Pulsed-field fingerprinting of listeriae: identification of genomic divisions for Listeria monocytogenes and their correlation with serovar

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.

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.

Guidelines for DNA recombination and repair studies: Cellular assays of DNA repair pathways

Understanding the plasticity of genomes has been greatly aided by assays for recombination, repair and mutagenesis. These assays have been developed in microbial systems that provide the advantages of genetic and molecular reporters that can readily be manipulated. Cellular assays comprise genetic, molecular, and cytological reporters. The assays are powerful tools but each comes with its particular advantages and limitations. Here the most commonly used assays are reviewed, discussed, and presented as the guidelines for future studies.

Prevalence, Serotype Diversity, Genotype and Antibiotic Resistance of Listeria monocytogenes Isolated from Carcasses and Human in Korea

This study investigated the prevalence of Listeria monocytogenes in slaughterhouses, and determined serovars and genotypes, and antibiotic resistance of the isolates obtained from slaughterhouses and humans in Korea. Two hundred ninety samples were collected from feces (n=136), carcasses [n=140 (cattle: n=61, swine: n=79)], and washing water (n=14) in nine slaughterhouses. Eleven human isolates were obtained from hospitals and the Korea Center for Disease Control and Prevention. Listeria monocytogenes was enriched and identified, using polymerase chain reaction (PCR) and 16S rRNA sequencing. Serovars and presence of virulence genes were determined, and genetic correlations among the isolates were evaluated by the restriction digest patterns of AscI. Antibiotic resistance of L. monocytogenes isolates were examined against 12 different antibiotics. Of 290 slaughterhouse samples, 15 (5.17%) carcass samples were L. monocytogenes positive. Most L. monocytogenes isolates possessed all the virulence genes, while polymorphisms in the actA gene were found between carcass and human isolates. Serovars 1/2a (33.3%) and 1/2b (46.7%) were the most frequent in carcass isolates. Genetic correlations among the isolates from carcass and clinical isolates were grouped within serotypes, but there were low geographical correlations. Most L. monocytogenes isolates were antibiotic resistant, and some strains showed resistance to more than four antibiotics. These results indicate that L. monocytogenes are isolated from carcass and human in Korea, and they showed high risk serotypes and antibiotic resistance. Therefore, intensive attentions are necessary to be aware for the risk of L. monocytogenes in Korea.

Diversity of Listeria monocytogenes strains isolated from Agaricus bisporus mushroom production

AIMS

The aims of this study were to characterize the genetic diversity of Listeria monocytogenes isolates obtained from commercial mushroom production, to establish the persistence, recontamination and the risk of cross-contamination from the working environment to the final products, creating awareness about the presence of L. monocytogenes thus helping to prevent the possibility of cross-contamination.

METHODS AND RESULTS

From an extensive analysis of commercial mushroom production, analysed with BS EN ISO 11290-1:1996/Amd 1:2004 and BS EN ISO 11290-2:1998/Amd 1:2004, 279 L. monocytogenes isolates were obtained. All of the isolates were characterized by pulsed-field gel electrophoresis, species PCR and serogroup PCR. All the isolates were confirmed as L. monocytogenes; 30·1% were serogroup 1/2b-3b-7, 40·8% were serogroup 1/2a-3a and 29·1% were serogroup 4b-4d-4e. There were 77 pulsotypes from the 279 isolates, 40 of the pulsotypes had only one strain and 37 had two or more strains, indicating great diversity in the isolates.

CONCLUSIONS

The high genetic diversity is indicative of the fact that current hygiene practices are successful at removing L. monocytogenes but that recontamination of the production environment is frequent.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results obtained are very valuable in creating awareness of L. monocytogenes in mushroom production and for the improvement of hygiene practices.

An Assessment of Different Genomic Approaches for Inferring Phylogeny of Listeria monocytogenes

Background/objectives: Whole genome sequencing (WGS) has proven to be a powerful subtyping tool for foodborne pathogenic bacteria like L. monocytogenes. The interests of genome-scale analysis for national surveillance, outbreak detection or source tracking has been largely documented. The genomic data however can be exploited with many different bioinformatics methods like single nucleotide polymorphism (SNP), core-genome multi locus sequence typing (cgMLST), whole-genome multi locus sequence typing (wgMLST) or multi locus predicted protein sequence typing (MLPPST) on either core-genome (cgMLPPST) or pan-genome (wgMLPPST). Currently, there are little comparisons studies of these different analytical approaches. Our objective was to assess and compare different genomic methods that can be implemented in order to cluster isolates of L. monocytogenes.

Methods: The clustering methods were evaluated on a collection of 207 L. monocytogenes genomes of food origin representative of the genetic diversity of the Anses collection. The trees were then compared using robust statistical analyses.

Results: The backward comparability between conventional typing methods and genomic methods revealed a near-perfect concordance. The importance of selecting a proper reference when calling SNPs was highlighted, although distances between strains remained identical. The analysis also revealed that the topology of the phylogenetic trees between wgMLST and cgMLST were remarkably similar. The comparison between SNP and cgMLST or SNP and wgMLST approaches showed that the topologies of phylogenic trees were statistically similar with an almost equivalent clustering.

Conclusion: Our study revealed high concordance between wgMLST, cgMLST, and SNP approaches which are all suitable for typing of L. monocytogenes. The comparable clustering is an important observation considering that the two approaches have been variously implemented among reference laboratories.

The Continuous Challenge of Characterizing the Foodborne Pathogen Listeria monocytogenes

Listeria monocytogenes is an important foodborne pathogen commonly isolated from food processing environments and food products. This organism can multiply at refrigeration temperatures, form biofilms on different materials and under various conditions, resist a range of environmental stresses, and contaminate food products by cross-contamination. L. monocytogenes is recognized as the causative agent of listeriosis, a serious disease that affects mainly individuals from high-risk groups, such as pregnant women, newborns, the elderly, and immunocompromised individuals. Listeriosis can be considered a disease that has emerged along with changing eating habits and large-scale industrial food processing. This disease causes losses of billions of dollars every year with recalls of contaminated foods and patient medical treatment expenses. In addition to the immune status of the host and the infecting dose, the virulence potential of each strain is crucial for the development of disease symptoms. While many isolates are naturally virulent, other isolates are avirulent and unable to cause disease; this may vary according to the presence of molecular determinants associated with virulence. In the last decade, the characterization of genetic profiles through the use of molecular methods has helped track and demonstrate the genetic diversity among L. monocytogenes isolates obtained from various sources. The purposes of this review were to summarize the main methods used for isolation, identification, and typing of L. monocytogenes and also describe its most relevant virulence characteristics.

Processing-Dependent and Clonal Contamination Patterns of Listeria monocytogenes in the Cured Ham Food Chain Revealed by Genetic Analysis

The quantitative and qualitative patterns of environmental contamination by Listeria monocytogenes were investigated in the production chain of dry-cured Parma ham. Standard arrays of surfaces were sampled in processing facilities during a single visit per plant in the three compartments of the food chain, i.e., ham production (19 plants) and postproduction, which was divided into deboning (43 plants) and slicing (25 plants) steps. The numbers of sampled surfaces were 384 in ham production, with 25 positive for L. monocytogenes, and 1,084 in postproduction, with 83 positives. Statistical analysis of the prevalence of contaminated surfaces showed that in ham production, contamination was higher at the beginning of processing and declined significantly toward the end, while in postproduction, prevalence rose toward the end of processing. Prevalence was higher in the deboning facilities than in slicing facilities and was dependent on the type of surface (floor/drainage > clothing > equipment). The qualitative pattern of contamination was investigated through an analysis of the survey isolates and a set of isolates derived from routine monitoring, including longitudinal isolations. Pulsed-field gel electrophoresis (PFGE) and whole-genome single-nucleotide polymorphism (SNP) analysis revealed a remarkable clonality of L. monocytogenes within plants, with the detection of 16 plant-specific clones out of 17 establishments with multiple isolates. Repeated detections of clonal isolates >6 months apart were also observed. Six was the maximum number of between-isolate differences in core SNPs observed within these clones. Based on the same six-SNP threshold, three clusters of clonal isolates, shared by six establishments, were also identified. The spread of L. monocytogenes within and between plants, as indicated by its clonal behavior, is a matter of concern for the hygienic management of establishments.

Identification of high-risk Listeria monocytogenes serotypes in lineage I (serotype 1/2a, 1/2c, 3a and 3c) using multiplex PCR

AIMS

Using molecular subtyping techniques, Listeria monocytogenes is divided into three major phylogenetic lineages, and a multiplex PCR method can differentiate five L. monocytogenes subgroups: 1/2a-3a, 1/2c-3c, 1/2b-3b-7, 4b-4d-4e and 4a-4c. In this study, we conducted genome comparisons and evaluated serotype-associated genes for their utility as a multiplex PCR-based method for distinguishing high-risk serotypes 1/2a and 1/2c in lineage I from low-risk serotypes 3a and 3c.

METHODS AND RESULTS

Primer sets were developed that are specific for serotype 1/2c (LMOSLCC2372_0308) and serotype 3a (LMLG_0742). These primers were then tested in a multiplex format with primers specific for serotype 1/2a (flaA) to separate serotypes 1/2a, 1/2c, 3a and 3c using 25 strains of lineage I L. monocytogenes.

CONCLUSIONS

Here, for the first time, we report primers specific for L. monocytogenes serotype 1/2c and serotype 3a, and we demonstrate a multiplex PCR method for separating the four serotypes of lineage I L. monocytogenes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The described multiplex PCR assay consistently showed successful separation of 1/2a and 1/2c strains from 3a and 3c strains. PCR is routinely performed in many diagnostic and epidemiologic investigations for L. monocytogenes, and these primers should increase the feasibility and accessibility of L. monocytogenes serotyping.

Increased spread and replication efficiency of Listeria monocytogenes in organotypic brain-slices is related to multilocus variable number of tandem repeat analysis (MLVA) complex

BACKGROUND

Listeria (L.) monocytogenes causes fatal infections in many species including ruminants and humans. In ruminants, rhombencephalitis is the most prevalent form of listeriosis. Using multilocus variable number tandem repeat analysis (MLVA) we recently showed that L. monocytogenes isolates from ruminant rhombencephalitis cases are distributed over three genetic complexes (designated A, B and C). However, the majority of rhombencephalitis strains and virtually all those isolated from cattle cluster in MLVA complex A, indicating that strains of this complex may have increased neurotropism and neurovirulence. The aim of this study was to investigate whether ruminant rhombencephalitis strains have an increased ability to propagate in the bovine hippocampal brain-slice model and can be discriminated from strains of other sources. For this study, forty-seven strains were selected and assayed on brain-slice cultures, a bovine macrophage cell line (BoMac) and a human colorectal adenocarcinoma cell line (Caco-2). They were isolated from ruminant rhombencephalitis cases (n = 21) and other sources including the environment, food, human neurolisteriosis cases and ruminant/human non-encephalitic infection cases (n = 26).

RESULTS

All but one L. monocytogenes strain replicated in brain slices, irrespectively of the source of the isolate or MLVA complex. The replication of strains from MLVA complex A was increased in hippocampal brain-slice cultures compared to complex C. Immunofluorescence revealed that microglia are the main target cells for L. monocytogenes and that strains from MLVA complex A caused larger infection foci than strains from MLVA complex C. Additionally, they caused larger plaques in BoMac cells, but not CaCo-2 cells.

CONCLUSIONS

Our brain slice model data shows that all L. monocytogenes strains should be considered potentially neurovirulent. Secondly, encephalitis strains cannot be conclusively discriminated from non-encephalitis strains with the bovine organotypic brain slice model. The data indicates that MLVA complex A strains are particularly adept at establishing encephalitis possibly by virtue of their higher resistance to antibacterial defense mechanisms in microglia cells, the main target of L. monocytogenes.

Division of Human Listeria monocytogenes Pulsed-Field Gel Electrophoresis (PFGE) Types Belonging to Lineage I (Serovar 4b, 1/2b, and 3b) into PFGE Groups

The 63 pulsed-field gel electrophoresis (PFGE) types identified among 427 clinical isolates of Listeria monocytogenes that were characterized in a previous study by serotyping and PFGE (AscI) could be further divided into 17 PFGE groups. While the 63 PFGE types, all part of lineage I, were established based on the number and distribution of all bands in each DNA profile, the 17 PFGE groups were based on the configuration of small bands with sizes <145.5 kb. The 30 PFGE types of L. monocytogenes serovar 4b isolates (n=334) were divided into 8 PFGE groups; the 32 PFGE types of serovar 1/2b isolates (n=90) and the serovar 3b isolates (n=3, 1 PFGE type) were divided into 9 PFGE groups. An association was observed between PFGE groups and serovars. L. monocytogenes isolates belonging to PFGE groups I, J, Q, R, X, Z, Ö-4, and Ö-5 all shared serovar 4b, whereas isolates from PFGE groups D, G, O, P, T, U, Ö-1, Ö-2, and Ö-3 shared serovar 1/2b. Small fragments <33.3 kb were nonvisible in all L. monocytogenes isolates. From the results of the present study, a procedure for accelerating the identification of PFGE types when analyzing new PFGE profiles can be suggested. Therefore, we propose a stepwise procedure to PFGE profiling by first identifying the PFGE group using the smaller band patterns <145.5 kb, and then determining PFGE types based on the band patterns >145.5 kb.

Molecular approaches to the identification of pathogenic and nonpathogenic listeriae

The genus Listeria consists of a closely related group of Gram-positive bacteria that commonly occur in the environment and demonstrate varied pathogenic potential. Of the 10 species identified to date, L. monocytogenes is a facultative intracellular pathogen of both humans and animals, L. ivanovii mainly infects ungulates (eg., sheep and cattle), while other species (L. innocua, L. seeligeri, L. welshimeri, L. grayi, L. marthii, L. rocourtiae, L. fleischmannii and L. weihenstephanensis) are essentially saprophytes. Within the species of L. monocytogenes, several serovars (e.g., 4b, 1/2a, 1/2b and 1/2c) are highly pathogenic and account for a majority of clinical isolations. Due to their close morphological, biological, biochemical and genetic similarities, laboratory identification of pathogenic and nonpathogenic Listeria organisms is technically challenging. With the development and application of various molecular approaches, accurate and rapid discrimination of pathogenic and nonpathogenic Listeria organisms, as well as pathogenic and nonpathogenic L. monocytogenes strains, has become possible.

Trapping and breaking of in vivo nicked DNA during pulsed field gel electrophoresis

Pulsed field gel electrophoresis (PFGE) offers a high-resolution approach to quantify chromosomal fragmentation in bacteria, measured as percentage of chromosomal DNA entering the gel. The degree of separation in pulsed field gel (PFG) depends on the size of DNA as well as various conditions of electrophoresis such as electric field strength, time of electrophoresis, switch time, and buffer composition. Here we describe a new parameter, the structural integrity of the sample DNA itself, that influences its migration through PFGs. We show that subchromosomal fragments containing both spontaneous and DNA damage-induced nicks are prone to breakage during PFGE. Such breakage at single-strand interruptions results in artifactual decrease in molecular weight of linear DNA making accurate determination of the number of double-strand breaks difficult. Although breakage of nicked subchromosomal fragments is field strength independent, some high-molecular-weight subchromosomal fragments are also trapped within wells under the standard PFGE conditions. This trapping can be minimized by lowering the field strength and increasing the time of electrophoresis. We discuss how breakage of nicked DNA may be mechanistically linked to trapping. Our results suggest how to optimize conditions for PFGE when quantifying chromosomal fragmentation induced by DNA damage.

Prevalent and persistent Escherichia coli O157:H7 strains on farms are selected by bovine passage

Escherichia coli O157:H7 is a human pathogen capable of causing hemorrhagic colitis and in some cases hemolytic uremic syndrome. Cattle are an asymptomatic carrier and a major reservoir of this pathogen that can be transmitted by contaminated foods like beef products and vegetables. To further understand persistence in cattle and on farms, a total of 1716 samples over a two-year period were collected from a Wisconsin dairy farm (Farm R) and 91 were positive for the presence of E. coli O157:H7. Seventy-six of 1373 (4.8%) fecal samples and 10/190 (5.3%) water samples were positive. Genotyping of the 341 E. coli O157 isolates by pulsed-field gel electrophoresis showed nine different restriction enzyme digestion profile (REDP) types, seven of which were 93-98% similar (comprised of serotype O157:H7 isolates) and two that were dissimilar (serotype O157:H-isolates). The REDP 31 strain dominated and was isolated from 59 fecal and 9 water samples; 75% of the positive samples (68/91) contained this strain. Growth studies of representative strains from each the REDP groups in Luria broth at 25 and 39 °C found no significant differences between the strains. In LB supplemented with bile salts (3, 6, and 9%; 39 °C, 48 h), the REDP 30 strain had a longer lag phase and achieved a lower maximum density than the other strains in the presence of 6 and 9% bile salts. Likewise, the survival of the strains in low-pH conditions (HCl, pH 2.0 and acetic acid, pH 3.0) were similar except for the REDP 30 strain which was significantly less acid tolerant at pH 2.0. A screening for differences in carbohydrate utilization found that the dominant strain (REDP 31) utilized the most carbon sources and was the only strain that oxidized amygdalin, citraconic acid, α-ketoglutarate, and γ-cyclodextrin. The inoculation of Holstein calves with a three-strain mixture (REDP 30, 31, and 36 strains) found the REDP 31 strain (FRIK 2455) dominated in fecal and rectal swab samples throughout the durations of shedding. These results suggested that carbohydrate utilization and host factors encountered during animal passage select for persistent and predominant strains on farms.

Illuminating the landscape of host-pathogen interactions with the bacterium Listeria monocytogenes

Listeria monocytogenes has, in 25 y, become a model in infection biology. Through the analysis of both its saprophytic life and infectious process, new concepts in microbiology, cell biology, and pathogenesis have been discovered. This review will update our knowledge on this intracellular pathogen and highlight the most recent breakthroughs. Promising areas of investigation such as the increasingly recognized relevance for the infectious process, of RNA-mediated regulations in the bacterium, and the role of bacterially controlled posttranslational and epigenetic modifications in the host will also be discussed.

Ruminant rhombencephalitis-associated Listeria monocytogenes alleles linked to a multilocus variable-number tandem-repeat analysis complex

Listeria monocytogenes is among the most important food-borne pathogens and is well adapted to persist in the environment. To gain insight into the genetic relatedness and potential virulence of L. monocytogenes strains causing central nervous system (CNS) infections, we used multilocus variable-number tandem-repeat analysis (MLVA) to subtype 183 L. monocytogenes isolates, most from ruminant rhombencephalitis and some from human patients, food, and the environment. Allelic-profile-based comparisons grouped L. monocytogenes strains mainly into three clonal complexes and linked single-locus variants (SLVs). Clonal complex A essentially consisted of isolates from human and ruminant brain samples. All but one rhombencephalitis isolate from cattle were located in clonal complex A. In contrast, food and environmental isolates mainly clustered into clonal complex C, and none was classified as clonal complex A. Isolates of the two main clonal complexes (A and C) obtained by MLVA were analyzed by PCR for the presence of 11 virulence-associated genes (prfA, actA, inlA, inlB, inlC, inlD, inlE, inlF, inlG, inlJ, and inlC2H). Virulence gene analysis revealed significant differences in the actA, inlF, inlG, and inlJ allelic profiles between clinical isolates (complex A) and nonclinical isolates (complex C). The association of particular alleles of actA, inlF, and newly described alleles of inlJ with isolates from CNS infections (particularly rhombencephalitis) suggests that these virulence genes participate in neurovirulence of L. monocytogenes. The overall absence of inlG in clinical complex A and its presence in complex C isolates suggests that the InlG protein is more relevant for the survival of L. monocytogenes in the environment.

Significant shift in median guinea pig infectious dose shown by an outbreak-associated Listeria monocytogenes epidemic clone strain and a strain carrying a premature stop codon mutation in inlA

Listeria monocytogenes contains (i) epidemic clone (EC) strains, which have been linked to the majority of listeriosis outbreaks worldwide and are overrepresented among sporadic cases in the United States, and (ii) strains commonly isolated from ready-to-eat foods that carry a mutation leading to a premature stop codon (PMSC) in inlA, which encodes the key virulence factor internalin A (InlA). Internalin A binds certain isoforms of the cellular receptor E-cadherin to facilitate crossing the intestinal barrier during the initial stages of an L. monocytogenes infection. Juvenile guinea pigs, which express the human isoform of E-cadherin that binds InlA, were intragastrically challenged with a range of doses of (i) an EC strain associated with a listeriosis outbreak or (ii) a strain carrying a PMSC mutation in inlA. Recovery of L. monocytogenes from tissues (i.e., liver, spleen, mesenteric lymph nodes, and ileum) was used to develop strain-specific dose-response curves on the basis of individual and combined organ data. Modeling of individual and combined organ data revealed an approximate 1.2 to 1.3 log(10) increase in the median infectious dose for the strain carrying a PMSC in inlA relative to that for the EC strain. Inclusion of the strain parameter significantly improved the goodness of fit for individual and combined organ models, indicating a significant shift in median infectious dose for guinea pigs challenged with an inlA PMSC strain compared to that for guinea pigs challenged with an EC strain. Results from this work provide evidence that the L. monocytogenes dose-response relationship is strain specific and will provide critical data for enhancement of current risk assessments and development of future risk assessments.

Semi-automated repetitive-sequence-based polymerase chain reaction compared to pulsed-field gel electrophoresis for Listeria monocytogenes subtyping

Listeriosis is a severe infection that mainly affects pregnant women, neonates, and immuno-compromised adults. The commercially available semi-automated repetitive-sequence-based polymerase chain reaction assay system, DiversiLab, has been successfully used for subtyping several species of bacteria. In this article we compare the DiversiLab System with macrorestriction analysis by pulsed-field gel electrophoresis (PFGE), which is currently the gold standard for molecular subtyping of Listeria monocytogenes. We used a panel of 116 human and food L. monocytogenes isolates for the comparative evaluation. Among these isolates, there were 4 pairs of duplicates, 13 strains were epidemiologically related, and the remaining food isolates were epidemiologically unrelated. The isolates of different serotypes represented distinct DiversiLab types (DTs) and ApaI/AscI-PFGE types except for one DT-containing isolates of two serotypes, 4b and 1/2b. The four duplicates displayed the same DT and ApaI/AscI PFGE type demonstrating the good reproducibility of the two methods. The epidemiologically related strains were clustered in the same DT and PFGE type. The Simpson's index of diversity was 0.954; 0.988; 0.994; and 0.998 for DiversiLab, AscI-PFGE, ApaI-PFGE, and AscI/ApaI-PFGE, respectively. Thus, PFGE was more discriminating than DiversiLab. However, for 1/2a serotype strains, six AscI-PFGE, three ApaI-PFGE, and one ApaI/AscI PFGE type were divided into different DTs. DiversiLab enabled a good discrimination between serotype 1/2a strains. DiversiLab is less labor intensive than PFGE and provides results in <24 hours compared with 30 hours to 3 days for PFGE from the time a pure culture of the bacteria has been obtained. On the basis of these results, DiversiLab may be useful for tracking the source of contamination in food-processing facilities and their environments. Also, DiversiLab may be more appropriate for long-term epidemiological studies where less discrimination is needed.

High-throughput genome sequencing of two Listeria monocytogenes clinical isolates during a large foodborne outbreak

BACKGROUND

A large, multi-province outbreak of listeriosis associated with ready-to-eat meat products contaminated with Listeria monocytogenes serotype 1/2a occurred in Canada in 2008. Subtyping of outbreak-associated isolates using pulsed-field gel electrophoresis (PFGE) revealed two similar but distinct AscI PFGE patterns. High-throughput pyrosequencing of two L. monocytogenes isolates was used to rapidly provide the genome sequence of the primary outbreak strain and to investigate the extent of genetic diversity associated with a change of a single restriction enzyme fragment during PFGE.

RESULTS

The chromosomes were collinear, but differences included 28 single nucleotide polymorphisms (SNPs) and three indels, including a 33 kbp prophage that accounted for the observed difference in AscI PFGE patterns. The distribution of these traits was assessed within further clinical, environmental and food isolates associated with the outbreak, and this comparison indicated that three distinct, but highly related strains may have been involved in this nationwide outbreak. Notably, these two isolates were found to harbor a 50 kbp putative mobile genomic island encoding translocation and efflux functions that has not been observed in other Listeria genomes.

CONCLUSIONS

High-throughput genome sequencing provided a more detailed real-time assessment of genetic traits characteristic of the outbreak strains than could be achieved with routine subtyping methods. This study confirms that the latest generation of DNA sequencing technologies can be applied during high priority public health events, and laboratories need to prepare for this inevitability and assess how to properly analyze and interpret whole genome sequences in the context of molecular epidemiology.

Impact of -2 degrees C superchilling before refrigerated storage (4 and 8 degrees C) on the microbiological and sensory qualities of cold-smoked salmon

Detection and enumeration of Listeria monocytogenes and total spoilage bacteria in 40 batches of cold-smoked salmon (one batch = 42 products from the same day of manufacture) straight from the factory were carried out. If L. monocytogenes was detected in at least one of the nine samples analyzed on receipt at the laboratory, 9 products of the same batch were stored for 10 days at 4 degrees C, which was followed by 18 days at 8 degrees C (control), 12 products were superchilled for 14 days at -2 degrees C, and 12 other products were superchilled for 28 days at -2 degrees C and then stored under the same conditions as the control was stored. L. monocytogenes was detected in 7% of the 40 batches analyzed immediately after receipt at the laboratory. L. monocytogenes prevalence was similar (approximately 25%) throughout the storage at 4 and 8 degrees C, both in control and super-chilled products at -2 degrees C for 14 days. After superchilling for 28 days at -2 degrees C, L. monocytogenes was found in 9% of products, and in 39% at the end of the storage above 0 degree C. Moreover, the L. monocytogenes count was higher after 3 and 4 weeks of storage at 4 and 8 degrees C in products superchilled 28 days at -2 degrees C than in control products or in products superchilled for 14 days. Serotype 1/2a-3a and nine genetic groups were identified and found throughout the storage scenario. At the end of shelf life, sensory characteristics of products superchilled for 28 days at -2 degrees C were slightly modified. A decrease in firmness associated with increased tearing of salmon slices was observed as well as a slight amine odor.

Different contamination patterns of lineage I and II strains of Listeria monocytogenes in a Spanish broiler abattoir

The purpose of this study was to determine whether genetically similar or diverse strains of Listeria monocytogenes colonize the environment and carcasses in a single Spanish broiler abattoir over time. The study was composed of 5 surveys over a 1.5-yr period and included the monitoring of cleaning and disinfection procedures. Overall, a total of 212 samples were tested for the presence of L. monocytogenes, and 31% of the samples were found to be positive. Listeria monocytogenes was isolated from carcasses and product contact and noncontact sites in the evisceration and carcass classification areas of the abattoir. A total of 132 L. monocytogenes isolates were characterized by PCR-based serotyping and pulsed-field gel electrophoresis (PFGE) restriction analysis with the endonucleases ApaI and AscI. Molecular serotyping showed that L. monocytogenes isolates were of serotypes 1/2a and 1/2b. Isolates of serotype 1/2b (89.4%) were contaminating carcasses as well as environmental product contact and noncontact sites, whereas isolates of serotype 1/2a (10.6%) were recovered only from environmental product noncontact sites. A relatively low genetic diversity was found in this group of L. monocytogenes isolates from the abbatoir; only 14 different PFGE types (A1 to A14) were obtained. Nine pulsotypes belonging to serotype 1/2b (lineage I) were grouped in only one PFGE genetic cluster, whereas 5 pulsotypes belonging to serotype 1/2a (lineage II) were grouped into 4 PFGE genetic clusters. Two genetically related pulsotypes of serotype 1/2b (A1 and A2, 64.4% of the isolates) predominated and persisted in the abattoir. Our study indicated that a few strains of L. monocytogenes lineage I that were genetically very closely related might be specifically adapted to colonizing the evisceration zone of the abattoir and were predominant on carcasses over 1 yr. On the other hand, a genetically diverse group of lineage II strains were present in the abattoir environment, but never contaminated carcasses.

Comparative analysis of extracellular and intracellular proteomes of Listeria monocytogenes strains reveals a correlation between protein expression and serovar

Listeria monocytogenes, the etiologic agent of listeriosis, remains a serious public health concern, with its frequent occurrence in food environments coupled with a high mortality rate. Among the 13 serovars, human listeriosis is mostly associated with the serovar 4b, 1/2b, and 1/2a strains. To investigate the diversity of L. monocytogenes, the intracellular and extracellular proteins of 12 strains were analyzed by two-dimensional gel electrophoresis. These strains had different origins, belonged to different serovars (4b, 1/2a, and 1/2b), and presented with different levels of virulence in chicken embryos. The clustering of the strains in two groups based on proteomic patterns is in agreement with the L. monocytogenes phylogenetic lineages. Statistical analysis did not allow for identification of proteins specific to the isolate origin or the virulence level of the strains, but 26 and 21 protein spots were shown to be significantly overexpressed and underexpressed, respectively, in the six strains of serovar 1/2a (lineage II) compared to strains of serovar 1/2b or 4b. Moreover, a penicillin-binding protein was specific for serovar 1/2b and two protein spots identified as a serine protease were specific to serovar 4b. These protein spots, identified through peptide mass fingerprinting using matrix-assisted laser desorption ionization-time of flight mass spectrometry, were essentially found in the extracellular proteome and may have uses as potential markers for serotyping and risk analysis.

Pulsed-field gel electrophoresis (PFGE) analysis of Listeria monocytogenes isolates from different sources and geographical origins and representative of the twelve serovars

Multiplex-PCR (MPCR) serogrouping and pulsed-field gel electrophoresis (PFGE) subtyping analysis are currently used by several public and private laboratories for the characterization of Listeria monocytogenes. In this study a set of 80 L. monocytogenes isolates belonging to the twelve serovars was used to investigate (i) the typeability of the rare serovars, (ii) the ability of PFGE analysis with ApaI and AscI to differentiate serovars within MPCR serogroups and (iii) the association of molecular types with the specific source or geographical origin of the isolates. With the exception of three isolates (rare serovars 4a and 4c) that were not amenable to restriction with ApaI, all the other analyzed isolates were subtyped by both enzymes. PFGE discriminated the 80 isolates into 62 combined ApaI and AscI PFGE patterns (pulsotypes), but could not differentiate serovars within MPCR serogroups, in which isolates from different serovars displaying the same pulsotype were found. Clustering analysis suggests that for some pulsotypes grouping according to Portuguese origin or source can be suggested. On the other hand, some L. monocytogenes clones are widely distributed. Two pulsotypes from Portuguese human isolates were identical to the ones displayed by human outbreak clones in the UK and in the USA and Switzerland, respectively, although they were not temporally matched. Computer-assisted data analysis of large and diverse PFGE type databases will improve the correct interpretation of subtyping data in epidemiological studies and in tracing routes and sources of contamination in the food industry.

A new perspective on Listeria monocytogenes evolution

Listeria monocytogenes is a model organism for cellular microbiology and host-pathogen interaction studies and an important food-borne pathogen widespread in the environment, thus representing an attractive model to study the evolution of virulence. The phylogenetic structure of L. monocytogenes was determined by sequencing internal portions of seven housekeeping genes (3,288 nucleotides) in 360 representative isolates. Fifty-eight of the 126 disclosed sequence types were grouped into seven well-demarcated clonal complexes (clones) that comprised almost 75% of clinical isolates. Each clone had a unique or dominant serotype (4b for clones 1, 2 and 4, 1/2b for clones 3 and 5, 1/2a for clone 7, and 1/2c for clone 9), with no association of clones with clinical forms of human listeriosis. Homologous recombination was extremely limited (r/m<1 for nucleotides), implying long-term genetic stability of multilocus genotypes over time. Bayesian analysis based on 438 SNPs recovered the three previously defined lineages, plus one unclassified isolate of mixed ancestry. The phylogenetic distribution of serotypes indicated that serotype 4b evolved once from 1/2b, the likely ancestral serotype of lineage I. Serotype 1/2c derived once from 1/2a, with reference strain EGDe (1/2a) likely representing an intermediate evolutionary state. In contrast to housekeeping genes, the virulence factor internalin (InlA) evolved by localized recombination resulting in a mosaic pattern, with convergent evolution indicative of natural selection towards a truncation of InlA protein. This work provides a reference evolutionary framework for future studies on L. monocytogenes epidemiology, ecology, and virulence.

Retail survey of Brazilian milk and Minas frescal cheese and a contaminated dairy plant to establish prevalence, relatedness, and sources of Listeria monocytogenes isolates

A study was designed to recover Listeria monocytogenes from pasteurized milk and Minas frescal cheese (MFC) sampled at retail establishments (REs) and to identify the contamination source(s) of these products in the corresponding dairy processing plant. Fifty milk samples (9 brands) and 55 MFC samples (10 brands) were tested from REs located in Juiz de Fora, Minas Gerais, Brazil. All milk samples and 45 samples from 9 of 10 MFC brands tested negative for L. monocytogenes; however, "brand F" of MFC obtained from REs 119 and 159 tested positive. Thus, the farm/plant that produced brand F MFC was sampled; all samples from the milking parlor tested negative for L. monocytogenes, whereas several sites within the processing plant and the MFC samples tested positive. All 344 isolates recovered from retail MFC, plant F MFC, and plant F environmental samples were serotype 1/2a and displayed the same AscI or ApaI fingerprints. Since these results established that the storage coolers served as the contamination source of the MFC, plant F was closed so that corrective renovations could be made. Following renovation, samples from sites that previously tested positive for the pathogen were collected from the processing environment and from MFC on multiple visits; all tested negative for L. monocytogenes. In addition, on subsequent visits to REs 159 and 119, all MFC samples tested negative for the pathogen. Studies are ongoing to quantify the prevalence, levels, and types of L. monocytogenes in MFC and associated processing plants to lessen the likelihood of listeriosis in Brazil.

Development of a multilocus variable-number of tandem repeat typing method for Listeria monocytogenes serotype 4b strains

Listeria monocytogenes serotype 4b strains have been identified as the causative agent in many human listeriosis epidemics as well as in a considerable number of sporadic cases. Due to the genetic homogeneity of serotype 4b isolates, development of rapid subtyping methods with high discriminatory power for serotype 4b isolates is required to allow for improved outbreak detection and source tracking. In this study, multilocus variable-number tandem repeat analysis (MLVA) was developed and used to characterize 60 serotype 4b isolates from various sources. All isolates were also characterized by automated EcoRI ribotyping, single enzyme pulsed-field gel electrophoresis (PFGE) with ApaI, and a multilocus sequence typing (MLST) scheme targeting six virulence and virulence-associated genes. Discriminatory power of MLVA (as determined by Simpson Index of Discrimination) was higher than the discriminatory power of any of the other three methods. MLVA markers targeted were found to be stable and did not change when three isolates were passaged daily for 70 days. Cluster analyses of MLVA, PFGE and MLST consistently grouped the same isolates into three major clusters, each of which includes one of the three major L. monocytogenes epidemic clones (i.e., ECI, ECIa and ECII). We conclude that the MLVA method described here (i) provides for more discriminatory subtyping of L. monocytogenes serotype 4b strains than the other three methods, (ii) identifies three major groups within the serotype 4b, which are consistent with the groups identified by other subtyping methods, and (iii) is easy to interpret. Use of MLVA may thus be recommended for subtyping of serotype 4b isolates, including as a secondary more discriminatory subtyping method that could be used after initial isolate characterization by PFGE or ribotyping.

Comparative transcriptome analysis of Listeria monocytogenes strains of the two major lineages reveals differences in virulence, cell wall, and stress response

Listeria monocytogenes is a food-borne, opportunistic, bacterial pathogen causing a wide spectrum of diseases, including meningitis, septicemia, abortion, and gastroenteritis, in humans and animals. Among the 13 L. monocytogenes serovars described, human listeriosis is mostly associated with strains of serovars 4b, 1/2b, and 1/2a. Within the species L. monocytogenes, three phylogenetic lineages are described. Serovar 1/2a belongs to phylogenetic lineage I, while serovars 4b and 1/2b group in phylogenetic lineage II. To explore the role of gene expression in the adaptation of L. monocytogenes strains of these two major lineages to different environments, as well as in virulence, we performed whole-genome expression profiling of six L. monocytogenes isolates of serovars 4b, 1/2b, and 1/2a of distinct origins, using a newly constructed Listeria multigenome DNA array. Comparison of the global gene expression profiles revealed differences among strains. The expression profiles of two strains having distinct 50% lethal doses, as assessed in the mouse model, were further analyzed. Gene ontology term enrichment analysis of the differentially expressed genes identified differences in protein-, nucleic acid-, carbon metabolism-, and virulence-related gene expression. Comparison of the expression profiles of the core genomes of all strains revealed differences between the two lineages with respect to cell wall synthesis, the stress-related sigma B regulon and virulence-related genes. These findings suggest different patterns of interaction with host cells and the environment, key factors for host colonization and survival in the environment.

Development of multilocus single strand conformation polymorphism (MLSSCP) analysis of virulence genes of Listeria monocytogenes and comparison with existing DNA typing methods

Development of rapid and simple typing methods is required for analyzing the distribution and contamination routes of food-borne pathogens. We established a simple typing method for Listeria monocytogenes using MLSSCP (Multilocus Single Strand Conformation Polymorphism) analysis. Four virulence genes, hlyA, iap, actA and inlB were amplified by PCR, digested with endonucleases and applied to gels for SSCP. As banding patterns have been shown to reflect even a single nucleotide difference, this method has a potential discriminatory power comparable to that of sequencing analysis. The 64 strains isolated from five meat processing plants were divided into 18 groups by this MLSSCP. Additionally, clustering obtained with this method showed strong correspondence with phylogenetic lineages I and II, and was achieved with much less expenditure in time and cost than is required for other methods, such as MLST. The validity of the MLSSCP lineage classification was confirmed by PFGE, AFLP and ribotyping results. This newly developed MLSSCP method is suitable when obtaining accurate results quickly and simply is crucial.

Traceback identification of an ingredient (pork dewlap) as the possible source of Listeria monocytogenes serotype 4b contamination in raw chicken products

In surveys conducted on finished product samples from a single poultry processing plant in Spain, Listeria monocytogenes was found in 14 different uncooked products. To track contamination patterns, 77 L. monocytogenes isolates were characterized by PCR-based serotyping, pulsed-field gel electrophoresis (PFGE) restriction analysis, and PCR-based allelic analysis of the virulence gene actA. Serotyping revealed that 12 isolates (15.6%) were of the L. monocytogenes serotype 4b complex (serotype 4b or the closely related serotypes 4d and 4e). A combination of endonucleases AscI and ApaI PFGE patterns yielded 15 different pulsotypes among all 77 tested isolates. All the serotype 4b isolates belonged to one pulsotype. Sequencing of the actA gene confirmed that all serotype 4b isolates corresponded to the same allelic subtype. The subtype was recovered from five product types, but its presence was not correlated with the production line or the date of isolation, suggesting a possible association of this strain with a common ingredient. This traceback investigation established that pork dewlap, an ingredient common to all the products contaminated with this strain, was the most probable source of L. monocytogenes 4b. The same 4b strain was isolated from four samples of pork dewlap from one specific supplier. After replacement of this contaminated ingredient in the fresh products, this strain of L. monocytogenes serotype 4b was not detected. This study confirms the effectiveness of molecular subtyping to control contamination by specific strains of L. monocytogenes and the importance of testing the different ingredients added to the food products.

Biodiversity of the species Listeria monocytogenes and the genus Listeria

This review describes the Listeria monocytogenes genome sequences available today and their comparison with that of Listeria innocua and Listeria welshimeri by highlighting their characteristic features and common traits. The diversity present among them is analysed with emphasis on putative virulence and host-pathogen interaction related functions. Then large-scale studies comparing gene content of Listeria and how these studies contributed to typing applications will be discussed. Finally, evolutionary conclusions and future perspectives in Listeria genomics are presented.

The presence of the internalin gene in natural atypically hemolytic Listeria innocua strains suggests descent from L. monocytogenes

The atypical hemolytic Listeria innocua strains PRL/NW 15B95 and J1-023 were previously shown to contain gene clusters analogous to the pathogenicity island (LIPI-1) present in the related foodborne gram-positive facultative intracellular pathogen Listeria monocytogenes, which causes listeriosis. LIPI-1 includes the hemolysin gene, thus explaining the hemolytic activity of the atypical L. innocua strains. No other L. monocytogenes-specific virulence genes were found to be present. In order to investigate whether any other specific L. monocytogenes genes could be identified, a global approach using a Listeria biodiversity DNA array was applied. According to the hybridization results, the isolates were defined as L. innocua strains containing LIPI-1. Surprisingly, evidence for the presence of the L. monocytogenes-specific inlA gene, previously thought to be absent, was obtained. The inlA gene codes for the InlA protein which enables bacterial entry into some nonprofessional phagocytic cells. PCR and sequence analysis of this region revealed that the flanking genes of the inlA gene at the upstream, 5'-end region were similar to genes found in L. monocytogenes serotype 4b isolates, whereas the organization of the downstream, 3'-end region was similar to that typical of L. innocua. Sequencing of the inlA region identified a small stretch reminiscent of the inlB gene of L. monocytogenes. The presence of two clusters of L. monocytogenes-specific genes makes it unlikely that PRL/NW 15B95 and J1-023 are L. innocua strains altered by horizontal transfer. It is more likely that they are distinct relics of the evolution of L. innocua from an ancestral L. monocytogenes, as postulated by others.

Combined sigB allelic typing and multiplex PCR provide improved discriminatory power and reliability for Listeria monocytogenes molecular serotyping

Conventional serotyping has traditionally been used to subtype Listeria monocytogenes, but has several limitations, including low discriminatory power and poor reproducibility. Molecular serotyping methods have been developed for L. monocytogenes, but generally show limited discriminatory power and high misclassification rates. We selected 157 Listeria isolates to evaluate a combination of a previously described multiplex PCR assay and sigB allelic typing as an alternative molecular serotyping and subtyping strategy for L. monocytogenes. While the multiplex PCR assay differentiated five L. monocytogenes subtypes (Simpson's Index of Discrimination [SID]=0.78), including classification of the most common disease-associated serotypes (1/2a, 1/2b, 1/2c, and lineage I 4b) into four distinct groups, it misclassified 3.8% of the isolates studied here. sigB allelic typing differentiated 29 subtypes (SID=0.87) and also allowed identification of lineage III L. monocytogenes, which could not be differentiated from the other Listeria spp. by the multiplex PCR assay. sigB allelic typing failed to differentiate serotype 1/2c and 1/2a isolates and one sigB allelic type included serotype 4b and 1/2b isolates. A molecular serotyping approach that combines multiplex PCR and sigB sequence data showed increased discriminatory power (SID=0.91) over either method alone as well as conventional serotyping (SID=0.87) and classifies the four major serotypes (i.e., 1/2a, 1/2b, 1/2c, and 4b) into unique subgroups with a lower misclassification rate as compared to the multiplex PCR assay. This combined approach also differentiates lineage I serotype 4b isolates from the genetically distinct serotype 4b isolates classified into lineage III.

The use of multiplex PCR to detect and differentiate food- and beverage-associated microorganisms: a review

Regarding food safety, rapid detection of microbial species is crucial to develop effective preventive and/or adjustment measures. Classical methods for determining the presence of certain species are time-consuming and labor-intensive, hence, molecular methods, which offer speed, sensitivity and specificity, have been developed to address this problem. Multiplex PCR (MPCR) is widely applied in the various fields of microbiology for the rapid differentiation of microbial species without compromising accuracy. This paper describes the method and reports on the state-of-the-art application of this technique to the identification of microorganisms vehiculated with foods and beverages. The identification of both pathogens and probiotics and the species important for food fermentation or deterioration will be discussed. Applications of MPCR in combination with other techniques are also reviewed. Potentials, pitfalls, limitations and future prospects are summarised.

Serological and molecular ecology of Listeria monocytogenes isolates collected from 13 French pork meat salting–curing plants and their products

The purpose of this study was dual: 1. to evaluate the serotype distribution of 1028 Listeria monocytogenes isolates collected in 13 French salting factories and their products and 2. to identify sources of L. monocytogenes contamination in these factories and trace the routes of spread by PFGE (Pulsed-Field Gel Electrophoresis) typing. Serotypes 1/2a, 1/2b, 1/2c, 4b and 4e occurred. Pulsotype diversity was high among strains collected in plants and products. Furthermore, strains showing similar pulsotypes occurred on the same surfaces after an interval of at least two weeks and in unrelated factories. Forty five strains were genetically closely related to a 4b serotype L. monocytogenes strain isolated from a human clinical case of listeriosis. Our results highlighted the fact that L. monocytogenes is introduced into meat processing plants through raw meat. To overcome such contamination, suppliers of raw material should adhere to specific microbiological control measures. In addition, more attention should be focused on the appropriateness and compliance with procedures of cleaning and disinfection.

Comparative and functional genomics of Listeria spp

The genus Listeria comprises a group of non-sporulating, Gram-positive, soil bacteria belonging to the low G+C group of microorganisms. The genus consists of only six species, L. monocytogenes, L. ivanovii, L. seeligeri, L. innocua, L. welshimeri, and L. grayi.L. monocytogenes and L. ivanovii are the only known pathogens of this group. Comparative whole-genome sequencing of representative strains comprising the entire genus is currently being performed and nearing completion. In the genus Listeria, genome reduction has led to the generation of non-pathogenic species from pathogenic progenitor strains. Indeed, many of the regions absent in the non-pathogenic species represent commonly deleted genes. Speciation and diversity of strains has been achieved by horizontal gene transfer of DNA encoding novel genes probably required for niche specific survival. The sequencing of several listerial genomes has also been accompanied by studies using global strategies involving whole-genome transcriptional profiling and proteomics to examine the adaptative changes of L. monocytogenes to growth in different environments and to catalogue the genes mediating these responses. We review this data and present information on the expression profile of L. monocytogenes EGD-e inside the vacuolar and the cytosolic environments of the host cell using whole-genome microarray analysis. Of the 484 genes regulated during intracellular growth 41 genes are species-specific, being absent from the genome of the non-pathogenic L. innocua CLIP 11262 strain. There were 25 genes that are strain-specific i.e. absent from the genome of the L. monocytogenes F2365 serotype 4b strain suggesting heterogeneity in the gene pool required for intracellular survival of L. monocytogenes in host cells.

Towards the definition of pathogenic microbe

Identification and typing of spoilage and pathogenic microorganisms have become major objectives over the past decade in microbiology. In food, strain typing is necessary to ensure food safety and for linking cases of foodborne infections to suspected items. Recent advances in molecular biology have resulted in the development of numerous DNA-based methods for discrimination among bacterial strains. Here, we present the use of Simple Sequence Repeats (SSR, or Microsatellites) for bacterial typing. SSRs are a class of short DNA sequence motifs that are tandemly repeated at a specific locus. Computer-based screen of the complete genomic DNA sequences of various prokaryotes showed the existence of tens of thousands well distributed SSR tracts. Mono Nucleotides Repeats (MNRs) are the majority of SSR tracts in bacteria, therefore selected MNR loci were analyzed for variation among strains belonging to three bacterial species: Escherichia coli, Listeria monocytogenes and Vibrio cholerae. High levels of polymorphism in the number of repeats was observed. The finding that most of the MNR tracts are variable in bacterial genomes, but stable at the strain level, allows the use of MNRs for bacterial strains identification. The variation in MNR tracts enables the separation between virulent and non-virulent strain groups and further discriminates among bacterial isolates, in the three tested bacterial species. The uncovered MNR polymorphism is important as a genome-wide source of variation, both in practical applications (e.g. rapid strain identification) and in evolutionary studies. This multi-locus MNR strategy could be applied for high throughput bacterial typing by assigning an "identity number" for each strain based on MNR variations. The developed typing technology should include the fingerprint database for large bacterial strain collections and a high throughput scanner. This accurate and rapid tool can have a major role in decreasing the incidences of food-related outbreaks and will contribute to limit epidemics.

A comparison of selected methods for measuring the virulence properties of Listeria spp

The comparative ability of different methods to assess virulence of Listeria species was investigated in ten Listeria strains. All strains were initially subjected to pulsed-field gel electrophoresis analysis to determine their relatedness. Virulence characteristics were subsequently tested for by (i) determining the presence of six virulence genes by polymerase chain reaction; (ii) testing for the production of listeriolysin O, phosphatidylcholine phospholipase C, and phosphatidylinositol-specific phospholipase C; (iii) investigating the hydrophobicity of the strains; (iv) determining the strains ability to attach to, enter, and replicate within the Caco-2 cells. Variations in most of the virulence characteristics were obvious across the strains for the range of tests performed. A wide range of anomalous results among methods were apparent. In particular, the presence of virulence genes was found to be unrelated to the production of virulence-associated proteins in vitro, while virulence protein production and hydrophobicity in Listeria monocytogenes were found to be unrelated or marginally related, respectively, to the ability to invade the Caco-2 cell line. It was concluded that the methods investigated were unable to consistently and unequivocally measure the differences in the virulence properties of the strains.

Comparison of Listeria monocytogenes strain types in Irish smoked salmon and other foods

An investigation of Listeria monocytogenes in Irish retail smoked salmon products and other unrelated food products was undertaken. Serotyping and genotyping methods were applied. Twenty-six L. monocytogenes isolates cultured from ready-to-eat smoked salmon and an additional 20 L. monocytogenes isolates from various commercially available food products (other than smoked salmon) were compared. Four serotypes, 12 ribotypes, 12 amplified fragment length polymorphism (AFLP) types and 17 pulsed-field gel electrophoresis (PFGE) types were identified among the 46 isolates studied. Genotyping identified a single dominant strain that accounted for 65% of those cultured from smoked salmon and this strain was present in product obtained from three out of five of the manufacturers surveyed. When compared to the food products obtained from a variety of sources, those from smoked salmon appeared to cluster as a single group. In Irish smoked salmon this strain may have adapted, and be capable of persisting in this food product. All isolates were grouped into genetic lineages based on their EcoR1 ribotypes. The attendant risk to public health following consumption of these foods is discussed.

Identification, subtyping and virulence determination of Listeria monocytogenes, an important foodborne pathogen

Listeria monocytogenes is an opportunistic intracellular pathogen that has become an important cause of human foodborne infections worldwide. Given its close relationship to other Listeria species and its tendency to produce non-specific clinical symptoms, the availability of rapid, sensitive and specific diagnostic tests for the differentiation of L. monocytogenes from other Listeria species is helpful for selecting appropriate treatment regimens. In addition, with L. monocytogenes comprising a diversity of strains of varying pathogenicity, the ability to precisely track the strains involved in listeriosis outbreaks and speedily determine their pathogenic potential is critical for the control and prevention of further occurrences of this deadly disease. Extensive research in recent decades has revealed significant insights regarding the molecular mechanisms of L. monocytogenes infection. This in turn has facilitated the development of laboratory procedures for enhanced detection and identification of L. monocytogenes, and has also contributed to the implementation of improved control and prevention strategies against listeriosis. The purpose of this review is to summarize recent progress in the species-specific identification, subtyping and virulence determination of L. monocytogenes strains, and to discuss future research needs pertaining to these important areas of listeriosis.

Application of molecular techniques to the study of hospital infection

Nosocomial infections are an important source of morbidity and mortality in hospital settings, afflicting an estimated 2 million patients in United States each year. This number represents up to 5% of hospitalized patients and results in an estimated 88,000 deaths and 4.5 billion dollars in excess health care costs. Increasingly, hospital-acquired infections with multidrug-resistant pathogens represent a major problem in patients. Understanding pathogen relatedness is essential for determining the epidemiology of nosocomial infections and aiding in the design of rational pathogen control methods. The role of pathogen typing is to determine whether epidemiologically related isolates are also genetically related. To determine molecular relatedness of isolates for epidemiologic investigation, new technologies based on DNA, or molecular analysis, are methods of choice. These DNA-based molecular methodologies include pulsed-field gel electrophoresis (PFGE), PCR-based typing methods, and multilocus sequence analysis. Establishing clonality of pathogens can aid in the identification of the source (environmental or personnel) of organisms, distinguish infectious from noninfectious strains, and distinguish relapse from reinfection. The integration of molecular typing with conventional hospital epidemiologic surveillance has been proven to be cost-effective due to the associated reduction in the number of nosocomial infections. Cost-effectiveness is maximized through the collaboration of the laboratory, through epidemiologic typing, and the infection control department during epidemiologic investigations.

[Molecular epidemiology of Listeria monocytogenes infections in a health district of Madrid in a 3-year period (2001-2003)]

INTRODUCTION

During 2003, an increase in the number of cases of listeriosis was observed in a tertiary hospital in Madrid. The objectives of this study were to review the clinical characteristics of the cases diagnosed from 2001 to 2003 and to investigate clonal relationships among the clinical isolates of Listeria monocytogenes.

METHODS

A retrospective analysis was performed of all cases of listeriosis diagnosed in Hospital 12 de Octubre (Madrid) during 2001-2003. Clinical records for each patient were reviewed and all clinical isolates were compared using serotyping and pulsed-field gel electrophoresis (PFGE) with the AscI and SmaI restriction enzymes.

RESULTS

A total of 18 patients were diagnosed: 4 in 2001, 2 in 2002, and 12 in 2003. The estimated incidence rates were 7.3 cases per 1,000,000 inhabitants in 2001, 3.6 in 2002 and 21.8 in 2003. The most frequent serotype during the study period was 4b (66.7%), and this serotype represented 83.3% of the cases diagnosed in 2003. PFGE yielded 12 different genotypes; one of them (PFGE type G) was common to 5 cases diagnosed in 2003.

CONCLUSION

The expansion of a single clone of L. monocytogenes during 2003 partially contributed to increasing the incidence of listeriosis that year. Molecular epidemiology techniques are useful for detecting outbreaks of a possible foodborne origin and their results should promote epidemiological studies to investigate the food products involved.

Concentrations and tracking of listeria monocytogenes strains in a seafood-processing environment using a most-probable-number enrichment procedure and randomly amplified polymorphic DNA analysis

Concentrations of environmental microflora and Listeria monocytogenes were monitored at multiple environmental locations within a seafood-processing facility over the course of 6 months. Concentrations of L. monocytogenes were determined using a most-probable-number (MPN) enrichment procedure. Two floor drains had persistent low concentrations of L. monocytogenes (0.03 to >1,100 MPN/cm2). In comparison, concentrations of the other organisms in the drain were much higher (heterotrophic plate count range of 10(5) to 10(8) CFU/cm2). Concentrations of environmental organisms (heterotrophic aerobic plate counts and counts of pseudomonads, Shewanella spp., Aeromonas hydrophila, and coliforms) were not correlated with concentrations of L. monocytogenes. The 178 confirmed L. monocytogenes isolates from the MPN procedure were further characterized by randomly amplified polymorphic DNA analysis. Sixteen different banding patterns were identified, and nine of the patterns were identified from samples collected on two or more collection dates. From all locations, banding type A was observed in 98 confirmed isolates (55%). Although present, L. monocytogenes was a relatively minor component in the ecosystem of the floor drains in this seafood-processing facility.

Genetic and phenotypic characterization of Listeria monocytogenes lineage III

Listeria monocytogeneshas been previously grouped into three evolutionary groups, termed lineages I, II and III. While lineages I and II are commonly isolated from various sources, lineage III isolates are rare and have several atypical and unique phenotypic characteristics. Relative to their prevalence in other sources, lineage III strains are overrepresented among isolates from food-production animals, and underrepresented among isolates from human clinical cases and foods. This work describes an extensive genotypic and phenotypic characterization of 46 lineage III isolates. Phylogenetic analyses of partialsigBandactAsequences showed that lineage III represents three distinct subgroups, which were termed IIIA, IIIB and IIIC. Each of these lineage III subgroups is characterized by differentiating genotypic and phenotypic characteristics. Unlike typicalL. monocytogenes, all subgroup IIIB and IIIC isolates lack the ability to ferment rhamnose. While all IIIC and most IIIB isolates carry the putative virulence genelmaA, the majority of subgroup IIIA isolates lack this gene. All three lineage III subgroups contain isolates from human clinical cases as well as isolates that are cytopathogenic in a cell culture plaque assay, indicating that lineage III isolates have the potential to cause human disease. The identification of specific genotypic and phenotypic characteristics among the three lineage III subgroups suggests that these subgroups may occupy different ecological niches and, therefore, may be transmitted by different pathways.