New Aspects on Listeria monocytogenes ST5-ECVI Predominance in a Heavily Contaminated Cheese Processing Environment

Unveiling the persistent threat: recent insights into Listeria monocytogenes adaptation, biofilm formation, and pathogenicity in foodborne infections

Listeriosis is a severe disease caused by the foodborne pathogen Listeria monocytogenes, posing a significant risk to vulnerable populations such as the elderly, pregnant women, and newborns. While relatively uncommon, it has a high global mortality rate of 20-30%. Recent research indicates that smaller outbreaks of the more severe, invasive form of the disease occur more frequently than previously thought, despite the overall stable infection rates of L. monocytogenes over the past 10 years. The ability of L. monocytogenes to form biofilm structures on various surfaces in food production environments contributes to its persistence and challenges in eradication, potentially leading to contamination of food and food production facilities. To address these concerns, this review focuses on recent developments in epidemiology, risk evaluations, and molecular mechanisms of L. monocytogenes survival in adverse conditions and environmental adaptation. Additionally, it covers new insights into strain variability, pathogenicity, mutations, and host vulnerability, emphasizing the important events framework that elucidates the biochemical pathways from ingestion to infection. Understanding the adaptation approaches of L. monocytogenes to environmental stress factors is crucial for the development of effective and affordable pathogen control techniques in the food industry, ensuring the safety of food production.

Raw milk cheeses from Beira Baixa, Portugal-A contributive study for the microbiological hygiene and safety assessment

Due to specific bacterial microbiota, raw milk cheeses have appreciated sensory properties. However, they may pose a threat to consumer safety due to potential pathogens presence. This study evaluated the microbiological contamination of 98 raw milk cheeses from Beira Baixa, Portugal. Presence and enumeration of Coagulase Positive Staphylococci (CPS), Listeria monocytogenes, Salmonella spp., pathogenic Escherichia coli, and indicator microorganisms (non-pathogenic E. coli and Listeria spp.) was attained. E. coli antimicrobial resistance (AMR) was also evaluated. PCR and/or Whole genome sequencing (WGS) was used to characterize E. coli, Salmonella spp. and L. monocytogenes isolates. Sixteen cheeses (16.3%) were classified as Satisfactory, 59 (60.2%) as Borderline and 23 (23.5%) as Unsatisfactory/Potential Injurious to Health. L. monocytogenes, CPS > 104 cfu g-1, Extraintestinal pathogenic E. coli (ExPEC) and Salmonella spp. were detected in 4.1%, 6.1%, 3.1% and 1.0% of the samples, respectively. Listeria innocua (4.1%) and E. coli > 104 cfu g-1 (16.3%) were also detected. AMR E. coli was detected in 23/98 (23.5%) of the cheese samples, of which two were multidrug resistant. WGS identified genotypes already associated to human disease and Listeria spp. cluster analysis indicated that cheese contamination might be related with noncompliance with Good Hygiene Practices during cheese production.

Listeria monocytogenes prevalence and genomic diversity along the pig and pork production chain

The facultative intracellular bacterium Listeria monocytogenes (L. monocytogenes) is the causative agent of listeriosis, a severe invasive illness. This ubiquitous species is widely distributed in the environment, but infection occurs almost exclusively through ingestion of contaminated food. The pork production sector has been heavily affected by a series of L. monocytogenes-related foodborne outbreaks in the past around the world. Ready-to-eat (RTE) pork products represent one of the main food sources for strong-evidence listeriosis outbreaks. This pathogen is known to be present throughout the entire pig and pork production chain. Some studies hypothesized that the main source of contamination in final pork products was either living pigs or the food-processing environment. A detailed genomic picture of L. monocytogenes can provide a renewed understanding of the routes of contamination from pig farms to the final products. This review provides an overview of the prevalence, the genomic diversity and the genetic background linked to virulence of L. monocytogenes along the entire pig and pork production chain, from farm to fork.

Persistence of microbiological hazards in food and feed production and processing environments

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

Environmental persistence of Listeria monocytogenes and its implications in dairy processing plants

Listeriosis, an invasive illness with a fatality rate between 20% and 30%, is caused by the ubiquitous bacterium Listeria monocytogenes. Human listeriosis has long been associated with foods. This is because the ubiquitous nature of the bacteria renders it a common food contaminant, posing a significant risk to the food processing sector. Although several sophisticated stress coping mechanisms have been identified as significant contributing factors toward the pathogen's persistence, a complete understanding of the mechanisms underlying persistence across various strains remains limited. Moreover, aside from genetic aspects that promote the ability to cope with stress, various environmental factors that exist in food manufacturing plants could also contribute to the persistence of the pathogen. The objective of this review is to provide insight into the challenges faced by the dairy industry because of the pathogens' environmental persistence. Additionally, it also aims to emphasize the diverse adaptation and response mechanisms utilized by L. monocytogenes in food manufacturing plants to evade environmental stressors. The persistence of L. monocytogenes in the food processing environment poses a serious threat to food safety and public health. The emergence of areas with high levels of L. monocytogenes contamination could facilitate Listeria transmission through aerosols, potentially leading to the recontamination of food, particularly from floors and drains, when sanitation is implemented alongside product manufacturing. Hence, to produce safe dairy products and reduce the frequency of outbreaks of listeriosis, it is crucial to understand the factors that contribute to the persistence of this pathogen and to implement efficient control strategies.

Listeria monocytogenes Biofilms in Food-Associated Environments: A Persistent Enigma

Listeria monocytogenes (LM) is a bacterial pathogen responsible for listeriosis, a foodborne illness associated with high rates of mortality (20-30%) and hospitalisation. It is particularly dangerous among vulnerable groups, such as newborns, pregnant women and the elderly. The persistence of this organism in food-associated environments for months to years has been linked to several devastating listeriosis outbreaks. It may also result in significant costs to food businesses and economies. Currently, the mechanisms that facilitate LM persistence are poorly understood. Unravelling the enigma of what drives listerial persistence will be critical for developing more targeted control and prevention strategies. One prevailing hypothesis is that persistent strains exhibit stronger biofilm production on abiotic surfaces in food-associated environments. This review aims to (i) provide a comprehensive overview of the research on the relationship between listerial persistence and biofilm formation from phenotypic and whole-genome sequencing (WGS) studies; (ii) to highlight the ongoing challenges in determining the role biofilm development plays in persistence, if any; and (iii) to propose future research directions for overcoming these challenges.

Identification by High-Throughput Real-Time PCR of 30 Major Circulating Listeria monocytogenes Clonal Complexes in Europe

Listeria monocytogenes is a ubiquitous bacterium that causes a foodborne illness, listeriosis. Most strains can be classified into major clonal complexes (CCs) that account for the majority of outbreaks and sporadic cases in Europe. In addition to the 20 CCs known to account for the majority of human and animal clinical cases, 10 CCs are frequently reported in food production, thereby posing a serious challenge for the agrifood industry. Therefore, there is a need for a rapid and reliable method to identify these 30 major CCs. The high-throughput real-time PCR assay presented here provides accurate identification of these 30 CCs and eight genetic subdivisions within four CCs, splitting each CC into two distinct subpopulations, along with the molecular serogroup of a strain. Based on the BioMark high-throughput real-time PCR system, our assay analyzes 46 strains against 40 real-time PCR arrays in a single experiment. This European study (i) designed the assay from a broad panel of 3,342 L. monocytogenes genomes, (ii) tested its sensitivity and specificity on 597 sequenced strains collected from 24 European countries, and (iii) evaluated its performance in the typing of 526 strains collected during surveillance activities. The assay was then optimized for conventional multiplex real-time PCR for easy implementation in food laboratories. It has already been used for outbreak investigations. It represents a key tool for assisting food laboratories to establish strain relatedness with human clinical strains during outbreak investigations and for helping food business operators by improving their microbiological management plans. IMPORTANCE Multilocus sequence typing (MLST) is the reference method for Listeria monocytogenes typing but is expensive and takes time to perform, from 3 to 5 days for laboratories that outsource sequencing. Thirty major MLST clonal complexes (CCs) are circulating in the food chain and are currently identifiable only by sequencing. Therefore, there is a need for a rapid and reliable method to identify these CCs. The method presented here enables the rapid identification, by real-time PCR, of 30 CCs and eight genetic subdivisions within four CCs, splitting each CC into two distinct subpopulations. The assay was then optimized on different conventional multiplex real-time PCR systems for easy implementation in food laboratories. The two assays will be used for frontline identification of L. monocytogenes isolates prior to whole-genome sequencing. Such assays are of great interest for all food industry stakeholders and public agencies for tracking L. monocytogenes food contamination.

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

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

Pervasive Listeria monocytogenes Is Common in the Norwegian Food System and Is Associated with Increased Prevalence of Stress Survival and Resistance Determinants

To investigate the diversity, distribution, persistence, and prevalence of stress survival and resistance genes of Listeria monocytogenes clones dominating in food processing environments in Norway, genome sequences from 769 L. monocytogenes isolates from food industry environments, foods, and raw materials (512 of which were sequenced in the present study) were subjected to whole-genome multilocus sequence typing (wgMLST), single-nucleotide polymorphism (SNP), and comparative genomic analyses. The data set comprised isolates from nine meat and six salmon processing facilities in Norway collected over a period of three decades. The most prevalent clonal complex (CC) was CC121, found in 10 factories, followed by CC7, CC8, and CC9, found in 7 factories each. Overall, 72% of the isolates were classified as persistent, showing 20 or fewer wgMLST allelic differences toward an isolate found in the same factory in a different calendar year. Moreover, over half of the isolates (56%) showed this level of genetic similarity toward an isolate collected from a different food processing facility. These were designated as pervasive strains, defined as clusters with the same level of genetic similarity as persistent strains but isolated from different factories. The prevalence of genetic determinants associated with increased survival in food processing environments, including heavy metal and biocide resistance determinants, stress response genes, and inlA truncation mutations, showed a highly significant increase among pervasive isolates but not among persistent isolates. Furthermore, these genes were significantly more prevalent among the isolates from food processing environments compared to in isolates from natural and rural environments (n = 218) and clinical isolates (n = 111) from Norway.

IMPORTANCEListeria monocytogenes can persist in food processing environments for months to decades and spread through the food system by, e.g., contaminated raw materials. Knowledge of the distribution and diversity of L. monocytogenes is important in outbreak investigations and is essential to effectively track and control this pathogen in the food system. The present study presents a comprehensive overview of the prevalence of persistent clones and of the diversity of L. monocytogenes in Norwegian food processing facilities. The results demonstrate extensive spread of highly similar strains throughout the Norwegian food system, in that 56% of the 769 collected isolates from food processing factories belonged to clusters of L. monocytogenes identified in more than one facility. These strains were associated with an overall increase in the prevalence of plasmids and determinants of heavy metal and biocide resistance, as well as other genetic elements associated with stress survival mechanisms and persistence.

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

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

Biofilm formation and genomic features of Listeria monocytogenes strains isolated from meat and dairy industries located in Piedmont (Italy)

Listeria monocytogenes is considered a major challenge for the food industry as it can persist for long periods in food processing plants by forming biofilms. The aims of this study were: i) to assess the biofilm producing ability of 57 Listeria monocytogenes isolates previously subjected to whole-genome sequencing (WGS); ii) to compare the levels of biofilm formation with the presence or absence of biofilm associated genes. To determine the presence or absence of a known set of biofilm associated genes, a comparative genomic analysis was performed on each strain. Among Listeria monocytogenes isolates, 58 %, 38.5 % and 3.5 % exhibited weak, moderate or strong biofilm production, respectively. No difference in biofilm production was observed between food and environmental isolates. The percentage of Listeria monocytogenes strains isolated from meat products (57 %) classified as moderate or strong biofilm producers was higher than the percentage obtained for strains isolated from dairy products (28 %). The presence of the Stress Survival Islet 1, the arsD stress gene and the truncated inlA protein was significantly associated with increased levels of biofilm. Combining biofilm phenotype with molecular and genotyping data may provide the opportunity to better understand the relationship between genes linked to biofilm formation in Listeria monocytogenes.

A Whole Genome Sequencing-Based Epidemiological Investigation of a Pregnancy-Related Invasive Listeriosis Case in Central Italy

Listeriosis is currently the fifth most common foodborne disease in Europe. Most cases are sporadic; however, outbreaks have also been reported. Compared to other foodborne infections, listeriosis has a modest incidence but can cause life-threatening complications, especially in elderly or immunocompromised people and pregnant women. In the latter case, the pathology can be the cause of premature birth or spontaneous abortion, especially if the fetus is affected during the first months of gestation. The causative agent of listeriosis, Listeria monocytogenes, is characterized by the innate ability to survive in the environment and in food, even in adverse conditions and for long periods. Ready-to-eat food represents the category most at risk for contracting listeriosis. This study presents the result of an investigation carried out on a case of maternal-fetal transmission of listeriosis which occurred in 2020 in central Italy and which was linked, with a retrospective approach, to other cases residing in the same city of the pregnant woman. Thanks to the use of next-generation sequencing methodologies, it was possible to identify an outbreak of infection, linked to the consumption of ready-to-eat sliced products sold in a supermarket in the investigated city.

Frozen Vegetable Processing Plants Can Harbour Diverse Listeria monocytogenes Populations: Identification of Critical Operations by WGS

Frozen vegetables have emerged as a concern due to their association with foodborne outbreaks such as the multi-country outbreak of Listeria monocytogenes serogroup IVb linked to frozen corn. The capacity of L. monocytogenes to colonize food-processing environments is well-known, making the bacteria a real problem for consumers. However, the significance of the processing environment in the contamination of frozen foods is not well established. This study aimed to identify potential contamination niches of L. monocytogenes in a frozen processing plant and characterize the recovered isolates. A frozen vegetable processing plant was monitored before cleaning activities. A total of 78 points were sampled, including frozen vegetables. Environmental samples belonged to food-contact surfaces (FCS); and non-food-contact surfaces (n-FCS). Positive L. monocytogenes samples were found in FCS (n = 4), n-FCS (n = 9), and the final product (n = 1). A whole-genome sequencing (WGS) analysis revealed two clusters belonging to serotypes 1/2a-3a and 1/2b-3b). The genetic characterization revealed the presence of four different sequence types previously detected in the food industry. The isolate obtained from the final product was the same as one isolate found in n-FCS. A multi-virulence-locus sequence typing (MVLST) analysis showed four different virulence types (VT). The results obtained highlight the relevant role that n-FCS such as floors and drains can play in spreading L. monocytogenes contamination to the final product.

Systematic review of Listeria monocytogenes from food and clinical samples in Chinese mainland from 2010 to 2019

Listeria monocytogenes, a foodborne pathogen, can cause human listeriosis. Listeriosis is a potentially fatal gastrointestinal illness, which is closely related to the spread of food to humans. We review the literature published during 2010 to 2019 to better understand the prevalence of L. monocytogenes in food products, incidence of human listeriosis, and their characteristics in Chinese mainland. We found the main sequence types (STs) strains from foods are similar globally, and the prevalence of L. monocytogenes from raw meat was the highest among all food products. The most common STs in food products and clinical cases were ST9 (serogroup Ⅰ.2) strains and ST87 (serogroup Ⅱ.2) strains, respectively. The ST87 strains being the most common STs of clinical cases might be related to the exist of Listeria pathogenicity islands 4 genes and Chinese eating habits for ready to eat foods, among which the prevalence of ST87 strain was the highest in ready to eat food. Therefore, more research should be conducted to explore the reasons for the L. monocytogenes isolates differences in food and clinic sources. Meanwhile, more research should be conducted to explore the reasons for differences among the L. monocytogenes isolates in food and clinical sources.

The Saprophytic Lifestyle of Listeria monocytogenes and Entry Into the Food-Processing Environment

Listeria monocytogenes is an environmentally adapted saprophyte that can change into a human and animal bacterial pathogen with zoonotic potential through several regulatory systems. In this review, the focus is on the occurrence of Listeria sensu stricto and sensu lato in different ecological niches, the detection methods, and their analytical limitations. It also highlights the occurrence of L. monocytogenes genotypes in the environment (soil, water, and wildlife), reflects on the molecular determinants of L. monocytogenes for the saprophytic lifestyle and the potential for antibiotic resistance. In particular, the strain-specific properties with which some genotypes circulate in wastewater, surface water, soil, wildlife, and agricultural environments are of particular interest for the continuously updating risk analysis.

Diversity of Listeria monocytogenes Strains Isolated from Food Products in the Central European Part of Russia in 2000-2005 and 2019-2020

Totally, 45 L. monocytogenes strains isolated from meat, poultry, dairy, and fish products in the Central European part of Russia in 2001–2005 and 2019–2020 were typed using a combined MLST and internalin profile (IP) scheme. Strains belonged to 14 clonal complexes (CCs) of the phylogenetic lineages I and II. Almost half of the strains (20 of 45) belonged to six CCs previously recognized as epidemic clones (ECs). ECI and ECV strains were isolated during both studied periods, and ECII, ECIV, ECVI, and ECVII strains were isolated in 2001–2005, but not in 2019–2020. ECI, ECIV, ECV, and ECVII strains were isolated from products of animal origin. ECII and ECVI were isolated from fish. Testing of invasion efficiencies of 10 strains isolated in different years and from different sources and belonging to distinct CCs revealed a statistically significant difference between phylogenetic lineage I and II strains but not between ECs and non-EC CCs or strains differing by year and source of isolation. Strains isolated in 2001–2005 were characterized by higher phylogenetic diversity and greater presentation of ECs and CCs non-typical for natural and anthropogenic environments of the European part of Russia comparatively to isolates obtained in 2019–2020.Closing of the Russian market in 2019–2020 for imported food might be responsible for these differences.

ON-rep-seq as a rapid and cost-effective alternative to whole-genome sequencing for species-level identification and strain-level discrimination of Listeria monocytogenes contamination in a salmon processing plant

Identification, source tracking, and surveillance of food pathogens are crucial factors for the food-producing industry. Over the last decade, the techniques used for this have moved from conventional enrichment methods, through species-specific detection by PCR to sequencing-based methods, whole-genome sequencing (WGS) being the ultimate method. However, using WGS requires the right infrastructure, high computational power, and bioinformatics expertise. Therefore, there is a need for faster, more cost-effective, and more user-friendly methods. A newly developed method, ON-rep-seq, combines the classical rep-PCR method with nanopore sequencing, resulting in a highly discriminating set of sequences that can be used for species identification and also strain discrimination. This study is essentially a real industry case from a salmon processing plant. Twenty Listeria monocytogenes isolates were analyzed both by ON-rep-seq and WGS to identify and differentiate putative L. monocytogenes from a routine sampling of processing equipment and products, and finally, compare the strain-level discriminatory power of ON-rep-seq to different analyzing levels delivered from the WGS data. The analyses revealed that among the isolates tested there were three different strains. The isolates of the most frequently detected strain (n = 15) were all detected in the problematic area in the processing plant. The strain level discrimination done by ON-rep-seq was in full accordance with the interpretation of WGS data. Our findings also demonstrate that ON-rep-seq may serve as a primary screening method alternative to WGS for identification and strain-level differentiation for surveillance of potential pathogens in a food-producing environment.

Use of an Ecosystem-Based Approach to Shed Light on the Heterogeneity of the Contamination Pattern of Listeria monocytogenes on Conveyor Belt Surfaces in a Swine Slaughterhouse in the Province of Quebec, Canada

The role of the accompanying microbiota in the presence of Listeria monocytogenes on meat processing surfaces is not yet understood, especially in industrial production conditions. In this study, 300 conveyor belt samples from the cutting room of a swine slaughterhouse were collected during production. The samples were subjected to the detection of L. monocytogenes. Recovered strains were characterized by serogrouping-PCR, InlA Sanger sequencing and for their ability to form biofilm. A selection of isolates was compared with core genome multi-locus sequence typing analysis (cgMLST). The sequencing of the V4 region of the 16S RNA gene of the microorganisms harvested from each sample was carried out in parallel using the Illumina MiSeq platform. Diversity analyses were performed and MaAsLin analysis was used to assess the link between L. monocytogenes detection and the surrounding bacteria. The 72 isolates collected showed a low genetic diversity and important persistence characteristics. L. monocytogenes isolates were not stochastically distributed on the surfaces: the isolates were detected on three out of six production lines, each associated with a specific meat cut: the half carcasses, the bostons and the picnics. MaAsLin biomarker analysis identified the taxa Veillonella (p ≤ 0.0397) as a bacterial determinant of the presence of L. monocytogenes on processing surfaces. The results of this study revealed a heterogenous contamination pattern of the processing surfaces by L. monocytogenes and targeted a bacterial indicator of the presence of the pathogen. These results could lead to a better risk assessment of the contamination of meat products.

Harnessing Whole Genome Sequence Data for Facility-Specific Signatures for Listeria monocytogenes: A Case Study With Turkey Processing Plants in the United States

Listeria monocytogenes is a Gram-positive foodborne pathogen responsible for the severe disease listeriosis and notorious for its ability to persist in food processing plants, leading to contamination of processed, ready-to-eat foods. L. monocytogenes persistence in various food processing environments (FPEs) has been extensively investigated by various subtyping tools, with increasing use of whole genome sequencing (WGS). However, major knowledge gaps remain. There is a need for facility-specific molecular signatures not only for adequate attribution of L. monocytogenes to a specific FPE but also for improved understanding of the ecology and evolution of L. monocytogenes in the food processing ecosystem. Furthermore, multiple strains can be recovered from a single FPE sample, but their diversity can be underestimated with common molecular subtyping tools. In this study we investigated a panel of 54 L. monocytogenes strains from four turkey processing plants in the United States. A combination of WGS and phenotypic assays was employed to assess strain persistence as well as identify facility-specific molecular signatures. Comparative analysis of allelic variation across the whole genome revealed that allelic profiles have the potential to be specific to individual processing plants. Certain allelic profiles remained associated with individual plants even when closely-related strains from other sources were included in the analysis. Furthermore, for certain sequence types (STs) based on the seven-locus multilocus sequence typing scheme, presence and location of premature stop codons in inlA, inlB length, prophage sequences, and the sequence content of a genomic hotspot could serve as plant-specific signatures. Interestingly, the analysis of different isolates from the same environmental sample revealed major differences not only in serotype and ST, but even in the sequence content of strains of the same ST. This study highlights the potential for WGS data to be deployed for identification of facility-specific signatures, thus facilitating the tracking of strain movement through the food chain. Furthermore, deployment of WGS for intra-sample strain analysis allows for a more complete environmental surveillance of L. monocytogenes in food processing facilities, reducing the risk of failing to detect strains that may be clinically relevant and potentially novel.

Certain Listeria monocytogenes plasmids contribute to increased UVC ultraviolet light stress

Listeria monocytogenes is the causative agent of the highly fatal foodborne disease listeriosis and can persist in food production environments. Recent research highlights the involvement of L. monocytogenes plasmids in different stress response mechanisms, which contribute to its survival in food production facilities. Ultraviolet (UV) light in the UVC spectrum (200–280 nm) is used in food production to control microbial contamination. Although plasmid-encoded UV resistance mechanisms have been described in other bacteria, no research indicates that L. monocytogenes plasmids contribute to the UV stress response. The plasmids of L. monocytogenes strains 6179, 4KSM and R479a are genetically distinct and were utilized to study the roles of plasmids in the UV response. Wild-type and plasmid-cured variant cells were grown to logarithmic or late-stationary phase, plated on agar plates and exposed to UVC for 60 or 90 s, and colony-forming units (CFUs) were determined. CFUs of 6179 and 4KSM, bearing pLM6179 and p4KSM, respectively, were significantly (P-value < 0.05) higher than those of the plasmid-cured strains in both logarithmic and stationary phases. No difference in survival was observed for the R479a strain. Our data show for the first time that certain L. monocytogenes plasmids contribute to the survival of UVC light stress.

Listeria monocytogenes Contamination Characteristics in Two Ready-to-Eat Meat Plants From 2019 to 2020 in Shanghai

Listeria monocytogenes is a ubiquitous foodborne pathogen that causes listeriosis and is mostly linked to consumption of ready-to-eat (RTE) foods. Lack of hygiene in food processing environments may be a primary reason for contamination by L. monocytogenes isolates. In this study, L. monocytogenes strains isolated from two RTE meat processing plants in the Shanghai municipality, China, were characterized during 2019–2020 using pulsed-field gel electrophoresis and whole-genome sequencing. Results showed that 29 samples (12.2%) out of 239 were positive for L. monocytogenes, with 21 (18.9%) and 8 (6.25%) isolates from plants A and B, respectively. The packaging room at plant A had the most contamination (14, 48.3%; p < 0.05), with a peak occurrence of 76.5% in processing environments. Nineteen L. monocytogenes isolates belonging to the pulsotype (PT) 7 group were indistinguishable (≥ 95.7%). Furthermore, core-genome multiple loci sequencing typing identified up to nine allelic differences, and the closet pairwise differences among these ST5 isolates included 0–16 small nucleotide polymorphisms. Therefore, L. monocytogenes likely persisted at plant A during 2019–2020 with ongoing clone transmission. In contrast, no L. monocytogenes isolates were identified from processing environments at plant B. Most L. monocytogenes isolates were sampled from raw materials (62.5%). Several isolates (ST378, ST8, and ST120) were detected only once in 2020 and were considered as transient isolates. However, three ST121 isolates with the same PT (PT2) were detected in 2020 and should be noted for their stronger survival ability in harsh environments. These results suggest that continuous monitoring, stringent surveillance, and source tracking are crucial to guaranteeing food safety in RTE food plants.

Monitoring by a Sensitive Liquid-Based Sampling Strategy Reveals a Considerable Reduction of Listeria monocytogenes in Smeared Cheese Production over 10 Years of Testing in Austria

Most Austrian dairies and cheese manufacturers participated in a Listeria monitoring program, which was established after the first reports of dairy product-associated listeriosis outbreaks more than thirty years ago. Within the Listeria monitoring program, up to 800 mL of product-associated liquids such as cheese smear or brine are processed in a semi-quantitative approach to increase epidemiological sensitivity. A sampling strategy within cheese production, which detects environmental contamination before it results in problematic food contamination, has benefits for food safety management. The liquid-based sampling strategy was implemented by both industrial cheese makers and small-scale dairies located in the mountainous region of Western Austria. This report considers more than 12,000 Listeria spp. examinations of liquid-based samples in the 2009 to 2018 timeframe. Overall, the occurrence of L. monocytogenes in smear liquid samples was 1.29% and 1.55% (n = 5043 and n = 7194 tested samples) for small and industrial cheese enterprises, respectively. The liquid-based sampling strategy for Listeria monitoring at the plant level appears to be superior to solid surface monitoring. Cheese smear liquids seem to have good utility as an index of the contamination of cheese up to that point in production. A modelling or validation process should be performed for the new semi-quantitative approach to estimate the true impact of the method in terms of reducing Listeria contamination at the cheese plant level.

Whole Genome-Based Characterization of Listeria monocytogenes Isolates Recovered From the Food Chain in South Africa

Listeria monocytogenes is an important foodborne pathogen which has the ability to adapt and survive in food and food processing facilities where it can persist for years. In this study, a total of 143 L. monocytogenes isolates in South Africa (SA) were characterized for their strain’s genetic relatedness, virulence profiles, stress tolerance and resistance genes associated with L. monocytogenes. The Core Genome Multilocus Sequence Typing (cgMLST) analysis revealed that the most frequent serogroups were IVb and IIa; Sequence Types (ST) were ST204, ST2, and ST1; and Clonal Complexes (CC) were CC204, CC1, and CC2. Examination of genes involved in adaptation and survival of L. monocytogenes in SA showed that ST1, ST2, ST121, ST204, and ST321 are well adapted in food processing environments due to the significant over-representation of Benzalkonium chloride (BC) resistance genes (bcrABC cassette, ermC, mdrL and Ide), stress tolerance genes (SSI-1 and SSI-2), Prophage (φ) profiles (LP_101, vB LmoS 188, vB_LmoS_293, and B054 phage), plasmids profiles (N1-011A, J1776, and pLM5578) and biofilm formation associated genes. Furthermore, the L. monocytogenes strains that showed hyper-virulent potential were ST1, ST2 and ST204, and hypo-virulent were ST121 and ST321 because of the presence and absence of major virulence factors such as LIPI-1, LIPI-3, LIPI-4 and the internalin gene family members including inlABCEFJ. The information provided in this study revealed that hyper-virulent strains ST1, ST2, and ST204 could present a major public health risk due to their association with meat products and food processing environments in SA.

Effect of Gaseous Ozone on Listeria monocytogenes Planktonic Cells and Biofilm: An In Vitro Study

Among food-borne pathogens, Listeria monocytogenes continues to pose concerns to food business operators due to its capacity to form biofilm in processing environments. Ozone may be an eco-friendly technology to control microbial contaminations, but data concerning its effect on Listeria monocytogenes biofilm are still limited. In this study, the effect of gaseous ozone at 50 ppm on planktonic cells and biofilm of reference and food-related Listeria monocytogenes strains was evaluated. Ozone caused a reduction in microbial loads of 3.7 ± 0.4 and 3.9 ± 0.4 Log10 CFU/mL after 10 and 30 min, respectively. A complete inactivation of planktonic cells after 6 h of treatment was observed. Biofilm inhibition and eradication treatments (50 ppm, 6 h) resulted in a significant decrease of the biofilm biomass for 59% of the strains tested, whilst a slight dampening of live cell loads in the biofilm state was observed. In conclusion, gaseous ozone is not sufficient to completely counteract Listeria monocytogenes biofilm, but it may be useful as an additional tool to contrast Listeria monocytogenes free-living cells and to improve the existing sanitization procedures in food processing environments.

A Large-Scale Sequencing-Based Survey of Plasmids in Listeria monocytogenes Reveals Global Dissemination of Plasmids

The food-borne pathogen Listeria monocytogenes is known for its capacity to cope with multiple stress conditions occurring in food and food production environments (FPEs). Plasmids can provide benefits to their host strains, and it is known that various Listeria strains contain plasmids. However, the current understanding of plasmid frequency and function in L. monocytogenes strains remains rather limited. To determine the presence of plasmids among L. monocytogenes strains and their potential contribution to stress survival, a comprehensive dataset was established based on 1,921 published genomes from strains representing 14 L. monocytogenes sequence types (STs). Our results show that an average of 54% of all L. monocytogenes strains in the dataset contained a putative plasmid. The presence of plasmids was highly variable between different STs. While some STs, such as ST1, ST2, and ST4, contained few plasmid-bearing strains (<15% of the strains per ST), other STs, such as ST121, ST5, ST8, ST3, and ST204, possessed a higher proportion of plasmid-bearing strains with plasmids found in >71% of the strains within each ST. Overall, the sizes of plasmids analyzed in this study ranged from 4 to 170 kbp with a median plasmid size of 61 kbp. We also identified two novel groups of putative Listeria plasmids based on the amino acid sequences of the plasmid replication protein, RepA. We show that highly conserved plasmids are shared among Listeria strains which have been isolated from around the world over the last few decades. To investigate the potential roles of plasmids, nine genes related to stress-response were selected for an assessment of their abundance and conservation among L. monocytogenes plasmids. The results demonstrated that these plasmid genes exhibited high sequence conservation but that their presence in plasmids was highly variable. Additionally, we identified a novel transposon, Tn7075, predicted to be involved in mercury-resistance. Here, we provide the largest plasmid survey of L. monocytogenes to date with a comprehensive examination of the distribution of plasmids among L. monocytogenes strains. Our results significantly increase our knowledge about the distribution, composition, and conservation of L. monocytogenes plasmids and suggest that plasmids are likely important for the survival of L. monocytogenes in food and FPEs.

Identification of Listeria monocytogenes Contamination in a Ready-to-Eat Meat Processing Plant in China

Listeria monocytogenes is the etiologic agent of listeriosis, which remains a significant public health concern in many countries due to its high case-fatality rate. The constant risk of L. monocytogenes transmission to consumers remains a central challenge in the food production industry. At present, there is very little known about L. monocytogenes contamination in ready-to-eat (RTE) processing plants in China. In this study, L. monocytogenes in an RTE meat processing plant in Shanghai municipality was characterized using pulsed-field gel electrophoresis (PFGE) and whole genome sequencing (WGS). Furthermore, the biofilm formation ability of the pathogen was also tested. Results revealed that L. monocytogenes isolates were present in 12 samples out of the 48 samples investigated. Most of them (66.7%, 8/12) were identified from the processing facilities irrespective of observed hygiene levels of aerobic plate count (APC) and coliforms. Coliforms were present in only one processing area. ST5 (1/2b) isolates were predominant (83.3%, 10/12) and were identified in two dominant pulsotypes (PTs) (three in PT3 and seven in PT4, respectively). Results of the core-genome multi-locus sequence typing (cgMLST) showed that ST5 in three PTs (PT1, PT3, and PT4) had 0-8 alleles, which confirmed that clonal transmission occurred in the RTE meat processing facilities. In addition, the biofilm formation test confirmed that the isolates from the processing facilities could form biofilms, which helped them colonize and facilitate persistence in the environment. These results indicated that common sanitation procedures regularly applied in the processing environment were efficient but not sufficient to remove L. monocytogenes isolates, especially biofilm of L. monocytogenes. Furthermore, the ST5 isolates in this study exhibited 12 alleles with one ST5 clinical isolate, which contributes to the understanding of the potential pathogenic risk that L. monocytogenes in RTE meat processing equipment posed to consumers. Therefore, strong hygienic measures, especially sanitation procedures for biofilms eradication, should be implemented to ensure the safety of raw materials. Meanwhile, continuous surveillance might be vital for the prevention and control of listeriosis caused by L. monocytogenes.

Sampling the Food-Processing Environment: Taking Up the Cudgel for Preventive Quality Management in Food Processing (FP)

The Listeria monitoring program for Austrian dairies and cheese factories was established in 1988. The aim was to control the entrance of L. monocytogenes into the food-processing environment (FPE), preventing the contamination of food under processing. The Austrian Listeria monitoring program comprises four levels of investigation, dealing with routine monitoring of samples and consequences of finding a positive sample. Preventive quality control concepts attempt to detect a foodborne hazard along the food-processing chain, prior to food delivery, retailing, and consumption. The implementation of a preventive food safety concept provokes a deepened insight by the manufacturers into problems concerning food safety. The development of preventive quality assurance strategies contributes to the national food safety status and protects public health.

Extraction and Analysis of Plasmid DNA from Listeria monocytogenes

A plasmid preparation is a method used to extract and purify plasmid DNA. Methods developed to purify plasmid DNA from bacteria generally involve harvesting and alkaline lysis of the bacteria, precipitation of chromosomal DNA and protein, followed by purification of the plasmid DNA. Here, we describe the mini-preparation of plasmid DNA by a rapid small-scale method, adapted for Listeria monocytogenes. The quality of plasmid DNA isolated using this method is sufficient for analytical purposes but may be upscaled for further downstream analysis. Electrophoretic separation of the resultant lysate allows conclusions to be made on the presence, number, copy number, and size of the plasmids in the analyzed bacterial strains.

Genomic Characterization of Listeria monocytogenes Isolated From Ready-to-Eat Meat and Meat Processing Environments in Poland

Listeria monocytogenes is one of the major foodborne pathogens. Isolates of PCR-serogroups IIb (n = 17) and IVb (n = 31) recovered from food (n = 33) and food processing environment (n = 15) in Poland were characterized using whole genome sequencing. Most isolates belonged to Multi-Locus Sequence Type (MLST) ST2 (31.3%) and ST5 (22.9%). Core genome MLST (cgMLST) analysis classified isolates into seven sublineages (SL) and 25 different cgMLST types (CT). Consistent with the MLST results, most sublineages were SL2 and SL5. Eleven isolates harbored aacA4 encoding resistance to aminoglycosides, three isolates harbored emrC (n = 3) and one brcABC (n = 1) encoding tolerance to benzalkonium chloride. Isolates belonging to SL5 CT2323 carried a so far unreported inlB allele with a deletion of 141 nucleotides encoding the β-repeat sheet and partially the GW1 domain of InlB. Comparison with publicly available genome sequences from L. monocytogenes isolated from human listeriosis cases in Poland from 2004 to 2013 revealed five common CTs, suggesting a possible epidemiological link with these strains. The present study contributes to characterize the diversity of L. monocytogenes in ready-to-eat (RTE) meat and meat processing environments in Poland and unravels previously unnoticed links with clinical cases in Europe.

Evolution of Listeria monocytogenes in a Food Processing Plant Involves Limited Single-Nucleotide Substitutions but Considerable Diversification by Gain and Loss of Prophages

Knowledge about the genetic evolution of L. monocytogenes in food processing facilities over multiple years is generally lacking. This information is critical to interpret WGS findings involving food or food-associated isolates. This study suggests that L. monocytogenes that persists in processing facilities may evolve with a low single-nucleotide mutation rate mostly driven by negative (i.e., purifying) selection but with rapid diversification of prophages. Hence, isolation of L. monocytogenes with few single-nucleotide polymorphism (SNP) differences in different locations (e.g., supplier plants and receiving plants) is possible, highlighting the importance of epidemiological and detailed isolate metadata for interpreting WGS data in traceback investigation. Our study also shows how advanced WGS data analyses can be used to support root cause analysis efforts and may, for example, pinpoint the time when a persistence event started (which then potentially could be linked to facility changes, introduction of new equipment, etc.).

Whole-genome sequencing (WGS) is becoming the standard method for subtyping Listeria monocytogenes. Interpretation of WGS data for isolates from foods and associated environments is, however, challenging due to a lack of detailed data on Listeria evolution in processing facilities. Here, we used previously collected WGS data for 40 L. monocytogenes isolates obtained from a cold-smoked salmon processing facility between 1998 and 2015 to probe the L. monocytogenes molecular evolution in this facility, combined with phenotypic assessment of selected isolates. Isolates represented three clusters (1, 2, and 3); cluster 3 isolates (n = 32) were obtained over 18 years. The average mutation rate for cluster 3 was estimated as 1.15 × 10⁻⁷ changes per nucleotide per year (∼0.35 changes per genome per year); the most recent common ancestors (MRCAs) of subclusters 3a and 3b were estimated to have occurred around 1958 and 1974, respectively, within the age of the facility, suggesting long-term persistence in this facility. Extensive prophage diversity was observed within subclusters 3a and 3b, which have one shared and six unique prophage profiles for each subcluster (with 16 prophage profiles found among all 40 isolates). The plasmid-borne sanitizer tolerance operon bcrABC was found in all cluster 2 and 3 isolates, while the transposon-borne sanitizer tolerance gene qacH was found in one cluster 1 isolate; presence of these genes was correlated with the ability to survive increased concentrations of sanitizers. Selected isolates showed significant variation in the ability to attach to surfaces, with persistent isolates attaching better than transient isolates at 21°C.

IMPORTANCE Knowledge about the genetic evolution of L. monocytogenes in food processing facilities over multiple years is generally lacking. This information is critical to interpret WGS findings involving food or food-associated isolates. This study suggests that L. monocytogenes that persists in processing facilities may evolve with a low single-nucleotide mutation rate mostly driven by negative (i.e., purifying) selection but with rapid diversification of prophages. Hence, isolation of L. monocytogenes with few single-nucleotide polymorphism (SNP) differences in different locations (e.g., supplier plants and receiving plants) is possible, highlighting the importance of epidemiological and detailed isolate metadata for interpreting WGS data in traceback investigation. Our study also shows how advanced WGS data analyses can be used to support root cause analysis efforts and may, for example, pinpoint the time when a persistence event started (which then potentially could be linked to facility changes, introduction of new equipment, etc.).

Predominance of Distinct Listeria Innocua and Listeria Monocytogenes in Recurrent Contamination Events at Dairy Processing Facilities

: The genus Listeria now comprises up to now 21 recognized species and six subspecies, with L. monocytogenes and L. innocua as the most prevalent sensu stricto associated species. Reports focusing on the challenges in Listeria detection and confirmation are available, especially from food-associated environmental samples. L. innocua is more prevalent in the food processing environment (FPE) than L. monocytogenes and has been shown to have a growth advantage in selective enrichment and agar media. Until now, the adaptive nature of L. innocua in FPEs has not been fully elucidated and potential persistence in the FPE has not been observed. Therefore, the aim of this study is to characterize L. innocua (n = 139) and L. monocytogenes (n = 81) isolated from FPEs and cheese products collected at five dairy processing facilities (A-E) at geno- and phenotypic levels. Biochemical profiling was conducted for all L. monocytogenes and the majority of L. innocua (n = 124) isolates and included a rhamnose positive reaction. L. monocytogenes isolates were most frequently confirmed as PCR-serogroups 1/2a, 3a (95%). Pulsed-field gel electrophoresis (PFGE)-typing, applying the restriction enzymes AscI, revealed 33 distinct Listeria PFGE profiles with a Simpson's Index of Diversity of 0.75. Multi-locus sequence typing (MLST) resulted in 27 STs with seven new L. innocua local STs (ST1595 to ST1601). L. innocua ST1597 and ST603 and L. monocytogenes ST121 and ST14 were the most abundant genotypes in dairy processing facilities A-E over time. Either SSI-1 (ST14) or SSI-2 (ST121, all L. innocua) were present in successfully FPE-adapted strains. We identified housekeeping genes common in Listeria isolates and L. monocytogenes genetic lineage III. Wherever there are long-term contamination events of L. monocytogenes and other Listeria species, subtyping methods are helpful tools to identify niches of high risk.

Dynamics of mobile genetic elements of Listeria monocytogenes persisting in ready-to-eat seafood processing plants in France

BACKGROUND

Listeria monocytogenes Clonal Complexes (CCs) have been epidemiologically associated with foods, especially ready-to-eat (RTE) products for which the most likely source of contamination depends on the occurrence of persisting clones in food-processing environments (FPEs). As the ability of L. monocytogenes to adapt to environmental stressors met in the food chain challenges the efforts to its eradication from FPEs, the threat of persistent strains to the food industry and public health authorities continues to rise. In this study, 94 food and FPEs L. monocytogenes isolates, representing persistent subtypes contaminating three French seafood facilities over 2-6 years, were whole-genome sequenced to characterize their genetic diversity and determine the biomarkers associated with long-term survival in FPEs.

RESULTS

Food and FPEs isolates belonged to five CCs, comprising long-term intra- and inter-plant persisting clones. Mobile genetic elements (MGEs) such as plasmids, prophages and transposons were highly conserved within CCs, some of which harboured genes for resistance to chemical compounds and biocides used in the processing plants. Some of these genes were found in a 90.8 kbp plasmid, predicted to be" mobilizable", identical in isolates from CC204 and CC155, and highly similar to an 81.6 kbp plasmid from isolates belonging to CC7. These similarities suggest horizontal transfer between isolates, accompanied by deletion and homologous recombination in isolates from CC7. Prophage profiles characterized persistent clonal strains and several prophage-loci were plant-associated. Notably, a persistent clone from CC101 harboured a novel 31.5 kbp genomic island that we named Listeria genomic island 3 (LGI3), composed by plant-associated loci and chromosomally integrating cadmium-resistance determinants cadA1C.

CONCLUSIONS

Genome-wide analysis indicated that inter- and intra-plant persisting clones harbour conserved MGEs, likely acquired in FPEs and maintained by selective pressures. The presence of closely related plasmids in L. monocytogenes CCs supports the hypothesis of horizontal gene transfer conferring enhanced survival to FPE-associated stressors, especially in hard-to-clean harbourage sites. Investigating the MGEs evolutionary and transmission dynamics provides additional resolution to trace-back potentially persistent clones. The biomarkers herein discovered provide new tools for better designing effective strategies for the removal or reduction of resident L. monocytogenes in FPEs to prevent contamination of RTE seafood.

Genomic Diversity of Common Sequence Types of Listeria monocytogenes Isolated from Ready-to-Eat Products of Animal Origin in South Africa

Listeria monocytogenes is a highly fatal foodborne causative agent that has been implicated in numerous outbreaks and related deaths of listeriosis in the world. In this study, six L. monocytogenes isolated from ready-to-eat (RTE) meat products were analysed using Whole Genome Sequencing (WGS) to identify virulence and resistance genes, prophage sequences, PCR-serogroups, and sequence types (STs). The WGS identified four different STs (ST1, ST121, ST204, and ST876) that belonged to serogroup 4b (lineage I) and 1/2a (lineage II). Core genome, and average nucleotide identity (ANI) phylogenetic analyses showed that the majority of strains from serogroup 4b (lineage I) clustered together. However, two isolates that belong to serogroup 1/2a (lineage II) grouped far from each other and the other strains. Examination of reference-guided scaffolds for the presence of prophages using the PHAge Search Tool Enhanced Release (PHASTER) software identified 24 diverse prophages, which were either intact or incomplete/questionable. The National Center for Biotechnology Information- Nucleotide Basic Local Alignment Search Tool (NCBI-BLASTn) revealed that Listeria monocytogenes strains in this study shared some known major virulence genes that are encoded in Listeria pathogenicity islands 1 and 3. In general, the resistance profiles for all the isolates were similar and encoded for multidrug, heavy metal, antibiotic, and sanitizer resistance genes. All the isolates in this study possessed genes that code for resistance to common food processing antiseptics such as Benzalkonium chloride.

Genetic Diversity of Listeria monocytogenes Isolates from Invasive Listeriosis in China

Listeria monocytogenes is a deadly foodborne pathogen, and infections can result in meningoencephalitis and sepsis with mortality rates of up to 30%. In this study, we performed comparative whole-genome analysis of 30 clinical isolates sequenced together with 32 previously sequenced clinical and food isolates from China. The data indicate that L. monocytogenes isolates belonging to the clonal complexes (CC) -1, -8, -9, -87, -121, and -155 are present in human clinical cases. The majority of isolates are from CC-87, 9, and 8 and overlap with those CCs previously reported on the basis of multilocus sequence typing for isolates from Chinese food products. Detailed genome analysis of isolates, representative of CCs in clinical and food products, revealed strong similarities both in their core- and accessory genomes indicating that they are highly related. When compared to genome sequences of isolates of a given CC worldwide, clinical isolates of China were distinct and clustered in unified clades. Our data indicate that epidemic clones of L. monocytogenes (CC-87, 9, and 8) with unusually high occurrence of plasmids are unique to China and suggest that common populations of L. monocytogenes clones are present in both clinical and food products in China.

Characterization of Virulence and Persistence Abilities of Listeria monocytogenes Strains Isolated from Food Processing Premises

We report the characterization of 15 Listeria monocytogenes strains isolated from various food processing plants by multivirulence locus sequence typing to determine virulence types (VTs) and epidemic clones. Molecular mechanisms involved in adaptation to food processing environments and related to virulence were also studied. Phenotypic behaviors associated with various antimicrobials, biofilm formations, and invasiveness were assessed. There were 11 VTs among the 15 L. monocytogenes strains. Strains belonging to six VTs were stress survival islet 1 (SSI-1) and one strain of VT94 was SSI-2. Tn6188 was found in VT6 and VT94 strains, and bcrABC cassette genes were identified in VT21, VT60, and VT63 strains. Only one strain, in VT20, showed llxS, whereas a full-size inlA was detected in strains belonging to VT8, VT20, VT21, and VT63. VT10, VT20, VT21, VT60, and VT63 strains were the most tolerant to studied disinfectants. A VT6 strain showed the strongest biofilm formation ability in polyvinyl chloride, and strains belonging to VT10, VT11, VT20, and VT94 had moderate abilities. Antimicrobial sensitivity tests showed that all the L. monocytogenes strains were multidrug resistant. F tests revealed that only strains of VT10, VT60, and VT94 were significantly noninvasive (P < 0.05) in Caco-2 cells. Our findings illustrate how L. monocytogenes isolates exploit diverse mechanisms to adapt to adverse conditions. Consequently, detailed characterization of L. monocytogenes isolates is required for comprehensive elimination of this pathogenic bacterium in food processing environments.

Distribution and Persistence of Listeria monocytogenes in a Heavily Contaminated Poultry Processing Facility

We studied the colonization and distribution of Listeria monocytogenes in a heavily contaminated poultry processing plant over a 1-year period. A total of 180 nonfood contact surfaces, 70 food contact surfaces, 29 personnel, and 40 food samples were analyzed. L. monocytogenes isolates were subtyped by PCR serotyping, pulsed-field gel electrophoresis (PFGE), and multilocus sequence typing. L. monocytogenes was detected in samples collected at every visit to the plant, and 43.8% (visit 4) to 65.6% (visit 7) of samples were positive, for an overall prevalence of 55.2%. The deboning area had the highest prevalence of positive samples (83.3%), and the processing area had the highest diversity of PFGE types. Ninety percent of the final products were positive for L. monocytogenes. Most of the isolates belonged to well-known persistent L. monocytogenes sequence types (ST9 and ST121). This study illustrates a well-established L. monocytogenes contamination problem in a poultry processing plant associated with a generalized failure of the food safety system as a whole. These findings reflect the potential for L. monocytogenes contamination when the food safety and quality management system is unsatisfactory, as described in the present study. It is essential to revise food safety and quality management systems to eliminate L. monocytogenes from food processing facilities, to control the entrance of sporadic sequence types, and to prevent L. monocytogenes spread within such facilities, especially in those premises with higher L. monocytogenes prevalence in the environment and final food products.

Plasmids contribute to food processing environment-associated stress survival in three Listeria monocytogenes ST121, ST8, and ST5 strains

Listeria monocytogenes is a food-borne pathogen responsible for the disease listeriosis and is commonly isolated from food and food production facilities. Many L. monocytogenes strains contain plasmids, though the contributions of plasmids to survival in food production environments are unknown. Three L. monocytogenes ST5, ST8, and ST121 strains containing plasmids, which harbor putative stress response genes, were cured of their plasmids. Wildtype (WT) and plasmid-cured strains were exposed to disinfectant, oxidative, heat, acid, or salt stress. After stress exposure, cells were plated for colony forming unit (CFU) counts to determine survivors. L. monocytogenes WT strains exposed to 0.01% (vol/vol) H₂O₂, 1% (vol/vol) lactic acid, and 15% (wt/vol) NaCl, pH 5 showed significantly higher counts of survivors compared to the plasmid-cured strains. The number of survivors for the ST5 WT strain exposed to 10 μg/mL benzalkonium chloride (BC) was significantly higher than in the plasmid-cured strain. The ST8 and ST5 strains were exposed to elevated temperature (50° and 55 °C respectively); only the ST5 WT strain had significantly higher numbers of survivors than the plasmid-cured strains. Our data revealed that L. monocytogenes ST5, ST8, and ST121 plasmids contribute to tolerance against elevated temperature, salinity, acidic environments, oxidative stress and disinfectants.

Listeria monocytogenes colonization in a newly established dairy processing facility

The presence and colonization of Listeria monocytogenes were investigated in a newly established dairy processing plant during a one-year period. A total of 250 non-food contact surfaces, 163 food contact surfaces, 46 personnel and 77 food samples were analyzed in two different buildings according to the cheese production chain. Initial steps, including salting, are performed in building I (old facility), while the final steps, including ripening, cutting and packaging, are performed in building II (new facility). Overall, 218 samples were collected from building I and 318 from building II. L. monocytogenes isolates were subtyped by PFGE and MLST, and a questionnaire about quality measures was completed. The overall prevalence of L. monocytogenes was 8.40%, and while the presence of the pathogen was observed just during the first sampling in building I, L. monocytogenes was found in building II at the third sampling event. The salting area in building I had the highest proportion of positive samples with the highest diversity of PFGE types. Moreover, L. monocytogenes PFGE type 3 (sequence type -ST- 204) was first detected in building II in the third visit, and spread through this building until the end of the study. The answers to the questionnaire implied that lack of hygienic barriers in specific parts of the facilities and uncontrolled personnel flow were the critical factors for the spread of L. monocytogenes within and between buildings. Knowledge of the patterns of L. monocytogenes colonization can help a more rational design of new cheesemaking facilities, and improve the food safety within current facilities.

Listeria monocytogenes Isolated from Illegally Imported Food Products into the European Union Harbor Different Virulence Factor Variants

Unregulated international flow of foods poses a danger to human health, as it may be contaminated with pathogens. Recent studies have investigated neglected routes of pathogen transmission and reported the occurrence of Listeria monocytogenes in food illegally imported into the European Union (EU), either confiscated at four international airports or sold illegally on the Romanian black market. In this study we investigated the genotype diversity and the amino acid sequence variability of three main virulence factors of 57 L. monocytogenes isolates. These isolates were derived from 1474 food samples illegally imported into the EU and originated from 17 different countries. Multilocus sequence typing revealed 16 different sequence types (STs) indicating moderate genotype diversity. The most prevalent STs were ST2, ST9, and ST121. The pulsed-field gel electrophoresis (PFGE) analysis resulted in 34 unique pulsotypes. PFGE types assigned to the most prevalent STs (ST2, ST9, and ST121) were highly related in their genetic fingerprint. Internalin A (InlA) was present in 20 variants, including six truncated InlA variants, all harbored by isolates of ST9 and ST121. We detected eight ST-specific listeriolysin O (LLO) variants, and among them, one truncated form. The actin-assembly-inducing protein ActA was present in 15 different ST-specific variants, including four ActA variants with an internal truncation. In conclusion, this study shows that L. monocytogenes, isolated from illegally imported food, have moderate genotype diversity, but diverse virulence factors variants, mainly of InlA.