Prevalence ofListeriain Chinese Food Products from 13 Provinces Between 2000 and 2007 and Virulence Characterization ofListeria monocytogenesIsolates

Detection of Pathogenic Serogroups and Virulence Genes in Listeria monocytogenes Strains Isolated from Beef and Beef Products Retailed in Gauteng Province, South Africa, Using Phenotypic and Polymerase Chain Reaction (PCR)-Based Methods

South Africa recently (2017-18) experienced the largest outbreak of human listeriosis in the world caused by L. monocytogenes following the consumption of "polony," a ready-to-eat meat product. Most (59%) cases originated from Gauteng province, South Africa. As a follow-up study to the outbreak, we used standard bacteriological and molecular methods to determine the prevalence of pathogenic and virulent serogroups of L. monocytogenes in various beef and beef products retailed in Gauteng province, South Africa. The overall prevalence of Listeria spp. was 28% (112/400), comprising Listeria monocytogenes (9.3%), Listeria innocua (16.3%), and Listeria welshimeri (2.5%) (p < 0.001). It is crucial to have detected that the region (p=0.036), type of product (p=0.032), and temperature at storage (p=0.011) significantly affected the occurrence of L. monocytogenes in beef products. It is alarming that pathogenic serogroups 4b-4d-4e (51.4%) and 1/2a-3a (43.2%) were detected among the isolates of L. monocytogenes. Importantly, they were all carriers of seven virulence-associated genes (hlyA, inlB, plcA, iap, inlA, inlC, and inlJ). Our study also demonstrated that 16.7% of "polony" samples investigated were contaminated with L. monocytogenes. Considering that pathogenic and virulent L. monocytogenes contaminated beef and beef products retailed in South Africa, the food safety risk posed to consumers remains and cannot be ignored. Therefore, it is imperative to reduce the contamination of these products with L. monocytogenes during beef production, processing, and retailing to avoid future outbreaks of human listeriosis in the country.

Distribution and Antimicrobial Resistance Characterization of Listeria monocytogenes in Poultry Meat in Jiading District, Shanghai

To investigate the distribution, contamination status, and antibiotic resistance of Listeria monocytogenes in four types of retail poultry meat, including chicken, duck, goose, and pigeon, sold in Jiading District, Shanghai, a total of 236 retail poultry meat samples were collected, and L. monocytogenes isolates were obtained for identification and antibiotic susceptibility testing against 14 common antibiotics. Forty-one L. monocytogenes isolates were detected from the 236 retail poultry meat samples, with detection rates of 24.47%, 19.44%, 14.75%, and 4.44% in chicken, goose, duck, and pigeon meat, respectively. Among refrigerated, frozen, and room temperature samples, refrigerated poultry had the highest detection rate at 25.40%, while frozen poultry had the lowest at 13.33%. The detection rate of L. monocytogenes in chicken meat differed significantly between the storage temperatures, while no significant differences were found for other poultry types. No significant differences in detection rates were observed between different retail locations or packaging methods. Isolates exhibited complete resistance to cefoxitin (FOX) and increasing resistance over time to tetracycline (TET) and clindamycin (CLI), while low levels of resistance were found for penicillin (PEN), oxacillin (OXA), and erythromycin (ERY). Resistance to ERY and TET suggests the potential for multidrug resistance. Significant differences in antibiotic resistance profiles were observed among L. monocytogenes from the various poultry types. In summary, contamination status and antibiotic resistance profiles differed among retail chicken, duck, goose, and pigeon meat sold and the resistance rate of strains continues to increase in Jiading District, Shanghai. Targeted control measures should be implemented to reduce the emergence of resistant strains, as retail conditions had minimal impact on L. monocytogenes prevalence in poultry meat.

Surveillance and examination of microbial contamination in ice cream in China

This study investigated the microbial contamination of ice cream in China. A total of 2,887 ice cream samples were collected from different regions of China. Contamination by the aerobic plate count (APC), coliforms, and three foodborne pathogens, such as Listeria monocytogenes, Staphylococcus aureus, and Salmonella spp., was detected in the samples. L. monocytogenes isolates were further analyzed for antibiotic susceptibility and multi-locus sequence typing (MLST). The results showed that APC was more than 10 5 CFU/g in 6.10% (176/2887) and coliform was more than 10 2 CFU/g in 15.69% (453/2887) of all samples. The positive rates of S. aureus, L. monocytogenes, and Salmonella spp. were 0.66%, 0.62%, and 0.10%, respectively. Of these, S. aureus contamination was more than 10 2 CFU/g in two samples, and L. monocytogenes in the positive sample was in the range of 0.3–240 MPN/g, with a median of 4.3 MPN/g. The hygienic status of the packaged samples was much better than that of the bulk samples (P &lt; 0.05). Catered samples were more frequently and heavily contaminated than samples from retail and wholesale outlets (P &lt; 0.05). No significant differences were observed in samples bought from urban and rural areas (P &gt;0.05). For 18 L. monocytogenes strains isolated from ice cream, the resistance rate of nine antibiotics was 5.56% (1/18). By MLST, the L. monocytogenes strains were classified into 9 sequence types (STs), of which ST8 was most common (6 isolates). These above results indicate that a potential health risk to the public may be caused by ice cream, particularly susceptible populations.

Novel Approaches to Environmental Monitoring and Control of Listeria monocytogenes in Food Production Facilities

Listeria monocytogenes is a serious public health hazard responsible for the foodborne illness listeriosis. L. monocytogenes is ubiquitous in nature and can become established in food production facilities, resulting in the contamination of a variety of food products, especially ready-to-eat foods. Effective and risk-based environmental monitoring programs and control strategies are essential to eliminate L. monocytogenes in food production environments. Key elements of the environmental monitoring program include (i) identifying the sources and prevalence of L. monocytogenes in the production environment, (ii) verifying the effectiveness of control measures to eliminate L. monocytogenes, and (iii) identifying the areas and activities to improve control. The design and implementation of the environmental monitoring program are complex, and several different approaches have emerged for sampling and detecting Listeria monocytogenes in food facilities. Traditional detection methods involve culture methods, followed by confirmation methods based on phenotypic, biochemical, and immunological characterization. These methods are laborious and time-consuming as they require at least 2 to 3 days to obtain results. Consequently, several novel detection approaches are gaining importance due to their rapidness, sensitivity, specificity, and high throughput. This paper comprehensively reviews environmental monitoring programs and novel approaches for detection based on molecular methods, immunological methods, biosensors, spectroscopic methods, microfluidic systems, and phage-based methods. Consumers have now become more interested in buying food products that are minimally processed, free of additives, shelf-stable, and have a better nutritional and sensory value. As a result, several novel control strategies have received much attention for their less adverse impact on the organoleptic properties of food and improved consumer acceptability. This paper reviews recent developments in control strategies by categorizing them into thermal, non-thermal, biocontrol, natural, and chemical methods, emphasizing the hurdle concept that involves a combination of different strategies to show synergistic impact to control L. monocytogenes in food production environments.

Genomic Determinants of Pathogenicity and Antimicrobial Resistance for 60 Global Listeria monocytogenes Isolates Responsible for Invasive Infections

Listeria monocytogenes remains a significant public health threat, causing invasive listeriosis manifested as septicemia, meningitis, and abortion, with up to 30% of cases having a fatal outcome. Tracking the spread of invasive listeriosis requires an updated knowledge for virulence factors (VFs) and antimicrobial resistance features, which is an essential step toward its clinical diagnosis and treatment. Taking advantage of high-throughput genomic sequencing, we proposed that the differential genes based on the pathogenomic composition could be used to evaluate clinical observations and therapeutic options for listeriosis. Here, we performed the comparative genomic analysis of 60 strains from five continents with a diverse range of sources, representing serotypes 1/2a, 1/2b, 1/2c, and 4b, comprising lineage I and lineage II and including 13 newly contributed Chinese isolates from clinical cases. These strains were associated with globally distributed clonal groups linked with confirmed foodborne listeriosis outbreak and sporadic cases. We found that L. monocytogenes strains from clonal complex (CC) CC8, CC7, CC9, and CC415 carried most of the adherence and invasive genes. Conversely, CC1, CC2, CC4, and CC6 have the least number of adherence and invasive genes. Additionally, Listeria pathogenicity island-1 (LIPI-1), LIPI-2, intracellular survival, surface anchoring, and bile salt resistance genes were detected in all isolates. Importantly, LIPI-3 genes were harbored in CC3, CC224, and ST619 of the Chinese isolates and in CC1, CC4, and CC6 of other worldwide isolates. Notably, Chinese isolates belonging to CC14 carried antibiotic resistance genes (ARGs) against β-lactams (blaTEM-101, blaTEM-105) and macrolide (ermC-15), whereas CC7 and CC8 isolates harbored ARGs against aminoglycoside (aadA10_2, aadA6_1), which may pose a threat to therapeutic efficacy. Phylogenomic analysis showed that CC8, CC7, and CC5 of Chinese isolates, CC8 (Swiss and Italian isolates), and CC5 and CC7 (Canadian isolates) are closely clustered together and belonged to the same CC. Additionally, CC381 and CC29 of Chinese isolates shared the same genomic pattern as CC26 of Swiss isolate and CC37 of Canadian isolate, respectively, indicating strong phylogenomic relation between these isolates. Collectively, this study highlights considerable clonal diversity with well-recognized virulence and antimicrobial-resistant determinants among Chinese and worldwide isolates that stress to design improved strategies for clinical therapies.

The prevalence of Listeria species in different food items of animal and plant origin in Ethiopia: a systematic review and meta-analysis

Background

Listeriosis is one of the important emerging zoonotic disease affecting human health following the consumption of contaminated food of animal origin. It results in serious clinical complications in humans with a high case facility rate. Therefore, this systematic review and meta-analysis aimed to estimate the pooled prevalence of Listeria species in Ethiopia.

Methods

A systematic search was conducted on PubMed, Web of Science, EMBASE, Google Scholar and the Cochrane Library. All identified observational studies reporting the prevalence of Listeria species in different food items of animal and plant origin in Ethiopia were included. Three authors independently extracted data and analyzed them using STATA Version 13 statistical software. A random effects model was computed to estimate the pooled prevalence of Listeria species in Ethiopia.

Results

After reviewing 122 studies, five studies fulfilled the inclusion criteria were included in the meta-analysis. The findings from the five studies revealed that the pooled prevalence of Listeria species in different food items of animal and plant origin in Ethiopia was 27% (95% CI 25, 29). The highest prevalence of Listeria species was reported in beef meat followed by ice cream with prevalence rates of 62% (95% CI 50, 75) and 43% (95% CI 33, 53), respectively.

Conclusion

The presence of Listeria species in different food items of animal and plant origin in Ethiopia is an indicator of the presence of public health hazards to the consumer, particularly to the high-risk groups. Hence, awareness creation on food safety and implementation of regulations is strongly recommended.

Genomic analysis of high copy-number sequences for the targeted detection of Listeria species using a flow-through surveillance system

The bacterial foodborne pathogen Listeria monocytogenes has been implicated in fresh produce outbreaks with a significant economic impact. Given that L. monocytogenes is widespread in the environment, food production facilities constantly monitor for the presence of Listeria species. To develop a surveillance platform for food processing facilities, this study conducted a comparative genomic analysis for the identification of conserved high copy sequences in the ribosomal RNA of Listeria species. Simulated folding was performed to assess RNA accessibility in the identified genomic regions targeted for detection, and the developed singleplex assay accurately detected cell amounts lower than 5 cells, while no signals were detected for non-targeted bacteria. The singleplex assay was subsequently tested with a flow-through system, consisting of a DNA aptamer-capture step, followed by sample concentration and mechanical lysis for the detection of Listeria species. Validation experiments indicated the continuous flow-through system accurately detected Listeria species at low cell concentrations.

Antimicrobial activity of various essential oils and their application in active packaging of frozen vegetable products

Essential oils (EOs) have potential utility as clean-label food preservatives due to their antimicrobial and antioxidant properties. In this study, various EOs were screened for their antimicrobial activities against Listeria grayi in vitro. The susceptibility of L. monocytogenes to select EOs was compared with that of L. grayi. The effectiveness of the selected EOs in inhibiting the growth of L. grayi on vegetable products was also investigated. The results showed that cinnamon and oregano EOs and carvacrol were effective in the vapor phase in inhibiting the growth of L. grayi as well as L. monocytogenes, with the susceptibility of L. monocytogenes to cinnamon EO being slightly higher than that of L. grayi. The packaging of green peppers with cellulose stickers impregnated with cinnamon EO at 556 μL/L_headspace reduced the Listeria count to 1 log CFU/g after 2 days of storage as compared to 7.5 log CFU/g for controls.

10-Year Molecular Surveillance of Listeria monocytogenes Using Whole-Genome Sequencing in Shanghai, China, 2009-2019

Listeria monocytogenes is an etiologic agent of listeriosis, and has emerged as an important foodborne pathogen worldwide. In this study, the molecular characteristics of 155 L. monocytogenes isolates from seven food groups in Shanghai, the biggest city in China, were identified using whole-genome sequencing (WGS). Most L. monocytogenes isolates (79.3%) were obtained between May and October from 2009 to 2019. The serogroups and clonal complexes (CCs) of L. monocytogenes were found useful for identifying potential health risks linked to foods. Differences in distributions of serogroups and CCs among different food groups were analyzed using t-test. The results showed that the IIa and IVb serogroups were identified among most of food groups. However, the prevalence of serogroup IIb was significantly higher in ready-to-eat (RTE) food and raw seafood than in other food groups, similar to group IIc in raw meat and raw poultry than others. Meanwhile, the prevalence of CC9 in raw meat and raw poultry, CC8 in raw poultry, and CC87 in raw seafood significantly exceeded that of in other food groups. Specially, CC87 was the predominant CC in foodborne and clinical isolates in China, indicating that raw seafood may induce a high-risk to food safety. Also, hypervirulence pathogenicity islands LIPI-3 and LIPI-4 were found in CC3, CC1, and CC87, respectively. The clonal group CC619 carried LIPI-3 and LIPI-4, as previously reported in China. Core genome multilocus sequence typing (cgMLST) analysis suggested that CC87 isolates from the same food groups in different years had no allelic differences, indicating that L. monocytogenes could persist over years. These 10-year results in Shanghai underscore the significance of molecular epidemiological surveillance of L. monocytogenes in foodborne products in assessing the potential risk of this pathogen, and further address food safety issues in China.

Genomic and Phenotypic Diversity of Listeria monocytogenes Causing Pregnancy-Associated Listeriosis from Zhejiang Province, China, 2016-2018

Introduction

There are few investigations describing the pregnancy-associated listeriosis in China, and the molecular characteristics of Listeria monocytogenes causing such infections remain largely unknown. We aim to investigate the phenotypic and genomic profiles of pregnancy-associated L. monocytogenes isolates and their association with isolates recovered from human and non-human in China.

Materials and Methods

In this study, we conducted a 3-year surveillance of listeriosis in a women's hospital in Zhejiang province, using whole genome sequencing and bioinformatics tools.

Results

From 2016 to 2018, we identified 13 clinical L. monocytogenes isolates. Among these pregnancy-associated isolates, we found seven sequence types (STs), with the prevalent STs of ST87 and ST7. Serotyping divided the strains into four serotypes, including serotype 1/2a, 1/2b, 3a, and 4b. Antimicrobial resistance testing showed that all the isolates were susceptible to 10 antibiotics. Comparative genomics analysis clearly classified our genome collection into four distinct evolutionary lineages with most isolates grouping into lineages I and II. Interestingly, we found three pairs of isolates with high identity, although no evident epidemiological association was observed.

Conclusion

This study reports for the first time the surveillance of pregnancy-associated listeriosis in Zhejiang province, China, which indicates that the infection rate is low in this region. Our findings provide insight into the evolution and genetic diversity of pregnancy-associated L. monocytogenes from Zhejiang province. Additional investigations involving more human and non-human isolates with a "one health" strategy are needed for prediction of the listeriosis risk associated with a typical prevalent clone in Zhejiang province, such as ST87.

Carriage and potential long distance transmission of Listeria monocytogenes by migratory black-headed gulls in Dianchi Lake, Kunming

Listeria monocytogenes is a high risk pathogen which can cause invasive diseases in humans. We previously reported that black-headed gulls from Dianchi Lake of Kunming carrying L. monocytogenes, while the characteristics of these isolates and the relationship with habitats of migratory gulls have not been explored. In this study, we investigated the prevalence and molecular characteristics of Listeria monocytogenes from black-headed gulls in Dianchi Lake, and phylogenetic analysis based on core genome SNPs was used to determine the genetic relationship of the strains from Dianchi Lake and other regions. Occurrence of L. monocytogenes in black-headed gull feces in 2016, 2017 and 2018 was 1.0%, 1.0% and 0.6% respectively. The predominant serotype of 28 isolates was 4b, while the predominant sequence types were ST145 and ST201. Based on their prevalence and genomic relationships, ST5 and ST87 were likely to be sourced locally while ST145 and ST201 were likely to be non-local. L. monocytogenes may travel along the bird migration route leading to transmission over a large geographical span carried by black-headed gull. Although the prevalence of L. monocytogenes was low, its carriage by the migratory black-headed gulls poses potential public health risks in regions where the migratory birds passage and reside.

Sentinel Listeriosis Surveillance in Selected Hospitals, China, 2013-2017

During 2013-2017, a total of 211 cases of listeriosis were reported by 64 sentinel hospitals in China to a national foodborne disease surveillance network. The average case-fatality rate was 31.2% for perinatal cases and 16.4% for nonperinatal cases. Sequence types 87 and 8 were the most prevalent types.

The prevalence of Listeria monocytogenes in meat products in China: A systematic literature review and novel meta-analysis approach

Meat products are commonly regarded as one of the main sources of human listeriosis caused by Listeria monocytogenes. The objective of this study was to estimate the prevalence of L. monocytogenes in a range of meat products from 24 different Chinese regions by using meta-analysis of literature data and a novel sensitivity analysis approach. A total of 112 publications from five databases, published between 1 January 2007 and 31 December 2017, were systematically selected for relevance and covered meat products sampled between 2000 and 2016. Estimated by the random-effects model, the pooled prevalence of L. monocytogenes was 8.5% (95% CI: 7.1%-10.3%) in raw meats and 3.2% (95% CI: 2.7%-3.9%) in ready-to-eat (RTE) meats. The prevalence differed from high to low among raw meats including prefabricated raw meats 12.6% (95% CI: 6.9%-21.7%), fresh pork 11.4% (95% CI: 8.6%-14.9%), fresh beef 9.1% (95% CI: 6.3%-13.0%), fresh poultry 7.2% (95% CI:4.9%-10.4%), frozen raw meats 7.2% (95% CI: 5.7%-9.0%), and fresh mutton 5.4% (95% CI: 2.5%-11.0%). A higher L. monocytogenes prevalence level was shown in the meat products from central and northeastern China provincial regions. The entropy-based sensitivity analysis utilized in the meta-analysis indicated that the sampling period and location were two critical factors influencing the prevalence level of L. monocytogenes in meat products. A better understanding of differences in prevalence levels per geographic region and between meat product sources may allow the competent authorities, industry, and other relevant stakeholders to tailor their interventions to control the occurrence of L. monocytogenes in meat products effectively.

Coresistance to Benzalkonium Chloride Disinfectant and Heavy Metal Ions in Listeria monocytogenes and Listeria innocua Swine Isolates from China

The development of coresistance to disinfectants and heavy metals contributes to the fitness of Listeria spp. in foods or food processing environments, where life-threatening Listeria monocytogenes coexist and coevolve with other Listeria spp. Despite extensive research on L. monocytogenes, coresistance to disinfectants and heavy metals is less documented for other Listeria spp. In this study, we screened 30 L. monocytogenes and 27 Listeria innocua isolates recovered from 273 swine samples for resistance to quaternary ammonium compound benzalkonium chloride (BC) and to heavy metals cadmium (Cd) and arsenic (As). Moreover, we evaluated the potential mechanisms of resistance by detecting the efflux pump activity in BC resistance and the presence of resistance determinants. The average minimum inhibitory concentrations of BC in L. innocua (10.7 ± 2.0) were significantly higher than that in L. monocytogenes (6.9 ± 3.7) (p < 0.05). Resistance to BC and heavy metals was correlated, where all BC-resistant L. innocua and As-resistant L. monocytogenes isolates were coresistant to BC and Cd. Twenty percent and 66.7% of BC resistance in L. monocytogenes and L. innocua were related to reserpine-associated efflux pumps, whereas all cases of BC resistance were related to carbonyl cyanide 3-chlorophenylhydrazone-associated efflux pumps. The cadA1 and cadA2 genes were present in Cd-resistant isolates but not in Cd-sensitive isolates, and cadA3 was undetectable in all isolates examined. cadA4 conferring lower level of Cd resistance was copresent with arsA1 and arsA2 in the Cd-resistant and As-susceptible L. monocytogenes isolate LM3. Our findings suggest that swine serves as a reservoir for developing resistance to disinfectant and heavy metals in L. monocytogenes and L. innocua, which share common resistance mechanisms such as efflux pumps and resistance genes. This work provides new insight into the coresistance events of other Listeria as a potential contributor of the resistance in L. monocytogenes.

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.

Implicated Food Products for Listeriosis and Changes in Serovars of Listeria monocytogenes Affecting Humans in Recent Decades

Listeriosis is a foodborne disease with a high fatality rate, and infection is mostly transmitted through ready-to-eat (RTE) foods contaminated with Listeria monocytogenes, such as gravad/smoked fish, soft cheeses, and sliced processed delicatessen (deli) meat. Food products/dishes stored in vacuum or in modified atmospheres and with extended refrigerator shelf lives provide an opportunity for L. monocytogenes to multiply to large numbers toward the end of the shelf life. Elderly, pregnant women, neonates, and immunocompromised individuals are particularly susceptible to L. monocytogenes. Listeriosis in humans manifests primarily as septicemia, meningitis, encephalitis, gastrointestinal infection, and abortion. In the mid 1990s and early 2000s a shift from L. monocytogenes serovar 4b to serovar 1/2a causing human listeriosis occurred, and serovar 1/2a is becoming more frequently linked to outbreaks of listeriosis, particularly in Europe and Northern America. Consumer lifestyle has changed, and less time is available for food preparation. Modern lifestyle has markedly changed eating habits worldwide, with a consequent increased demand for RTE foods; therefore, more RTE and take away foods are consumed. There is a concern that many Listeria outbreaks are reported from hospitals. Therefore, it is vitally important that foods (especially cooked and chilled) delivered to hospitals and residential homes for senior citizens and elderly people are reheated to at least 72°C: cold food, such as turkey deli meat and cold-smoked and gravad salmon should be free from L. monocytogenes. Several countries have zero tolerance for RTE foods that support the growth of Listeria.

Characterization of Antimicrobial Resistance of Listeria monocytogenes Strains Isolated from a Pork Processing Plant and Its Respective Meat Markets in Southern China

A total of 78 Listeria monocytogenes isolates from a pork processing plant and the respective meat markets in southern China were examined. This number includes 60 isolates from pork at markets, 5 from cooked pork products at markets, 10 from pork at a processing plant, and 3 from food-contact surfaces at the processing plant. All isolates were subjected to serotyping, antimicrobial susceptibility testing, pulsed-field gel electrophoresis (PFGE), and molecular basis of antibiotic resistance. Four serogroups were identified among the 78 tested isolates, with serogroup I (serotypes: 1/2a and 3a) being predominant (42.3%, 33/78). Antimicrobial resistance was most frequently observed for tetracycline (20.5%, 16/78), streptomycin (9.0%, 7/78), cefotaxime (7.7%, 6/78), and gentamicin (6.4%, 5/78). Multiple resistances occurred among 10.2% (8/78) isolates. All strains were sensitive to ampicillin, ampicillin/sulbactam, imipenem, ciprofloxacin, levofloxacin, trimethoprim/sulfamethoxazole, and vancomycin. Two isolates were resistant to five antimicrobials. Twelve strains carried tet(M) and located on Tn916. PFGE analysis revealed genetic heterogeneity among individual serotypes. Two predominant PFGE types were found persistent from the processing plant to markets indicating that these two types of isolates were able to survive under environmental adverse conditions from the processing plant to markets, which need to be monitored. Compared to samples from the pork processing plant, the prevalence of L. monocytogenes in meat market samples tended to be higher, serovar was more variable, and the antibiotic resistance range was wider, probably due to secondary contamination. Therefore, stringent hygiene measures and bacteriological controls should be observed to reduce the risk of transmission of L. monocytogenes from food to humans.

Analysis of Multilocus Sequence Typing and Virulence Characterization of Listeria monocytogenes Isolates from Chinese Retail Ready-to-Eat Food

Eighty Listeria monocytogenes isolates were obtained from Chinese retail ready-to-eat (RTE) food and were previously characterized with serotyping and antibiotic susceptibility tests. The aim of this study was to characterize the subtype and virulence potential of these L. monocytogenes isolates by multilocus sequence typing (MLST), virulence-associate genes, epidemic clones (ECs), and sequence analysis of the important virulence factor: internalin A (inlA). The result of MLST revealed that these L. monocytogenes isolates belonged to 14 different sequence types (STs). With the exception of four new STs (ST804, ST805, ST806, and ST807), all other STs observed in this study have been associated with human listeriosis and outbreaks to varying extents. Six virulence-associate genes (inlA, inlB, inlC, inlJ, hly, and llsX) were selected and their presence was investigated using PCR. All strains carried inlA, inlB, inlC, inlJ, and hly, whereas 38.8% (31/80) of strains harbored the listeriolysin S genes (llsX). A multiplex PCR assay was used to evaluate the presence of markers specific to epidemic clones of L. monocytogenes and identified 26.3% (21/80) of ECI in the 4b-4d-4e strains. Further study of inlA sequencing revealed that most strains contained the full-length InlA required for host cell invasion, whereas three mutations lead to premature stop codons (PMSC) within a novel PMSCs at position 326 (GAA → TAA). MLST and inlA sequence analysis results were concordant, and different virulence potentials within isolates were observed. These findings suggest that L. monocytogenes isolates from RTE food in China could be virulent and be capable of causing human illness. Furthermore, the STs and virulence profiles of L. monocytogenes isolates have significant implications for epidemiological and public health studies of this pathogen.

Listeria monocytogenes: Strain Heterogeneity, Methods, and Challenges of Subtyping

Listeria monocytogenes is a food-borne bacterial pathogen that is associated with 20% to 30% case fatality rate. L. monocytogenes is a genetically heterogeneous species, with a small fraction of strains (serotypes 1/2a, 1/2b, 4b) implicated in human listeriosis. Monitoring and source tracking of L. monocytogenes involve the use of subtyping methods, with the performance of genetic-based methods found to be superior to phenotypic-based ones. Various methods have been used to subtype L. monocytogenes isolates, with the pulsed-field gel electrophoresis (PFGE) being the gold standard. Although PFGE has had a massive impact on food safety through the establishment of the PulseNet, there is no doubt that whole genome sequence (WGS) typing is accurate, has a discriminatory power superior to any known method, and allows genome-wide differences between strains to be quantified through the comparison of nucleotide sequences. This review focuses on the different techniques that have been used to type L. monocytogenes strains, their performance challenges, and the tremendous impact WGS typing could have on the food safety landscape.

Occurrence and Antibiotic Susceptibility of Listeria Species in Turkey Meats

The aims of this study were to investigate the occurrence of Listeria species in turkey meats and to check the antimicrobial susceptibility of the isolated strains. Hundred and fifteen raw turkey meat samples were randomly collected from the supermarkets, butchers and restaurants. Strain isolation and identification were made according to the ISO11290-1 method. Antimicrobial susceptibility was determined by the standard disc diffusion method. A total of 47 Listeria spp. were isolated from 115 (40.9%) raw turkey meat samples. The isolates were distributed between L. monocytogenes (25.53%), L. innocua (34.04%), L. grayi (31.91%) and L. welshimeri (8.51%). A total of 55.3 % of Listeria spp. isolates were multi-resistant to at least 3 of the antimicrobial agent tested. The level of multi-resistance was higher in L. monocytogenes strains (66.7%) than in L. innocua (62.5%) and L. grayi (53.3%). Listeria spp. isolates were highly resistant to ampicillin, cephalothin, penicillin, meticillin, oxacillin, and trimethoprime-sulfamethoxazole. The isolates particularly L. monocytogenes are increasingly resistant to one or more antibiotics and may represent a potential risk for public health because these antibiotics are common used in treatment of listeriosis. The correct and controlled use of antibiotics in veterinary medicine is important to the emergence of resistant strains.

Prevalence and Virulence Characterization of Listeria monocytogenes in Chilled Pork in Zhejiang Province, China

Listeria monocytogenes is an important foodborne pathogen that can grow in refrigeration temperature and causes severe human infections. The aims of this work were to estimate the prevalence of L. monocytogenes in chilled pork in Zhejiang, China and to examine the virulence features of the isolates. Of 331 meat samples, 196 were positive for Listeria spp., with L. innocua accounting for 54.4%, L. monocytogenes for 11.5%, and L. welshimeri for 4.2%. The most prevalent L. monocytogenes serotype was 1/2c (60.5%), followed by serotypes 1/2a (28.9%), 1/2b (7.9%), and 4b (2.6%). All L. monocytogenes isolates contained virulence-associated genes examined. Adhesion and invasion ability of serotype 1/2c isolates was much lower than those of other serotypes. Only one isolate was defective in cell-to-cell spread. These findings are important for risk assessment of chilled pork as a source of potential transmission of L. monocytogenes to other food products, particularly to ready-to-eat food products.

Listeria monocytogenes Prevalence and Characteristics in Retail Raw Foods in China

The prevalence and levels of Listeria monocytogenes in retail raw foods covering most provincial capitals in China were studied with testing of 1036 samples of vegetables, edible mushrooms, raw meat, aquatic products and quick-frozen products from September 2012 to January 2014. The total prevalence of Listeria monocytogenes was 20.0% (207/1036), and the most probable number (MPN) values of 65.7% of the positive samples ranged from 0.3 to 110 MPN/g. Geographical differences were observed in this survey, and the results of both qualitative and quantitative methods indicated that the levels in the samples from North China were higher than those in the samples from South China. A total of 248 isolates were analyzed, of which approximately half belonged to molecular serogroup 1/2a-3a (45.2%), followed by 1/2b-3b-7 (30.6%), 1/2c-3c (16.1%), 4b-4d-4e (5.2%) and 4a-4c (2.8%). Most of the isolates carried hly (100%), inlB (98.8%), inlA (99.6%), inlC (98.0%) and inlJ (99.2%), and 44.8% of the isolates were llsX-positive. Seventeen epidemic clones (ECs) were detected, with 7 strains belonging to ECI (2.8%) and 10 belonging to ECIII (4.03%). Resistance to clindamycin (46.8%) was commonly observed, and 59 strains (23.8%) were susceptible to all 14 tested antibiotics, whereas 84 (33.9%) showed an intermediate level of resistance or were resistant to two or more antibiotics, including 7 multi-resistant strains that exhibited resistance to more than 10 antibiotics. The data obtained in the present study provides useful information for assessment of the possible risk posed to Chinese consumers, and this information will have a significant public health impact in China. Furthermore, the presence of virulence markers, epidemic clones, as well as the antibiotic resistance amongst the isolates strongly implies that many of these strains might be capable of causing listeriosis, and more accurate treatment of human listeriosis with effective antibiotics should be considered. This research represents a more full-scale and systematical investigation of the prevalence of L. monocytogenes in retail raw foods in China, and it provides baseline information for Chinese regulatory authorities that will aid in the formulation of a regulatory framework for controlling L. monocytogenes with the aim of improving the microbiological safety of raw foods.

Activation of PrfA results in overexpression of virulence factors but does not rescue the pathogenicity of Listeria monocytogenes M7

Listeria monocytogenes encodes a transcriptional activator, PrfA, to positively regulate the expression of virulence factors. Several mutations in PrfA (PrfA*) have been found to contribute to increased regulatory activity. Here, we describe a strain, M7, containing a PrfA*(G145S) that activates expression of virulence factors but with low pathogenicity. To study this contradictory relationship, we exchanged the prfA genes between strains EGDe and M7 (designated EGDe-prfA(M7) and M7-prfA(EGDe)). The phospholipase B (PlcB) and listeriolysin O (LLO) activities were significantly upregulated in the strain EGDe-prfA(M7) (PrfA*). Constitutive activation of PrfA potentiated virulence of the pathogenic strain EGDe, shown as increased adhesion and invasion as well as enhanced cell-to-cell spread in cultured cell lines. However, the strain M7, though PrfA-activated, had significant defects in these virulence-related phenotypes and low pathogenicity in the murine infection model, as compared with EGDe or EGDe-PrfA(M7). To further uncover the possible mechanisms, we analysed abundance and distributions of InlA, InlB, LLO and ActA proteins, all regulated by PrfA, in EGDe, M7 and their prfA mutants. Western blotting showed that the PrfA-regulated genes of constitutively activated PrfA strains were overexpressed in vitro, while different distributions were observed. In contrast to the virulent strain EGDe-prfA(M7), the majority of InlB in M7 was detected in the culture supernatant and not on the bacterial surface. We suppose that the low virulence of strain M7 is due to its defects in infecting host cells, possibly as a result of failed anchorage on the bacterial cells of surface proteins like InlB, a major protein involved in adhesion and invasion of pathogenic L. monocytogenes strains. Further research is warranted to address why InlB detaches from the bacterial cells of this particular strain.

Essential oils of thyme and Rosemary in the control of Listeria monocytogenes in raw beef

This study was developed in order to evaluate two alternatives for the control of Listeria monocytogenes in raw bovine meat pieces, both based on the use of Thymus vulgaris and Rosmarinus officinalis essential oils (EOs). The antilisterial activity of different concentrations of the EOs was tested in vitro using agar dilution and disk volatilization techniques. In addition, L. monocytogenes was inoculated in meat pieces, which were submerged in edible gelatin coatings containing 2% (v/v) EOs or submitted to the vapor of EOs (0.74 μL.cm(-3)). L. monocytogenes was quantified after one, 48 and 96 hours of storage (7 °C). In the in vitro tests, the EO of T. vulgaris presented higher activity. The two options used (edible gelatin coating and vapor activity), in spite of exercising effects with differentiated behaviors, presented antibacterial activity against L. monocytogenes inoculated in raw bovine meat (p < 0.05). Greatest antibacterial activity were obtained in the experiment that used edible coatings containing EOs, at 48 hours of storage reductions in bacterial counts between 1.09 and 1.25 Log CFU.g(-1) were obtained. In the vapor effect experiment, the EO of T. vulgaris caused the highest reduction in the population of bacteria inoculated in raw bovine meat (p < 0.05), 0.40 Log CFU.g(-1) at 96 hours of storage. This study supplied important information regarding new and promising natural alternatives, based on the concept of active packaging, for the control of L. monocytogenes in the meat industry.

Systematic review of human listeriosis in China, 1964-2010

OBJECTIVES

Listeria is an important foodborne pathogen with severe manifestations and high case-fatality rate. However, listeriosis is not yet a notifiable disease in China, and there is no national monitoring system for cases. We conducted a systematic review to better understand the clinical and epidemiologic features of listeriosis in China.

METHODS

Both electronic and manual retrieval systems were used to search Chinese literature for cases and isolates of human listeriosis reported between 1964 and 2010. We recorded and analysed demographic, clinical and laboratory information available for reported cases.

RESULTS

A total of 147 clinical cases, 479 Listeria isolates and 82 outbreak-related cases were reported in 28 (90%) provinces in China from January 1964 to December 2010. Of the clinical cases, 45 (31%) were central nervous system infections, 68 (46%) were septicaemia and 34 (23%) were focal infections or gastroenteritis. The overall case-fatality rate was 26% (34/130) among clinical cases with known outcomes and 46% (21/46) among neonatal cases.

CONCLUSION

Listeriosis cases occurred in China throughout the study period between 1964 and 2010. Case-fatality was similar to published data from other countries. China should consider requiring notification of listeriosis cases to improve estimates of incidence, identification of risk factors and design of preventive measures.

Listeria monocytogenes ArcA contributes to acid tolerance

The foodborne pathogen Listeria monocytogenes is able to colonize the human and animal intestinal tracts and subsequently crosses the intestinal barrier, causing systemic infection. For successful establishment of infection, L. monocytogenes must survive and adapt to the low pH environment of the stomach. Gene sequence analysis indicates that lmo0043, an orthologue of arcA, encodes a protein containing conserved motifs and critical active amino acids characteristic of arginine deiminase that mediates an arginine deimination reaction. We attempted to characterize the role of ArcA in acid tolerance in vitro and in mice models. Transcription of arcA was significantly increased in L. monocytogenes culture subjected to acid stress at pH 4.8, as compared with that at pH 7.0. Deletion of arcA impaired growth of L. monocytogenes under mild acidic conditions at pH 5.5, and reduced its survival in synthetic human gastric fluid at pH 2.5 and in the murine stomach. Bacterial load in the spleen of mice intraperitoneally inoculated with an arcA deletion mutant was significantly lower than that of the wild-type strain. These phenotypic changes were recoverable by genetic complementation. Thus, we conclude that L. monocytogenes arcA not only mediates acid tolerance in vitro but also participates in gastric survival and virulence in mice.

Molecular subtyping and virulence gene analysis of Listeria monocytogenes isolates from food

A total of 67 Listeria monocytogenes isolates from 698 raw meat samples were characterized for molecular serogroup identification and antimicrobial susceptibility. Approximately one third (32.8%) of the isolates belonged to molecular serogroup 1/2a, 3a, followed by 1/2c, 3c (26.9%), 1/2b, 3b, 7 (22.4%), 4b, 4d, 4e (16.4%) and 4a, 4c (1.5%). Most of the L. monocytogenes isolates were susceptible to 14 antimicrobials tested but several were resistant to tetracycline, ciprofloxacin and nitrofurantoin. An additional 30 L. monocytogenes isolates from chicken and produce in our collection were also included to determine the presence of significant virulence markers. All 97 isolates carried inlC and inlJ except for a lineage III isolate 110-1. Most Listeriolysin S (LLS)-carrying isolates (11/12) belonged to lineage I, whereas the remaining one isolate belonged to lineage III. Five 4b, 4d, 4e isolates including two from turkey and three from produce belonged to Epidemic Clone I (ECI). Four molecular serogroup associated mutation types that lead to premature stop codons (PMSCs) in inlA were identified. PFGE and inlA sequence analysis results were concordant, and different virulence potential within 1/2a, 3a and 4b, 4d, 4e isolates were observed. The study revealed that a subset of isolates from meat and produce belonged to ECI, harbored inlC, inlJ and LLS, and produced full length InlA, suggesting that they be capable of causing human illness.

Examination of food chain-derived Listeria monocytogenes strains of different serotypes reveals considerable diversity in inlA genotypes, mutability, and adaptation to cold temperatures

Listeria monocytogenes strains belonging to serotypes 1/2a and 4b are frequently linked to listeriosis. While inlA mutations leading to premature stop codons (PMSCs) and attenuated virulence are common in 1/2a, they are rare in serotype 4b. We observed PMSCs in 35% of L. monocytogenes isolates (n = 54) recovered from the British Columbia food supply, including serotypes 1/2a (30%), 1/2c (100%), and 3a (100%), and a 3-codon deletion (amino acid positions 738 to 740) seen in 57% of 4b isolates from fish-processing facilities. Caco-2 invasion assays showed that two isolates with the deletion were significantly more invasive than EGD-SmR (P < 0.0001) and were either as (FF19-1) or more (FE13-1) invasive than a clinical control strain (08-5578) (P = 0.006). To examine whether serotype 1/2a was more likely to acquire mutations than other serotypes, strains were plated on agar with rifampin, revealing 4b isolates to be significantly more mutable than 1/2a, 1/2c, and 3a serotypes (P = 0.0002). We also examined the ability of 33 strains to adapt to cold temperature following a downshift from 37°C to 4°C. Overall, three distinct cold-adapting groups (CAG) were observed: 46% were fast (<70 h), 39% were intermediate (70 to 200 h), and 15% were slow (>200 h) adaptors. Intermediate CAG strains (70%) more frequently possessed inlA PMSCs than did fast (20%) and slow (10%) CAGs; in contrast, 87% of fast adaptors lacked inlA PMSCs. In conclusion, we report food chain-derived 1/2a and 4b serotypes with a 3-codon deletion possessing invasive behavior and the novel association of inlA genotypes encoding a full-length InlA with fast cold-adaptation phenotypes.

Genetic diversity and molecular typing of Listeria monocytogenes in China

BACKGROUND

Listeria monocytogenes can cause invasive diseases in humans and farm animals and is frequently isolated from dairy products and poultry. Listeriosis is uncommon in China but L. monocytogenes has been isolated from foods and food processing environments in China. However little is known of genetic diversity of Chinese L. monocytogenes isolates and their relationships with global isolates.

RESULTS

Two hundred and twelve isolates of L. monocytogenes from food sources from 12 provinces/cities in China were analysed by serotyping, Pulsed Field Gel Electrophoresis (PFGE) and Multi-locus Sequence Typing (MLST). The predominant serotypes are 1/2a, 1/2b and 1/2c accounting for 90.1% of the isolates. PFGE divided the isolates into 61 pulse types (PTs). Twenty nine PTs were represented by more than one isolates with PT GX6A16.0004 containing the most number of isolates. MLST differentiated the isolates into 36 STs, among which 15 were novel. The 3 most common STs were ST9 (29.1%), ST8 (10.7%) and ST87 (9.2%), accounting for 49.0% of the isolates.

CONCLUSIONS

STs prevalent in other parts of the world are also prevalent in China including 7 STs (ST1-ST3, ST5, ST6, ST8, ST9) which caused maternal fetal infections or outbreaks, suggesting that these STs potentially can also cause severe human infections or outbreaks in China. Surveillance of these STs will provide important information for prevention of listeriosis. This study also enhances our understanding of genetic diversity of L. monocytogenes in China.

Deciphering the biodiversity of Listeria monocytogenes lineage III strains by polyphasic approaches

Listeria monocytogenes is a foodborne pathogen of humans and animals. The majority of human listeriosis cases are caused by strains of lineages I and II, while lineage III strains are rare and seldom implicated in human listeriosis. We revealed by 16S rRNA sequencing the special evolutionary status of L. monocytogenes lineage III, which falls between lineages I and II strains of L. monocytogenes and the non-pathogenic species L. innocua and L. marthii in the dendrogram. Thirteen lineage III strains were then characterized by polyphasic approaches. Biochemical reactions demonstrated 8 biotypes, internalin profiling identified 10 internal-in types clustered in 4 groups, and multilocus sequence typing differentiated 12 sequence types. These typing schemes show that lineage III strains represent the most diverse population of L. monocytogenes, and comprise at least four subpopulations IIIA-1, IIIA-2, HIB, and IIIC. The in vitro and in vivo virulence assessments showed that two lineage IIIA-2 strains had reduced pathogenicity, while the other lineage III strains had comparable virulence to lineages I and II. The HIB strains are phylogenetically distinct from other sub-populations, providing additional evidence that this sublineage represents a novel lineage. The two biochemical reactions L-rhamnose and L-lactate alkalinization, and 10 internalins were identified as potential markers for lineage III subpopulations. This study provides new insights into the biodiversity and population structure of lineage III strains, which are important for understanding the evolution of the L. mono-cytogenes-L. innocua clade.

Lmo0036, an ornithine and putrescine carbamoyltransferase in Listeria monocytogenes, participates in arginine deiminase and agmatine deiminase pathways and mediates acid tolerance

Listeria monocytogenes is a foodborne pathogen causing listeriosis. Acid is one of the stresses that foodborne pathogens encounter most frequently. The ability to survive and proliferate in acidic environments is a prerequisite for infection. However, there is limited knowledge about the molecular basis of adaptation of L. monocytogenes to acid. Arginine deiminase (ADI) and agmatine deiminase (AgDI) systems are implicated in bacterial tolerance to acidic environments. Homologues of ADI and AgDI systems have been found in L. monocytogenes lineages I and II strains. Sequence analysis indicated that lmo0036 encodes a putative carbamoyltransferase containing conserved motifs and residues important for substrate binding. Lmo0036 acted as an ornithine carbamoyltransferase and putrescine carbamoyltransferase, representing the first example, to our knowledge, that catalyses reversible ornithine and putrescine carbamoyltransfer reactions. Catabolic ornithine and putrescine carbamoyltransfer reactions constitute the second step of ADI and AgDI pathways. However, the equilibrium of in vitro carbamoyltransfer reactions was overwhelmingly towards the anabolic direction, suggesting that catabolic carbamoyltransferase was probably the limiting step of the pathways. lmo0036 was induced at the transcriptional level when L. monocytogenes was subjected to low-pH stress. Its expression product in Escherichia coli exhibited higher catabolic carbamoyltransfer activities under acidic conditions. Consistently, absence of this enzyme impaired the growth of Listeria under mild acidic conditions (pH 4.8) and reduced its survival in synthetic human gastric fluid (pH 2.5), and corresponded to a loss in ammonia production, indicating that Lmo0036 was responsible for acid tolerance at both sublethal and lethal pH levels. Furthermore, Lmo0036 played a possible role in Listeria virulence.

Genome sequence of the nonpathogenic Listeria monocytogenes serovar 4a strain M7

This report presents the complete and annotated genome sequence of the naturally nonpathogenic Listeria monocytogenes serovar 4a strain M7, isolated from cow's milk in Zhejiang province, China.

Survival of Listeria monocytogenes in simulated gastrointestinal system and transcriptional profiling of stress- and adhesion-related genes

Food ingestion is the major route of exposure to the important human pathogen Listeria monocytogenes. An in vitro gastrointestinal model was used to (1) compare the survival rates of L. monocytogenes strains of serotypes 1/2a, 1/2c, and 4b; and (2) examine the transcription of stress- and adhesion-related genes after exposure to the conditions similar to those encountered in the mouth, stomach, and small intestine. None of the L. monocytogenes strains investigated could survive in the gastric juice at pH 2.5 or 3.0. Their survival increased at higher pH (3.5 and 4.0) in the gastric stress. Relative survival of L. monocytogenes serotypes 4b and 1/2a strains were higher than that of serotype 1/2c, suggesting that pathogenicity might be related to the viability in the gastrointestinal tract. The transcription levels of prfA and the general stress-related genes clpC, clpE, and clpP were upregulated after passing through the simulated gastrointestinal tract, whereas that of the adhesion-related gene ami was downregulated. Taken together, this study revealed that L. monocytogenes strains enhanced the expression of stress-related genes and decreased the transcription of adhesion-related gene in order to survive in the diverse microenvironments.

Serovar 4b complex predominates among Listeria monocytogenes isolates from imported aquatic products in China

Listeria monocytogenes, the causative organism of listeriosis, is primarily transmitted to humans through contaminated food. In this study, we examined 1275 batches of aquatic products imported from 29 countries and found that 36 batches from 8 countries were contaminated by Listeria (2.8%), with L. monocytogenes accounting for 2.6% (33/1275) and L. innocua for 0.2% (3/1275). Of the 23 selected L. monocytogenes isolates (from the 33 identified), 15 (65.2%) were of serovar 4b complex (4b, 4d, or 4e), three (13.0%) of 1/2a or 3a, four (17.4%) of 1/2b or 3b, and one (4.4%) of 1/2c or 3c. Notably, four of the 23 isolates belonged to epidemic clone I (ECI) and another four were associated with epidemic clone II (ECII), two highly clonal 4b clusters responsible for most of the documented listeriosis outbreaks. In the multilocus sequence typing scheme based on the concatenated genes gyrB-dapE-hisJ-sigB-ribC-purM-betL-gap-tuf, serovar 4b complex isolates from imported aquatic products exhibited significant genetic diversity. While the four ECI isolates were genetically related to those from Chinese diseased animals, both lacking one proline-rich repeat of ActA, the four ECII isolates were located between 1/2b or 3b strains. As the L. monocytogenes isolates from imported aquatic products possessed a nearly complete set of major infection-related genes, they demonstrated virulence potential in mouse model.