Prevalence, Potential Virulence, and Genetic Diversity of Listeria monocytogenes Isolates From Edible Mushrooms in Chinese Markets

Listeria monocytogenes Contamination Leads to Survival and Growth During Enoki Mushroom Cultivation

Two recent outbreaks of listeriosis have been linked to the consumption of enoki mushrooms. After the first outbreak, import sampling by the U.S. FDA identified that 43% of the samples evaluated were positive for Listeria monocytogenes (Lm). These observations raised questions about the potential sources of Lm contamination of enoki mushrooms. One potential source of contamination is during enoki mushroom cultivation, as growing conditions are comparatively cool and moist to induce mushroom germination, to which Lm is well adapted. Two varieties of enoki mushrooms were evaluated to determine the potential for Lm to contaminate enoki cultures when introduced at various points during cultivation (inoculation, scraping, pinning, and collaring). The results of two trials showed that Lm established contamination and grew to similar levels in the substrate regardless of when Lm was introduced and, with one exception, did not alter the rate of mushroom generation to below the control. Enumeration of Lm in enoki mushroom cultures at harvest found an average contamination of 103 cfu/g, though the results were variable. Refrigerated storage for six weeks was found to result in an increase in Lm. Additionally, no statistically significant difference in the levels of Lm was observed based on proximity to the substrate, though levels of Lm in the different enoki samples correlated with levels of Lm in the substrate at harvest, but not at scraping. The ability of Lm to grow independently in the media used to culture enoki was assessed, and Lm was found to be unable to grow but could sporadically survive in Masters Mix. No growth of Lm was observed in potato dextrose broth, though growth could occur on the agar. Overall, the data indicate a high potential for the establishment of Lm contamination at any point during enoki cultivation to result in Lm-contaminated mushrooms. These data indicate a need for active control mechanisms to prevent the introduction of Lm to enoki cultures.

Microbiological quality and safety of fresh mushroom products at retail level in Korea

Several investigations and recalls have demonstrated that Listeria monocytogenes can occur on mushrooms. This study aimed to assess the microbiological quality and safety of four types of edible mushrooms (Flammulina velutipes, Pleurotus ostreatus, Pleurotus eryngii, and Agaricus bisporus) available in the Korean market, and to evaluate the prevalence of Listeria spp., including L. monocytogenes. Results revealed that out of 100 samples tested, 16% (32/200) were positive for Listeria spp. Of the Listeria-positive samples, five strains of Listeria innocua were detected. The total microbial counts ranged from 0.79 to 5.84 log CFU/g, with F. velutipes exhibiting the highest microbial load (mean 5.03 log CFU/g). These findings provide significant data for risk assessment and emphasize the need for continued monitoring of the microbiological safety of edible mushrooms.

A novel multiplex PCR based method for the detection of Listeria monocytogenes clonal complex 8

Listeria monocytogenes is an important foodborne pathogen worldwide, which could cause listeriosis with a 20-30 % fatality rate in immunocompromised individuals. Listeria monocytogenes MLST clonal complex (CC) 8 strain is a common clone in food and clinical cases. The aim of this study was to develop multiplex PCR (mPCR) and high-resolution melting (HRM) qPCR to simultaneously detect L. monocytogenes CC8 and the other L. monocytogenes strains based on pan-genome analysis. A novel multiplex PCR and HRM qPCR targeted for the genes LM5578_1180 (specific for CC8) and LM5578_2262 (for L. monocytogenes) were developed. The specificity of this multiplex PCR and HRM qPCR were verified with other CCs of L. monocytogenes and other species strains. The detection limit of this multiplex PCR and HRM qPCR is 2.1 × 103 CFU/mL and 2.1 × 100 CFU/mL, respectively. This multiplex PCR and HRM qPCR could accurately detect CC8 strains with the interference of different ratios of L. monocytogenes CC9, CC87, CC121, CC155, and L. innocua strains. Subsequently, the detection ability of mPCR and HRM qPCR were also evaluated in spiked samples. The mPCR method could successfully detect 6.2 × 103 CFU/mL of CC8 L. monocytogenes after 6 h enrichment while the multiplex HRM qPCR method could successfully detect 6.2 × 104 CFU/mL of CC8 L. monocytogenes after 3 h enrichment. The feasibility of these methods were satisfactory in terms of sensitivity, specificity, and efficiency after evaluating 12 mushroom samples and was consistent with that of the National Standard Detection Method (GB4789.30-2016). In conclusion, the developed assays could be applied for rapid screening and detection of L. monocytogenes CC8 strains both in food and food production environments, providing accurate results to adopt monitoring measures to improve microbiological safety.

Genetic and Phenotypic Diversity of Listeria monocytogenes in Pig Slaughterhouses in Korea

Listeria monocytogenes is a foodborne pathogen that has variable subtypes associated with human listeriosis and occurs in food and processing environments. This study was conducted to provide the genetic and phenotypic characterization of L. monocytogenes in pig carcasses and environments of slaughterhouses in Korea. A total of 22 L. monocytogenes were isolated from eight of 26 pig slaughterhouses between 2020 and 2022, and the most common serotype was 1/2c (40.9%), followed by serotypes 1/2b (31.8%) and 1/2a (27.3%). The isolates showed a significantly high prevalence of virulence genes located in Listeria pathogenicity island-1 (LIPI-1) and internalins (90.9-100%; p < 0.05). However, the prevalence rates of llsX, ptsA, and stress survival islet-1 (SSI-1) located in LIPI-3, LIPI-4, and SSI were only 9.1%, 22.7%, and 31.8%, respectively. In addition, among the epidemic clones (EC), ECI, ECII, ECIII, and ECV, only one isolate was represented as ECV. Isolates identified from the same slaughterhouses were divided into two or more pulsotypes, except for two slaughterhouses. Furthermore, the seven STs were classified into seven clonal complexes (CCs) (CC8, CC9, CC37, CC87, CC121, CC155, and CC288), and all CCs belonged to lineages I (31.8%) and II (68.1%). Interestingly, the isolates showed a high prevalence of oxacillin resistance (59.1%), and most isolates of the serotypes 1/2a and 1/2b exhibited oxacillin resistance, whereas only one of nine serotype 1/2c isolates exhibited oxacillin resistance. These results provide the genetic diversity of L. monocytogenes in pig carcasses and environments of slaughterhouses, and continuous monitoring will be helpful in predicting food safety risks.

A rapid and inexpensive nucleic acid detection platform for Listeria monocytogenes based on the CRISPR/Cas12a system

Listeria monocytogenes (LM) is an important foodborne pathogen that is associated with a high mortality rate. Currently, there is an urgent need for an inexpensive and rapid assay for the large-scale diagnosis and monitoring of LM. To meet these requirements, we designed a one-step, low-cost platform for the simultaneous amplification and detection of LM based on the CRISPR/Cas12a system with a micro-amplification (named Cas12a-MA). This method utilizes a combination of CRISPR/Cas12a and recombinase polymerase amplification (RPA) in the same vessel to provide a contamination-free platform for rapid nucleic acid detection with high specificity and ultra-sensitivity. In this study, we screened for three specific genes and selected the hly gene in LM as the final target. Our data showed that the number of amplification products plays a crucial role in the function of the CRISPR/Cas12a system. Our method was then further optimized for the specific detection of target DNA on 4.4 CFU/g in 25min. These assays successfully detected LM in spiked pork samples and natural meat samples (pork, beef, and mutton). All results indicate that Cas12a-MA shows great promise for foodborne pathogen detection.

Prevalence and contamination patterns of Listeria monocytogenes in Pleurotus eryngii (king oyster mushroom) production plants

Listeria monocytogenes is a major foodborne pathogen that is well-known for its high mortality rate upon infection. In recent years, the edible mushroom has also been found to be an important source of L. monocytogenes, but the contamination sources in Pleurotus eryngii (the king oyster mushroom) were unclear. In this study, a total of 203 edible mushrooms and environmental samples from four P. eryngii production plants were obtained. As a result, 29 samples (14.3%) were positive for L. monocytogenes, including eight mushroom samples (13.3%, 8/60) and 21 associated environmental samples (14.7%, 21/143). The contamination of L. monocytogenes in plants A and B was more severe and was likely to originate from the mycelium stimulation machine. The isolates belonged to serogroups II.1 (4b-4d-4e), I.1 (1/2a-3a), and I.2 (1/2c-3c), and multilocus sequence typing (MLST) revealed that these L. monocytogenes strains belonged to five different sequence types (ST3, ST121, ST9, ST87, and ST224). The ST121 and ST3 isolates were only found in plants A and B, respectively. The isolates were carried by hly (29/29, 100%), inlB (23/29, 79.3%), inlA (29/29, 100%), inlC (29/29, 100%), inlJ (29/29, 100%), actA (19/29, 65.5%), iap (29/29, 100%), plcA (26/29, 100%), plcB (29/29, 100%), prfA (27/29, 93.1%), and mpl (29/29, 100%). Further study of inlA sequencing showed that 65.5% of strains (19/29) contained full-length InlA that was required for host cell invasion, whereas the mutation led to premature stop codons (PMSCs) at position 492 (type 6) on inlA alleles. All isolates in this survey were sensitive to gentamicin, kanamycin, sulbactam/ampicillin, trimethoprim-sulfamethoxazole, tetracycline, and doxycycline. The drug with the highest resistance is rifampicin (37.9%), followed by penicillin (24.1%) and ciprofloxacin (10.3%). Most multiply resistant strains are isolated from raw materials and equipment of the P. eryngii processing lines. Our study reflects the contamination patterns and potential risk of L. monocytogenes infection in P. eryngii production plants. The persistence of specific L. monocytogenes isolates (such as ST121 and ST3) may assist with contamination. In accordance with these results, the control of L. monocytogenes should focus on the environmental materials, especially in the mycelium stimulation stage. However, effective Listeria monitoring programs will allow for the improved development of Listeria control measures to minimize cross-contamination in the processing of P. eryngii.

Genetic diversity and known virulence genes in Listeria innocua strains isolated from cattle abortions and farm environment

Listeria innocua is considered as non-pathogenic bacteria living in an environment although several cases of immunocompromised humans and ruminant listeriosis infections have been reported. Previously, L. innocua was identified as a potential pathogen and virulence in association with L. monocytogenes PrfA dependent virulence (LIPI-1) gene cluster was demonstrated in hemolytic L. innocua. L. innocua usually considered non-pathogenic versus pathogenic L. monocytogenes and L. ivanovii because of the main virulence gene loss. There are limited studies and reports available about L. innocua-caused illness in cattle. A total of 18 STs were identified in cattle abortions while 17 STs in the farm environment with majority of STs were present in both abortions and environmental samples. Genome sequencing showed that in one farm identical L. innocua clones were represented in water, feed, soil, and faeces sample groups, suggesting that animals most likely through the faecal shedding may remain as the main source of L. innocua in a farm environment. Out of all L. innocua isolates PrfA-dependent virulence genes were not found in aborted foetuses isolates and environmental L. innocua isolate groups; however, in 20% of isolates a complete LIPI-3 pathogenicity island encoding listeriolysin S was identified. In this study, we demonstrated that genetically diverse L. innocua clones were widely distributed in cattle farm environment and certain isolates had a significant pathogenicity potential for cattle, thus causing adverse health effects, including abortions.

Assessment of the molecular epidemiology and genetic multiplicity of Listeria monocytogenes recovered from ready-to-eat foods following the South African listeriosis outbreak

Following the recent listeriosis outbreak in South Africa, this study was carried out to assess the safety level of various common ready-to-eat foods (RTE) obtained from supermarkets and grocery stores in major towns and cities within the Amathole, Chris Hani and Sarah Baartman Districts Municipalities, Eastern Cape Province, South Africa. A sum of 239 food samples was collected from these locations, and Listeria monocytogenes (Lm) was isolated in line with the recommended techniques by the International Organization for Standardization EN ISO 11290:2017 parts 1 and 2. Identification of the pathogen and detection of various associated virulence genes was done using Polymerase Chain Reaction (PCR) techniques. From the RTE food samples processed, Lm was detected in 107 (44.77%) of the samples. Russian sausage was the most contaminated (78.57%), followed by sliced polony (61.90%), muffins (58.33%), polony (52.63%), and pies (52.38%), while all vetkoek samples examined were negative for Lm. Although the prevalence of Lm in the food samples was very high, concentrations were generally < 100 CFU/g. Strains of Lm recovered from the RTE foods were predominantly epidemiological strains belonging to serotypes 1/2a, 1/2b and 4b. The prevalence of 10 virulence genes including the inlA, InlC, inlJ, plcA, hlyA, plcB, prfA, mpl, inlB, and actA were detected among Lm isolates. Most of the isolates (69.07%) demonstrated the potential for biofilm formation and were categorized as weak (14.95%), moderate (13.40%) and strong (40.72) biofilm formers. Furthermore, molecular typing revealed high levels of genetic diversity among Lm isolates. The findings of this investigation suggested that the presence of Lm in the RTE foods may constitute potential threats to the food sector and could pose public health hazards to consumers, particularly the high-risk group of the population. We, therefore, recommend that adequate food monitoring for safety and proper regulation enforcement in the food sector must be ensured to avoid any future listeriosis outbreak that could be linked to RTE foods in South Africa.

Identification and antibiotic resistance of Cronobacter spp. isolated from dried edible mushrooms

Cronobacter spp. is an important foodborne pathogen that can cause life-threatening diseases in infants and immunocompromised adults. The present study was carried out to understand the prevalence and characterization of Cronobacter spp. in dried edible mushrooms in Jiangsu province, China. Cronobacter isolates were identified and genotyped by multilocus sequence typing (MLST); the antimicrobial susceptibility of Cronobacter strains was determined by the disk diffusion method; the biofilm formation ability of Cronobacter spp. was assessed using the microtiter plate method. The overall prevalence of Cronobacter spp. in dried edible mushrooms was 14.8%, with the highest contamination rate of after 37.2% found in Auricularia auricular. The Cronobacter isolates were identified as C. sakazakii (n = 26), C. malonaticus (n = 2), C. dublinensis (n = 2) and C. turicensis (n = 1). The MLST scheme produced 20 sequence types (STs), two of which were newly identified. ST148 was the most prevalent ST (n = 5), followed by ST4 (n = 3), ST17 (n = 3), ST64 (n = 3), and ST540 (n = 2). One (3.2%) and 15 (48.4%) Cronobacter isolates were resistant to tetracycline and meropenem, respectively. In contrast, all of the tested isolates were susceptible to the remaining 14 antibiotics. Moreover, 20 (64.5%) Cronobacter isolates showed weak ability to produce biofilm, but no isolates showed strong or moderate biofilm-forming ability. PRACTICAL APPLICATION: Our findings revealed a high genetic diversity of Cronobacter spp. in dried edible mushrooms and provided new epidemiological evidence for the widespread existence of Cronobacter spp. in such products. The presence of Cronobacter spp. in dried edible mushrooms may pose potential risks to human health and enhancing the hygiene of such products are necessary to ensure food safety.

Assessment of multidrug-resistant Listeria monocytogenes in milk and milk product and One Health perspective

The occurrence and the antibiogram signatures of Listeria monocytogenes (Lm) recovered from 65 milk samples and its products within the Eastern Cape province were examined. The EN ISO 11290:2017 procedures Parts 1 and 2 described by the International Organization for Standardization for the enumeration and isolation of Lm was adopted for the study. Lm was detected in 18.46% of all the samples examined, and the strains recovered from the samples belong to serotypes 4b and 1/2b. The virulence determinants including prfA, plcA, plcB, inlA, inlC, hly, mpl, actA, inlJ and inlB were detected in all the isolates. About 95.24% of the studied Lm isolates demonstrated potential capacity for biofilm formation. The antibiogram profile revealed high resistance against sulfamethoxazole (71.43%), trimethoprim (52.86%); erythromycin, cefotetan and oxytetracycline (42.86% respectively). About 85.71% exhibited multiple antibiotic resistance phenotypes against the test antibiotics. The resistance determinants encoding resistance against the β-lactamase antibiotics [such as the bla_TEM, bla_SHV, bla_TEMvariants (TEM-1 and TEM-2) and the bla_Z], the tetracycline resistance genes (including tetA, tetD, tetG and tetM and tetK) were detected among resistant isolates. In addition, the aminoglycoside resistance gene aph (3)-IIa (aphA2)^a was detected only in one isolate. Finally, the sulfonamide resistance genes including the sul2 and the sul1 genes were the most frequently observed among Lm isolates. Generally, 71.43% of all Lm isolates recovered from the samples investigated harboured one or more resistance genes encoding resistance against various antibiotics. The antibiogram signatures of Lm isolates observed in this study is an indication that empirical treatment of listeriosis may be challenging in the future as the pathogen may obliterate the success of antibiotics. We, therefore, advocate for the recognition of the One Health approach to ensuring food safety and curbing the spread of antimicrobial resistance in food.

Prevalence and Clonal Diversity of over 1,200 Listeria monocytogenes Isolates Collected from Public Access Waters near Produce Production Areas on the Central California Coast during 2011 to 2016

A 5-year survey of public access surface waters in an agricultural region of the Central California Coast was done to assess the prevalence of the foodborne pathogen Listeria monocytogenes. In nature, L. monocytogenes lives as a saprophyte in soil and water, which are reservoirs for contamination of preharvest produce. Moore swabs were deployed biweekly in lakes, ponds, streams, and rivers during 2011 to 2016. L. monocytogenes was recovered in 1,224 of 2,922 samples, resulting in 41.9% prevalence. Multiple subtypes were isolated from 97 samples, resulting in 1,323 L. monocytogenes isolates. Prevalence was higher in winter and spring and after rain events in some waterways. Over 84% of the isolates were serotype 4b. Whole-genome sequencing was done on 1,248 isolates, and in silico multilocus sequence typing revealed 74 different sequence types (STs) and 39 clonal complexes (CCs). The clones most isolated, CC639, CC183, and CC1, made up 27%, 19%, and 13%, respectively, of the sequenced isolates. Other types were CC663, CC6, CC842, CC4, CC2, CC5, and CC217. All sequenced isolates contained intact copies of core L. monocytogenes virulence genes, and pathogenicity islands LIPI-3 and LIPI-4 were identified in 73% and 63%, respectively, of the sequenced isolates. The virulence factor internalin A was predicted to be intact in all but four isolates, while genes important for sanitizer and heavy metal resistance were found in <5% of the isolates. These waters are not used for crop irrigation directly, but they are available to wildlife and can flood fields during heavy rains. IMPORTANCE Listeria monocytogenes serotype 4b and 1/2a strains are implicated in most listeriosis, and hypervirulent listeriosis stems from strains containing pathogenicity islands LIPI-3 and LIPI-4. The waters and sediments in the Central California Coast agricultural region contain widespread and diverse L. monocytogenes populations, and all the isolates contain intact virulence genes. Emerging clones CC183 and CC639 were the most abundant clones, and major clones CC1, CC4, and CC6 were well represented. CC183 was responsible for three produce-related outbreaks in the last 7 years. Most of the isolates in the survey differ from those of lesser virulence that are often isolated from foods and food processing plants because they contain genes encoding an intact virulence factor, internalin A, and most did not contain genes for sanitizer and heavy metal resistance. This isolate collection is important for understanding L. monocytogenes populations in agricultural and natural regions.

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.

Landscape of Stress Response and Virulence Genes Among Listeria monocytogenes Strains

The pathogenic microorganism Listeria monocytogenes is ubiquitous and responsible for listeriosis, a disease with a high mortality rate in susceptible people. It can persist in different habitats, including the farm environment, the food production environments, and in foods. This pathogen can grow under challenging conditions, such as low pH, low temperatures, and high salt concentrations. However, L. monocytogenes has a high degree of strain divergence regarding virulence potential, environmental adaption, and stress response. This review seeks to provide the reader with an up-to-date overview of clonal and serotype-specific differences among L. monocytogenes strains. Emphasis on the genes and genomic islands responsible for virulence and resistance to environmental stresses is given to explain the complex adaptation among L. monocytogenes strains. Moreover, we highlight the use of advanced diagnostic technologies, such as whole-genome sequencing, to fine-tune quantitative microbiological risk assessment for better control of listeriosis.

Plants as a realized niche for Listeria monocytogenes

Listeria monocytogenes is a human pathogen. It is the causative agent of listeriosis, the leading cause of bacterial-linked foodborne mortality in Europe and elsewhere. Outbreaks of listeriosis have been associated with the consumption of fresh produce including vegetables and fruits. In this review we summarize current data providing direct or indirect evidence that plants can serve as habitat for L. monocytogenes, enabling this human pathogen to survive and grow. The current knowledge of the mechanisms involved in the interaction of this bacterium with plants is addressed, and whether this foodborne pathogen elicits an immune response in plants is discussed.

Bioactive components of mushrooms: Processing effects and health benefits

Mushrooms have been recognized for their culinary attributes for long and were relished in the most influential civilizations in history. Currently, they are the focus of renewed research because of their therapeutic abilities. Nutritional benefits from mushrooms are in the form of a significant source of essential proteins, dietary non-digestible carbohydrates, unsaturated fats, minerals, as well as various vitamins, which have enhanced its consumption, and also resulted in the development of various processed mushroom products. Mushrooms are also a crucial ingredient in traditional medicine for their healing potential and curative properties. The literature on the nutritional, nutraceutical, and therapeutic potential of mushrooms, and their use as functional foods for the maintenance of health was reviewed, and the available literature indicates the enormous potential of the bioactive compounds present in mushrooms. Future research should be focused on the development of processes to retain the mushroom bioactive components, and valorization of waste generated during processing. Further, the mechanisms of action of mushroom bioactive components should be studied in detail to delineate their diverse roles and functions in the prevention and treatment of several diseases.

Microbiological Safety and Sensory Quality of Cultivated Mushrooms (Pleurotus eryngii, Pleurotus ostreatus and Lentinula edodes) at Retail Level and Post-Retail Storage

In this study, the microbiological and sensory quality of cultivated mushrooms (Pleurotus ostreatus and eryngii and Lentinula edodes) available at the Austrian retail level were determined. Aerobic mesophilic bacteria (AMC), Enterobacteriaceae (EB), Pseudomonadaceae (PS), lactic acid bacteria (LAB), yeast, moulds and presumptive Bacillus cereus were enumerated at the day of purchase and after storage at 4 °C for 7 or 12 days. Additionally, the presence of Salmonella spp. and Listeria monocytogenes was investigated. Isolates of presumptive spoilage bacteria were confirmed by partial 16S rRNA sequencing. At the day of purchase, 71.2% of the samples were of high microbiological quality and grouped into the low contamination category (AMC < 5.0 log cfu/g), while the sensory quality of 67.1% was categorized as “very good or good”. After storage, the number of samples with high microbial quality was 46.6%, and only 37.0% of the samples scored as “very good or good”. The most abundant species across all mushroom samples were the Pseudomonas fluorescens species complex (58.4%) and the potential mushroom pathogen Ewingella americana (28.3%). All mushroom samples tested negative for Salmonella spp., L. monocytogenes and Bacillus cereus. The microbiological and sensory quality of the analysed mushrooms at the day of purchase and after storage was considered to be good overall. Longer transport distances were found to have a significant influence on the microbiological and sensory quality.

An ultrasensitive CRISPR/Cas12a based electrochemical biosensor for Listeria monocytogenes detection

Listeria monocytogenes is an important foodborne pathogen that can cause listeriosis with high patient mortality. Accordingly, it is necessary to develop a L. monocytogenes detection platform with high specificity, sensitivity, and exploitability. CRISPR/Cas systems have shown great potential in the development of next-generation biosensors for nucleic acid detection, owing to the trans-cleavage capabilities of the Cas effector proteins. Herein, we introduce the trans-cleavage activity of CRISPR/Cas12a into an electrochemical biosensor (E-CRISPR), combined with recombinase-assisted amplification (RAA), to establish a cost-effective, specific and ultrasensitive method; namely RAA-based E-CRISPR. The concept behind this approach is that the target will induce the number change of the surface signaling probe (containing an electrochemical tag), which leads to a variation in the electron transfer of the electrochemical tag. The introduction of an RAA-based Cas12a system into the E-CRISPR sensor achieves a more prominent signal change between the presence and absence of the target. Under optimized conditions, RAA-based E-CRISPR can detect as low as 0.68 aM of genomic DNA and 26 cfu/mL of L. monocytogenes in pure cultures. More importantly, the RAA-based E-CRISPR enables rapid and ultrasensitive detection of L. monocytogenes in spiked and natural Flammulina velutipes samples. Moreover, no cross-reactivity with other non-target bacteria was observed. This system thus demonstrates to be a simple, high-sensitivity, and high-accuracy platform for L. monocytogenes detection.

Multiplex PCR for the Identification of Pathogenic Listeria in Flammulina velutipes Plant Based on Novel Specific Targets Revealed by Pan-Genome Analysis

Listeria spp. is an important foodborne disease agent, often found in the fresh mushroom (Flammulina velutipes) and its production environment. The aim of this study was to develop multiplex PCR for rapid identification of Listeria monocytogenes and Listeria ivanovii, and nonpathogenic Listeria in F. velutipes plants. Pan-genome analysis was first used to identify five novel Listeria-specific targets: one for the Listeria genus, one for L. monocytogenes, and three for L. ivanovii. Primers for the novel targets were highly specific in individual reactions. The detection limits were 10³-10⁴ CFU/mL, meeting the requirements of molecular detection. A mPCR assay for the identification of pathogenic Listeria, with primers targeting the novel genes specific for Listeria genus (LMOSLCC2755_0944), L. monocytogenes (LMOSLCC2755_0090), and L. ivanovii (queT_1) was then designed. The assay specificity was robustly verified by analyzing nonpathogenic Listeria and non-Listeria spp. strains. The determined detection limits were 2.0 × 10³ CFU/mL for L. monocytogenes and 3.4 × 10³ CFU/mL for L. ivanovii, for pure culture analysis. Further, the assay detected 7.6 × 10⁴ to 7.6 × 10⁰ CFU/10 g of pathogenic Listeria spiked into F. velutipes samples following 4-12 h enrichment. The assay feasibility was evaluated by comparing with a traditional culture-based method, by analyzing 129 samples collected from different F. velutipes plants. The prevalence of Listeria spp. and L. monocytogenes was 58.1% and 41.1%, respectively. The calculated κ factors for Listeria spp., L. monocytogenes, and L. ivanovii were 0.97, 0.97, and 1, respectively. The results of the novel mPCR assay were highly consistent with those of the culture-based method. The new assay thus will allow rapid, specific, and accurate detection and monitoring of pathogenic Listeria in food and its production environment.

Microbiological safety of ready-to-eat fresh-cut fruits and vegetables sold on the Canadian retail market

Following implementation of Health Canada's Policy on Listeria monocytogenes in Ready-to-Eat Foods by Canadian food safety authorities in 2011, a four-year study (2012-2016) was carried out to gain baseline information on the occurrence of bacterial pathogens, notably the prevalence and levels of Listeria monocytogenes (L. monocytogenes) in various product types of ready-to-eat (RTE) fresh-cut fruits and fresh-cut vegetables sold at retail in Canada. A total of 10,070 pre-packaged samples, including 4691 fresh-cut fruit and 5379 fresh-cut vegetable samples were collected from retail stores across Canada and analyzed for bacterial pathogens and generic Escherichia coli (E. coli). Salmonella species (spp.), E. coli O157:H7, Shigella and Campylobacter were not detected in any of the tested samples. L. monocytogenes was identified in 0.51% (95% CI [0.34, 0.76]) of the fresh-cut fruit and in 0.24% (95% CI [0.14, 0.41]) of the fresh-cut vegetable samples. Of the 37 L. monocytogenes positive samples identified, levels of L. monocytogenes <5 CFU/g, 5-<100 CFU/g, and ≥100 CFU/g were found in 67.6% (25/37), 24.3% (9/37) and 8.1% (3/37) of the samples, respectively. The results of this study indicate that the vast majority of fresh-cut fruits and vegetables sold on the Canadian retail market are safe for consumption. However, contamination by L. monocytogenes can infrequently occur in fresh-cut fruits and vegetables, with certain types of fresh-cut fruits (i.e., melons, apples) and vegetables (i.e., mushrooms, cauliflower) being more likely to be contaminated than others. Safe handling practices are recommended for producers, retailers and consumers including storage at refrigerated temperatures.

Heterogeneity, Characteristics, and Public Health Implications of Listeria monocytogenes in Ready-to-Eat Foods and Pasteurized Milk in China

Listeria monocytogenes is a foodborne pathogen with a high mortality rate in humans. This study aimed to identify the pathogenic potential of L. monocytogenes isolated from ready-to-eat (RTE) foods and pasteurized milk in China on the basis of its phenotypic and genotypic characteristics. Approximately 7.7% (44/570) samples tested positive for L. monocytogenes among 10.8% (39/360) RTE and 2.4% (5/210) pasteurized milk samples, of which 77.3% (34/44) had < 10 MPN/g, 18.2% (8/44) had 10-110 MPN/g, and 4.5% (2/44) had > 110 MPN/g. A total of 48 strains (43 from RTE foods and five from milk samples) of L. monocytogenes were isolated from 44 positive samples. PCR-serogroup analysis revealed that the most prevalent serogroup was II.2 (1/2b-3b-7), accounting for 52.1% (25/48) of the total, followed by serogroup I.1 (1/2a-3a) accounting for 33.3% (16/48), serogroup I.2 (1/2c-3c) accounting for 12.5% (6/48), and serogroup II.1 (4b-4d-4e) accounting for 2.1%. All isolates were grouped into 11 sequence types (STs) belonging to 10 clonal complexes (CCs) and one singleton (ST619) via multi-locus sequence typing. The most prevalent ST was ST87 (29.2%), followed by ST8 (22.9%), and ST9 (12.5%). Virulence genes determination showed that all isolates harbored eight virulence genes belonging to Listeria pathogenicity islands 1 (LIPI-1) (prfA, actA, hly, mpl, plcA, plcB, and iap) and inlB. Approximately 85.4% isolates carried full-length inlA, whereas seven isolates had premature stop codons in inlA, six of which belonged to ST9 and one to ST5. Furthermore, LLS (encoded by llsX gene, representing LIPI-3) displays bactericidal activity and modifies the host microbiota during infection. LIPI-4 enhances neural and placental tropisms of L. monocytogenes. Results showed that six (12.5%) isolates harbored the llsX gene, and they belonged to ST1/CC1, ST3/CC3, and ST619. Approximately 31.3% (15/48) isolates (belonging to ST87/CC87 and ST619) harbored ptsA (representing LIPI-4), indicating the potential risk of this pathogen. Antimicrobial susceptibility tests revealed that > 95% isolates were susceptible to 16 antimicrobials; however, 60.4 and 22.9% isolates were intermediately resistant to streptomycin and ciprofloxacin, respectively. The results show that several isolates harbor LIPI-3 and LIPI-4 genes, which may be a possible transmission route for Listeria infections in consumers.

Association between edible mushroom intake and the prevalence of newly diagnosed non-alcoholic fatty liver disease: results from the Tianjin Chronic Low-Grade Systemic Inflammation and Health Cohort Study in China

Animal studies have suggested that mushroom intake can alleviate non-alcoholic fatty liver disease (NAFLD) due to its anti-inflammatory and antioxidant properties. However, the association between mushroom intake and NAFLD is unknown in humans. We aimed to investigate the association of mushroom intake with NAFLD among Chinese adults. This is a cross-sectional study of 24 236 adults (mean (standard deviation) age: 40·7 (sd 11·9) years; 11 394 men (47·0 %)). Mushroom intake was assessed via a validated FFQ. Newly diagnosed NAFLD was identified based on the results of annual health examinations, including ultrasound findings and a self-reported history of the disease. Multiple logistic models were used to examine the association between mushroom intake and NAFLD. The prevalence of newly diagnosed NAFLD was 19·0 %. Compared with those consuming mushrooms less frequently (≤1 time/week), the fully adjusted OR of newly diagnosed NAFLD were 0·95 (95 % CI 0·86, 1·05) for those consuming 2-3 times/week and 0·76 (95 % CI 0·63, 0·92) for those consuming ≥4 times/week (Pfor trend = 0·01). The inverse association was consistent in subgroups defined by age, sex and BMI. In conclusion, higher mushroom intake was significantly associated with lower prevalence of NAFLD among Chinese adults. Future research is required to understand the causal association between mushroom intake and NAFLD.

Luteolin Inhibits Listeriolysin O Translation by Directly Targeting the Coding Region of the hly mRNA

Listeriolysin O (LLO) is necessary for bacterial escape from the phagosome into the cytoplasm, which suggests that targeting LLO may be an alternative strategy to combat Listeria monocytogenes-mediated infection. Here, luteolin, a natural compound without anti-bacterial activity, as indentified as effective inhibitor of LLO by translationally inhibiting the production of LLO. Additionally, luteolin-treated L. monocytogenes displayed reductions in cytoplasmic growth, cytotoxicity and phagosome escape within macrophages. Molecular modeling and mutational analysis revealed a direct interaction between luteolin and the 5′ coding region (A818, U819, G820, and U830 located in nt 814–849) of the mRNA of hly, the gene encoding LLO, which interfered with its translation. Together, our data demonstrate that luteolin may be used as a novel therapeutic and lead compound for treating L. monocytogenes infection.

Comparative Genomic Analysis of a Multidrug-Resistant Listeria monocytogenes ST477 Isolate

Listeria monocytogenes is an opportunistic human foodborne pathogen that causes severe infections with high hospitalization and fatality rates. Clonal complex 9 (CC9) contains a large number of sequence types (STs) and is one of the predominant clones distributed worldwide. However, genetic characteristics of ST477 isolates, which also belong to CC9, have never been examined, and little is known about the detail genomic traits of this food-associated clone. In this study, we sequenced and constructed the whole-genome sequence of an ST477 isolate from a frozen food sample in China and compared it with 58 previously sequenced genomes of 25 human-associated, 5 animal, and 27 food isolates consisting of 6 CC9 and 52 other clones. Phylogenetic analysis revealed that the ST477 clustered with three Canadian ST9 isolates. All phylogeny revealed that CC9 isolates involved in this study consistently possessed the invasion-related gene vip. Mobile genetic elements (MGEs), resistance genes, and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas system were elucidated among CC9 isolates. Our ST477 isolate contained a Tn554-like transposon, carrying five arsenical-resistance genes (arsA-arsD, arsR), which was exclusively identified in the CC9 background. Compared with the ST477 genome, three Canadian ST9 isolates shared nonsynonymous nucleotide substitutions in the condensin complex gene smc and cell surface protein genes ftsA and essC. Our findings preliminarily indicate that the extraordinary success of CC9 clone in colonization of different geographical regions is likely due to conserved features harboring MGEs, functional virulence and resistance genes. ST477 and three ST9 genomes are closely related and the distinct differences between them consist primarily of changes in genes involved in multiplication and invasion, which may contribute to the prevalence of ST9 isolates in food and food processing environment.

Isolation and Characterization of Clinical Listeria monocytogenes in Beijing, China, 2014-2016

Listeria monocytogenes is an important foodborne pathogen with a significant impact on public health worldwide. A great number of outbreaks caused by L. monocytogenes has been reported, especially in the United States, and European countries. However, listeriosis has not yet been included in notifiable disease in China, and thus information on this infection has been scarce among the Chinese population. In this study, we described a 3-year surveillance of listeriosis in Beijing, China. Fifty-six L. monocytogenes strains isolated from 49 clinical infectious cases (27 pregnancy-associated infections and 22 non-pregnancy-associated infections) were analyzed by serotyping, pulsed field gel electrophoresis (PFGE), multilocus sequence typing (MLST), and antimicrobial susceptibility testing between 2014 and 2016 in Beijing. The predominant serogroups were 1/2a,3a and 1/2b,3b,7 which accounted for 92% of the overall isolates. Four strains were serogroup 4b,4d,4e, isolated from patients with pregnancy-associated infections. Based on PFGE, these isolates were divided into 32 pulsotypes (PTs) and 3 clusters associated with serogroups. Ten PTs were represented by more than one isolate with PT09 containing the most number of isolates. MLST differentiated the isolates into 18 STs, without new ST designated. The three most common STs were ST8 (18.4%), ST5 (16.3%), and ST87 (12.2%), accounting for 46.9% of the isolates. STs prevalent in other parts of the world were also present in China such as ST1, ST2, ST5, ST8, and ST9 which caused maternal fetal infections or outbreaks. However, the STs and serogroup distribution of clinical L. monocytogenes in Beijing, China was different from those in other countries. Strains of ST1 and ST2 were isolated from patients with pregnancy-associated infection, whereas none of ST155 isolates caused pregnancy-associated cases. Surveillance of molecular characterization will provide important information for prevention of listeriosis. This study also enhances our understanding of genetic diversity of clinical L. monocytogenes in China.

Isolation, Potential Virulence, and Population Diversity of Listeria monocytogenes From Meat and Meat Products in China

Listeria monocytogenes is a globally notorious foodborne pathogen. This study aimed to qualitatively and quantitatively detect L. monocytogenes from meat and meat products in China and to establish their virulence profiles and population diversity. From 1212 meat and meat product samples, 362 (29.9%) were positive for L. monocytogenes. Of these positive samples, 90.6% (328/362) had less than 10 MPN/g, 5.5% (20/364) samples had 10-110 MPN/g, and 3.9% (14/362) of the positive samples had over 110 MPN/g. Serogroup analysis showed that the most prevalent serogroup of L. monocytogenes was I.1 (1/2a-3a), which accounted for 45.0% (123/458) of the total, followed by serogroup I.2 (1/2c-3c) that comprised 26.9%, serogroup II.1 (4b-4d-4e) that comprised 4.8%, and serogroup II.2 (1/2b-3b-7) that comprised 23.3%. A total of 458 isolates were grouped into 35 sequence types (STs) that belonged to 25 clonal complexes (CCs) and one singleton (ST619) by multi-locus sequence typing. The most prevalent ST was ST9 (26.9%), followed by ST8 (17.9%), ST87 (15.3%), ST155 (9.4%), and ST121 (7.6%). Thirty-seven isolates harbored the llsX gene (representing LIPI-3), and they belonged to ST1/CC1, ST3/CC3, ST288/CC288, ST323/CC288, ST330/CC288, ST515/CC1, and ST619, among which ST323/CC288, ST330/CC288, and ST515/CC1 were newly reported to carry LIPI-3. Seventy-five isolates carried ptsA, and they belonged to ST87/CC87, ST88/CC88, and ST619, indicating that consumers may be exposed to potential hypervirulent L. monocytogenes. Antibiotics susceptibility tests revealed that over 90% of the isolates were susceptible to 11 antibiotics; however, 40.0% of the isolates exhibited resistance against ampicillin and 11.8% against tetracycline; further, 45.0 and 4.6% were intermediate resistant and resistant to ciprofloxacin, respectively. The rise of antibiotic resistance in L. monocytogenes suggests that stricter regulations should be formulated to restrict the use of antibiotic agents in human listeriosis treatment and livestock breeding.

Occurrence, Antibiotic Resistance, and Population Diversity of Listeria monocytogenes Isolated From Fresh Aquatic Products in China

Listeria monocytogenes is an important Gram-positive foodborne pathogen. However, limited information is available on the comprehensive investigation and potential risk of L. monocytogenes in fresh aquatic products, which are popular to consumers in China. This study aimed to determine the occurrence, virulence profiles, and population diversity of L. monocytogenes isolated from aquatic products in China. In total, 846 aquatic product samples were collected between July 2011 and April 2016 from 43 cities in China. Approximately 7.92% (67/846) aquatic product samples were positive for L. monocytogenes, 86.57% positive samples ranged from 0.3 to 10 MPN/g, whereas 5.97% showed over 110 MPN/g by the Most Probable Number method, which included two samples of products intended to be eaten raw. Serogroups I.1 (serotype 1/2a), I.2 (serotype 1/2b), and III (serotype 4c) were the predominant serogroups isolated, whereas serogroup II.1 (serotype 4b) was detected at much lower frequencies. Examination of antibacterial resistance showed that nine antibacterial resistance profiles were exhibited in 72 isolates, a high level susceptibility of 16 tested antibiotics against L. monocytogenes were observed, indicating these common antibacterial agents are still effective for treating L. monocytogenes infection. Multilocus sequence typing revealed that ST299, ST87, and ST8 are predominant in aquatic products, indicating that the rare ST299 (serotype 4c) may have a special ecological niche in aquatic products and associated environments. Except llsX and ptsA, the 72 isolates harbor nine virulence genes (prfA, actA, hly, plcA, plcB, iap, mpl, inlA, and inlB), premature stop codons (PMSCs) in inlA were found in four isolates, three of which belonged to ST9. A novel PMSC was found in 2929-1LM with a nonsense mutation at position 1605 (TGG→TGA). All ST87 isolates harbored the ptsA gene, whereas 8 isolates (11.11%) carried the llsX gene, and mainly belonged to ST1, ST3, ST308, ST323, ST330, and ST619. Taken together, these results first reported potential virulent L. monocytogenes isolates (ST8 and ST87) were predominant in aquatic products which may have implications for public health in China. It is thus necessary to perform continuous surveillance for L. monocytogenes in aquatic products in China.