Risk Factors and Level of Listeria monocytogenes Contamination of Raw Pork in Retail Markets in China

Clinical Outcome and Factors with Dietary Behaviors in Pregnant Women with Listeria monocytogenes: A Hospital-Based Case-Control Study in Shanghai

Listeriosis is a globally rare foodborne disease that causes fetal-placental infection, leading to adverse pregnancy outcome, while limited research among pregnant women is available in China. This study was therefore aimed at analyzing the incidence, clinical manifestations, and clinical outcome of listeriosis among pregnant women and its associated dietary behavior risk factors in prevention. A hospital-based case-control study had been conducted from January 2017 to December 2021. Clinical data, laboratory information, and questionnaires including dietary behaviors and personal hygiene were collected within 2 days after case diagnosis. There were 48 pregnant women, including 12 cases and 36 controls, with an average age of 31.19 ± 3.75 years. The incidence of admission-based listeriosis among pregnant women was 1.6058 per 10,000. The 12 strains were divided into 3 serotypes: 1/2a(83.33%), 1/2b(8.33%), and 4b(8.33%). Among the cases, 5 cases (41.67%) resulted in abortion, 3 cases (25%) induced preterm labor, and 4 cases (33.33%) had full-term deliveries after treatment. There were 7 live births in the case group, among which 6 were admitted to the neonatal intensive care unit (NICU), while 1 case had a healthy fetal outcome. All patients in the control group gave birth to live fetuses. Epidemiological investigation revealed that pregnant women dining at restaurants three or more times per week might increase the risk of having Listeria infection. There were no significant differences in dietary consumed behaviors, hand hygiene, and refrigerator usage behaviors between case and control groups. The study suggested that dining at restaurants might be associated with Listeria infection among pregnant women. Therefore, it is essential to enhance education on listeriosis serious consequences and promote healthy dietary and hygiene habits among pregnant women.

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.

Quality and safety of pork sold in the informal urban street markets of the Cape Metropole, South Africa

Pork in the informal market significantly contributes to food, nutrition and income security in low-income countries' urban areas but is a safety concern to value chain actors and public authorities due to potential contamination by pathogens. To evaluate the physicochemical quality, microbial and oxidative profiles of pork sold from the informal urban street market, 50 samples were collected from 40 street vendors and 10 supermarkets in five different low-income, high-density suburbs in the Cape Metropole District, South Africa. Results showed no differences (P > 0.05) in pH, colour, proximate attributes (except for lipid content), antioxidant activity, lipid oxidation, and Escherichia coli counts in pork collected from the formal and informal markets, and open-air and enclosed market stalls. Lipid content, Enterobacteriaceae, and total bacterial counts of pork collected from the informal market were higher (P ≤ 0.05) than those from the formal market. Positive cases of Listeria monocytogenes (6-8%) and Salmonella spp. (4%) were reported for pork sampled in the informal market, especially in open-air stalls. It was concluded that higher levels of microbial contamination in the informal market, particularly in open-air stalls compared to the formal market require constant monitoring, provision of appropriate market infrastructure, and hygiene behaviour change of vendors to ensure pork safety.

Association of Virulence, Biofilm, and Antimicrobial Resistance Genes with Specific Clonal Complex Types of Listeria monocytogenes

Precise classification of foodborne pathogen Listeria monocytogenes is a necessity in efficient foodborne disease surveillance, outbreak detection, and source tracking throughout the food chain. In this study, a total of 150 L. monocytogenes isolates from various food products, food processing environments, and clinical sources were investigated for variations in virulence, biofilm formation, and the presence of antimicrobial resistance genes based on their Whole-Genome Sequences. Clonal complex (CC) determination based on Multi-Locus Sequence Typing (MLST) revealed twenty-eight CC-types including eight isolates representing novel CC-types. The eight isolates comprising the novel CC-types share the majority of the known (cold and acid) stress tolerance genes and are all genetic lineage II, serogroup 1/2a-3a. Pan-genome-wide association analysis by Scoary using Fisher's exact test identified eleven genes specifically associated with clinical isolates. Screening for the presence of antimicrobial and virulence genes using the ABRicate tool uncovered variations in the presence of Listeria Pathogenicity Islands (LIPIs) and other known virulence genes. Specifically, the distributions of actA, ecbA, inlF, inlJ, lapB, LIPI-3, and vip genes across isolates were found to be significantly CC-dependent while the presence of ami, inlF, inlJ, and LIPI-3 was associated with clinical isolates specifically. In addition, Roary-derived phylogenetic grouping based on Antimicrobial-Resistant Genes (AMRs) revealed that the thiol transferase (FosX) gene was present in all lineage I isolates, and the presence of the lincomycin resistance ABC-F-type ribosomal protection protein (lmo0919_fam) was also genetic-lineage-dependent. More importantly, the genes found to be specific to CC-type were consistent when a validation analysis was performed with fully assembled, high-quality complete L. monocytogenes genome sequences (n = 247) extracted from the National Centre for Biotechnology Information (NCBI) microbial genomes database. This work highlights the usefulness of MLST-based CC typing using the Whole-Genome Sequence as a tool in classifying isolates.

Prevalence and Biological Characteristics of Listeria Species Isolated from Livestock and Poultry Meat in Gansu Province, China

Listeria monocytogenes is a widespread foodborne pathogen contaminating foods during their production or processing stages. Fresh meat is susceptible to such contamination if it is not properly preserved. Our study was conducted to reveal the level of contamination and prevalence of Listeria spp. present in livestock and poultry meat from Gansu province. A total of 1,387 samples were collected from five cities in Gansu Province according to standard sampling procedures, of which 174 samples (12.5%) were positive for Listeria species. Among them, 14 isolates of L. monocytogenes (1.0%), 150 isolates of Listeria innocua (10.8%), and ten isolates of Listeria welshimeri (0.7%) were identified by conventional bacteriological and molecular identification methods. All isolates were subjected to serological assays, antimicrobial susceptibility tests, growth curve assays, determination of biofilm-forming capacity, and cluster analysis of the 16S rRNA gene sequences. Four predominant serotypes of L. monocytogenes were identified, including 1/2a (35.7%, 5/14), 1/2b (14.3%, 2/14), 1/2c (42.9%, 6/14), and 4b (7.1%, 1/14). All L. monocytogenes isolates were resistant to tetracycline and cefoxitin. Most L. innocua isolates (63.6%, 14/22) and L. welshimeri (40%, 4/10) were resistant to tetracycline. The high biofilm-forming ability was observed among 1/2c and 1/2a serotype isolates. The cluster analysis of the 16S rRNA gene sequences revealed a close genetic relationship between the three Listeria species. This study fills the gap in the knowledge of livestock and poultry meat that carry Listeria in slaughterhouses and markets in Gansu Province.

Whole-genome sequencing reveals genomic characterization of Listeria monocytogenes from food in China

Introduction

Listeria monocytogenes is a foodborne bacterium that could persist in food and food processing environments for a long time. Understanding the population structure and genomic characterization of foodborne L. monocytogenes is essential for the prevention and control of listeriosis.

Methods

A total of 322 foodborne L. monocytogenes isolates from 13 geographical locations and four food sources in China between 2000 and 2018 were selected for whole-genome sequencing.

Results

In silico subtyping divided the 322 isolates into five serogroups, 35 sequence types (STs), 26 clonal complexes (CCs) and four lineages. Serogroup IIa was the most prevalent serogroup and ST9 was the most prevalent ST of foodborne L. monocytogenes strains isolated in China. The in-depth phylogenetic analysis on CC9 revealed that ST122 clone might be original from ST9 clone. Furthermore, 23 potentially relevant clusters were identified by pair-wised whole-genome single nucleotide polymorphism analysis, indicating that persistent- and/or cross-contamination had occurred in markets in China. ST8 and ST121 were the second and third top STs of L. monocytogenes in China, which had heterogeneity with that of L. monocytogenes isolates from other countries. The antibiotic resistance genes aacA4, tetM, tetS, dfrG carried by different mobile elements were found in L. monocytogenes strains. One lineage II strain carrying Listeria Pathogenicity Island 3 was first reported. In addition, a novel type of premature stop codon in inlA gene was identified in this study.

Discussion

These findings revealed the genomic characteristics and evolutionary relationship of foodborne L. monocytogenes in China on a scale larger than previous studies, which further confirmed that whole-genome sequencing analysis would be a helpful tool for routine surveillance and source-tracing investigation.

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

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

Genotyping of Listeria monocytogenes isolates by high-resolution melting curve (HRM) analysis of tandem repeat locus

Listeria monocytogenes is responsible for causing listeriosis, a type of food poisoning with high mortality. This bacterium is mainly transmitted to humans through the consumption of contaminated foods. Detection of L. monocytogenes through molecular methods is crucial for food safety and clinical diagnosis. Present techniques are characterized by low discrimination power and high cost, as well as being time-consuming and taking several days to give the final result. In our study, MLVA-HRM (Multiple-Locus Variable-number tandem repeats Analysis ‒ High-Resolution Melting) was investigated as an alternative method for a fast and precise method for the genotyping of L. monocytogenes isolates. Forty-eight isolates of L. monocytogenes obtained from the microbial bank of Department of Microbiology, Iran University of Medical Sciences, were typed by MLVA-HRM analysis using five Variable Numbers of Tandem Repeat (VNTR) loci. A total of 43 different types were obtained. This research demonstrated the usefulness of the MLVA-HRMA method and its ability to discriminate L. monocytogenes isolates. Since this method is easier and more efficient than existing methods, it can be widely used in food processing plants and diagnostic laboratories as a fast and accurate method.

Surveillance and Reduction Control of Escherichia coli and Diarrheagenic E. coli During the Pig Slaughtering Process in China

The microbial contamination of pork during the slaughter process, especially that of the hygiene indicator bacteria, Escherichia coli, is closely related to the safety and quality of the meat. Some diarrheagenic E. coli can cause serious foodborne diseases, and pose a significant threat to human life and health. In order to ascertain the current status of E. coli and diarrheagenic E. coli contamination during the pig slaughter process in China, we conducted thorough monitoring of large-sized slaughterhouses, as well as small- or medium-sized slaughterhouses, in different provinces of China from 2019 to 2020. The overall positive rate of E. coli on the pork surface after slaughter was very high (97.07%). Both the amount of E. coli contamination and the positive ratio of diarrheagenic E. coli in large-sized slaughterhouses (7.50–13.33 CFU/cm², 3.44%) were lower than those in small- or medium-sized slaughterhouses (74.99–133.35 CFU/cm², 5.71%). Combined with the current status of sanitary control in slaughterhouses, we determined that pre-cooling treatment significantly reduced E. coli and diarrheagenic E. coli in pork after slaughter, while microbiological testing reduced E. coli. Based on our monitoring data, China urgently needs to establish relevant standards to better control microbial contamination during pig slaughtering progress. This study provided a theoretical basis for the hygiene quality management of the pig slaughter industry in China.

Intensive Environmental Surveillance Plan for Listeria monocytogenes in Food Producing Plants and Retail Stores of Central Italy: Prevalence and Genetic Diversity

Listeria monocytogenes (Lm) can persist in food processing environments (FPEs), surviving environmental stresses and disinfectants. We described an intensive environmental monitoring plan performed in Central Italy and involving food producing plants (FPPs) and retail grocery stores (RSs). The aim of the study was to provide a snapshot of the Lm circulation in different FPEs during a severe listeriosis outbreak, using whole genome sequencing (WGS) to investigate the genetic diversity of the Lm isolated, evaluating their virulence and stress resistance profiles. A total of 1217 samples were collected in 86 FPEs with 12.0% of positive surfaces at FPPs level and 7.5% at RSs level; 133 Lm isolates were typed by multilocus sequencing typing (MLST) and core genome MLST (cgMLST). Clonal complex (CC) 121 (25.6%), CC9 (22.6%), CC1 (11.3%), CC3 (10.5%), CC191 (4.5%), CC7 (4.5%) and CC31 (3.8%) were the most frequent MLST clones. Among the 26 cgMLST clusters obtained, 5 of them persisted after sanitization and were re-isolated during the follow-up sampling. All the CC121 harboured the Tn6188_qac gene for tolerance to benzalkonium chloride and the stress survival islet SSI-2. The CC3, CC7, CC9, CC31 and CC191 carried the SSI-1. All the CC9 and CC121 strains presented a premature stop codon in the inlA gene. In addition to the Lm Pathogenicity Island 1 (LIPI-1), CC1, CC3 and CC191 harboured the LIPI-3. The application of intensive environmental sampling plans for the detection and WGS analysis of Lm isolates could improve surveillance and early detection of outbreaks.

Prevalence and antimicrobial resistance of Listeria species and molecular characterization of Listeria monocytogenes isolated from retail ready-to-eat foods

A wide variety of foods can be contaminated with Listeria species, especially L. monocytogenes. Ready-to-eat (RTE) foods are predominantly associated with human listeriosis caused by L. monocytogenes. Therefore, this study aimed to assess the presence of Listeria species in RTE foods and to characterize L. monocytogenes isolates by means of detection of virulence markers, serotypes and genetic relatedness. Of the 300 RTE food samples, 59 (19.7%) were positive for Listeria species: L. innocua (13.3%), L. monocytogenes (5%), L. welshimerii (2.3%), L. grayi subsp. murrayi (1.3%), L. grayi (1%), L. ivanovii (1%) and L. ivanovi subsp. londoniensis (0.3%). All L. monocytogenes isolates identified were positive for the actA, iap, inlA, inlB, inlC, inlJ, plcA and prfA virulence genes and biofilm. The isolates were serotyped as 1/2c (33.3%), 4b (26.7%), 1/2a (26.7%), 1/2b (6.7%) and 3c (6.7%) by the multiplex-PCR and agglutination methods. PCR-restriction fragment length polymorphism with AluI and MluCI resulted in three and two profiles, respectively. Pulsed-field gel electrophoresis differentiated the L. monocytogenes isolates into 15 ApaI and 12 AscI patterns. Antimicrobial resistance of all Listeria isolates was determined by the disk diffusion method. Most L. monocytogenes isolates were sensitive to antimicrobials used in the treatment of listeriosis. This study shows the presence of potential pathogenic and antimicrobial-resistant L. monocytogenes in RTE foods that may lead to consumer health risks.

Listeria monocytogenes Biofilms in the Food Industry: Is the Current Hygiene Program Sufficient to Combat the Persistence of the Pathogen?

Biofilms contain microbial cells which are protected by a self-produced matrix and they firmly attach themselves to many different food industry surfaces. Due to this protection, microorganisms within biofilms are much more difficult to eradicate and therefore to control than suspended cells. A bacterium that tends to produce these structures and persist in food processing plants is Listeria monocytogenes. To this effect, many attempts have been made to develop control strategies to be applied in the food industry, although there seems to be no clear direction on how to manage the risk the bacteria poses. There is no standardized protocol that is applied equally to all food sectors, so the strategies for the control of this pathogen depend on the type of surface, the nature of the product, the conditions of the food industry environment, and indeed the budget. The food industry performs different preventive and corrective measures on possible L. monocytogenes-contaminated surfaces. However, a critical evaluation of the sanitization methods applied must be performed to discern whether the treatment can be effective in the long-term. This review will focus on currently used strategies to eliminate biofilms and control their formation in processing facilities in different food sectors (i.e., dairy, meat, fish, chilled vegetables, and ready-to-eat products). The technologies employed for their control will be exemplified and discussed with the objective of understanding how L. monocytogenes can be improved through food safety management systems.

Unraveling the emergence and population diversity of Listeria monocytogenes in a newly built meat facility through whole genome sequencing

The food processing environments of a newly opened meat processing facility were sampled in ten visits carried out during its first 1.5 years of activity and analyzed for the presence of Listeria monocytogenes. A total of 18 L. monocytogenes isolates were obtained from 229 samples, and their genomes were sequenced to perform comparative genomic analyses. An increase in the frequency of isolation of L. monocytogenes and in the diversity of sequence types (STs) detected was observed along time. Although the strains isolated belonged to six different STs (ST8, ST9, ST14, ST37, ST121 and ST155), ST9 was the most abundant (8 out of 18 strains). Low (0 and 2) single nucleotide polymorphism (SNP) distances were found between two pairs of ST9 strains isolated in both cases 3 months apart from the same processing room (Lm-1267 and Lm-1705, with a 2 SNPs distance in the core genome; Lm-1265 and Lm-1706, with a 0 SNPs distance), which suggests that these strains may be persistent L. monocytogenes strains in the food processing environment. Most strains showed an in silico attenuated virulence potential either through the truncation of InlA (in 67% of the isolates) or the absence of other virulence factors involved in cell adhesion or invasion. Twelve of the eighteen L. monocytogenes isolates contained a plasmid, which ranged in size from 4 to 87 Kb and harbored stress survival, in addition to heavy metals and biocides resistance determinants. Identical or highly similar plasmids were identified for various sets of L. monocytogenes ST9 isolates, which suggests the clonal expansion and persistence of plasmid-containing ST9 strains in the processing environments of the meat facility. Finally, the analysis of the L. monocytogenes genomes available in the NCBI database, and their associated metadata, evidenced that strains from ST9 are more frequently reported in Europe, linked to foods, particularly to meat and pork products, and less represented among clinical isolates than other L. monocytogenes STs. It also showed that the ST9 strains here isolated were more closely related to the European isolates, which clustered together and separated from ST9 North American isolates.

Distribution, virulence, genotypic characteristics and antibiotic resistance of Listeria monocytogenes isolated over one-year monitoring from two pig slaughterhouses and processing plants and their fresh hams

Listeria monocytogenes contamination in raw pork and ready to eat foods is an important food safety concern, also for the increasing detection of antimicrobial-resistant isolates. Data on L. monocytogenes occurrence, persistence, distribution and genetic characterization in two different plants, namely in continuum from slaughtered pigs, environment and unfinished products (fresh hams) were observed by one-year monitoring and were integrated with their antimicrobial resistance patterns. A total of 98 samples out of the overall 1131 (8.7%) were positive for L. monocytogenes, respectively 2.6% and 13.2% in plants A and B: only three serotypes were identified, 1/2c (50%), 1/2b (36.7%) and 1/2a (13.27%), and strains were classified in 35 pulsotypes and 16 clusters by PFGE; a unique P-type was highlighted according to the detection of virulence genes. The contamination flow of L. monocytogenes has a low occurrence in slaughterhouse (Plant A = 1.1%, Plant B: 3.1%; p > 0.05) and increased throughout the processing chain with trimming area as the most contaminated (Plant A: 25%, Plant B: 57%; (p < 0.05)), both in the environment and in unfinished products (80% in hams before trimming in plant B). The dominant role of environmental contamination in post-slaughter processing is confirmed to be a significant cause of meat contamination by L. monocytogenes. Very high levels of resistance were observed for clindamycin (57%) and high resistance levels (>20-50%) to ciprofloxacin, oxacillin, levofloxacin and daptomycin, confirming the L. monocytogenes resistance trend to a wide range of antimicrobial agents. A total of 11 L. monocytogenes isolates were multidrug resistant and 7 out of them were isolated from slaughtered pigs. An interesting significant (p < 0.05) statistical correlation has been found between resistance to some antimicrobial agents and lineage/serotypes. Microbiological sampling of food and environments after sanitization are commonly used as verification procedure for the absence of L. monocytogenes in food plants and to give assurance of food safety, but strains characterization is necessary for industries to target specific control measures, like the enforcement of the hygiene program and of the control of operator activities, at least for permanent strains. The only presence of L. monocytogenes could not be considered as the conclusive assessment of a potential risk for public health, also in terms of emerging and emerged antimicrobial resistances.

Perinatal listeriosis patients treated at a maternity hospital in Beijing, China, from 2013-2018

Background

Listeriosis is a rare but severe foodborne infectious disease. Perinatal listeriosis is often associated with septicemia, central nervous system (CNS) infection, and serious adverse pregnancy outcomes (miscarriage and neonate death). Here we report the characteristics and outcomes of perinatal listeriosis cases treated over 6 years at Beijing Obstetrics and Gynecology Hospital (BOGH), the largest maternity hospital in China.

Methods

We retrospectively reviewed the records of laboratory-confirmed, pregnancy-associated listeriosis cases treated from January 1, 2013 to December 31, 2018. The clinical manifestations, laboratory results, perinatal complications and outcomes (post-natal follow-up of 6 months) were investigated.

Results

In BOGH, 12 perinatal listeriosis cases were diagnosed based on Listeria monocytogenes positive culture, including 10 single pregnancies and 2 twin pregnancies. The corresponding incidence of pregnancy-associated listeriosis was 13.7/100,000 deliveries. Among those cases, four pregnant women and four newborns had septicemia, and two of the neonates with septicemia also suffered CNS infection. All the maternal patients recovered. Two inevitable miscarriages and four fetal stillbirths occurred. Of the eight delivered newborns, six survived, and two died within 2 days from birth. None of the survivors had neurological sequelae during a 6-month follow-up. The overall feto-neonatal fatality rate was 57.1%; notably, this rate was 100% for infections occurring during the second trimester of pregnancy and only 14.3% for those occurring in the third trimester.

Conclusions

Perinatal listeriosis is associated with high feto-neonatal mortality, and thus, a public health concern. Additional large-scale studies are needed to strengthen the epidemiological understanding of listeriosis 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.