of Listeria has also been characterized. Listeria monocytogenes strains isolated from clinical, environmental, and foods showed biofilm-forming abilities. Listeria monocytogenes serotype 4b isolates of ST328, a predominant and unique ST observed in India, was repeatedly isolated from different sources, times, and geographical locations. Here, we reviewed the occurrence of Listeria in different sources in India, its resistance to biocides, and provide epidemiological analysis on its genomic landscape.