Characterization and pathogenic potential of Listeria monocytogenes isolates from the smoked fish industry

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.

Accurate classification of Listeria species by MALDI-TOF mass spectrometry incorporating denoising autoencoder and machine learning

Listeria monocytogenes belongs to the category of facultative anaerobic bacteria, and is the pathogen of listeriosis, potentially lethal disease for humans. There are many similarities between L. monocytogenes and other non-pathogenic Listeria species, which causes great difficulties for their correct identification. The level of L. monocytogenes contamination in food remains high according to statistics from the Food and Drug Administration. This situation leads to food recall and destruction, which has caused huge economic losses to the food industry. Therefore, the identification of Listeria species is very important for clinical treatment and food safety. This work aims to explore an efficient classification algorithm which could easily and reliably distinguish Listeria species. We attempted to classify Listeria species by incorporating denoising autoencoder (DAE) and machine learning algorithms in matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). In addition, convolutional neural networks were used to map the high dimensional original mass spectrometry data to low dimensional core features. By analyzing MALDI-TOF MS data via incorporating DAE and support vector machine (SVM), the identification accuracy of Listeria species was 100%. The proposed classification algorithm is fast (range of seconds), easy to handle, and, more importantly, this method also allows for extending the identification scope of bacteria. The DAE model used in our research is an effective tool for the extraction of MALDI-TOF mass spectrometry features. Despite the fact that the MALDI-TOF MS dataset examined in our research had high dimensionality, the DAE + SVM algorithm was still able to exploit the hidden information embedded in the original MALDI-TOF mass spectra. The experimental results in our work demonstrated that MALDI-TOF mass spectrum combined with DAE + SVM could easily and reliably distinguish Listeria species.

Assessment of Internalin A Gene Sequences and Cell Adhesion and Invasion Capacity of Listeria monocytogenes Strains Isolated from Foods of Animal and Related Origins

Listeria monocytogenes is a foodborne pathogen of global relevance that causes outbreaks and sporadic cases of listeriosis, acquired through the consumption of contaminated products, including milk or meat products and ready-to-eat meat products subjected to intensive handling. The objective of the present study was to classify L. monocytogenes isolated from various food-related sources in the Federal District of Brazil and surrounding areas to sequence internalin A (inlA) genes from these isolates and assess their adhesion and invasion capacity using Caco-2 cells. In addition, 15 were classified as group I, 3 as group II, and 7 classified as group IV. Premature stop codons (PMSCs) at the nucleotide position 976 (GAA→TAA) of the inlA gene were identified in 5 of the 25 isolates. Adhesion and invasion tests in Caco-2 cells showed that all the isolates were capable of adhesion and cellular invasion, with isolates containing PMSCs exhibiting on average higher invasion capacity than those without PMSCs (p = 0.041) and a median of adhesion very distinctive from those without stop codons. These results are the first report of PMSCs in the inlA gene of L. monocytogenes from the Federal District of Brazil and Brazil.

Strain Variability of Listeria monocytogenes under NaCl Stress Elucidated by a High-Throughput Microbial Growth Data Assembly and Analysis Protocol

Listeria monocytogenes causes the severe foodborne illness listeriosis and survives in food-associated environments due to its high stress tolerance. A data assembly and analysis protocol for microbial growth experiments was compiled to elucidate the strain variability of L. monocytogenes stress tolerance. The protocol includes measurement of growth ability under stress (step 1), selection of a suitable method for growth parameter calculation (step 2), comparison of growth patterns between strains (step 3), and biological interpretation of the discovered differences (step 4). In step 1, L. monocytogenes strains (n = 388) of various serovars and origins grown on media with 9.0% NaCl were measured using a Bioscreen C microbiology reader. Technical variability of the growth measurements was assessed and eliminated. In step 2, the growth parameters determined by Gompertz, modified-Gompertz, logistic, and Richards models and model-free splines were compared, illustrating differences in the suitability of these methods to describe the experimental data. In step 3, hierarchical clustering was used to describe the NaCl tolerance of L. monocytogenes measured by strain-specific variation in growth ability; tolerant strains had higher growth rates and maximum optical densities and shorter lag phases than susceptible strains. The spline parameter area under the curve best classified "poor," "average," and "good" growers. In step 4, the tested L. monocytogenes lineage I strains (serovars 4b and 1/2b) proved to be significantly more tolerant toward 9.0% NaCl than lineage II strains (serovars 1/2a, 1/2c, and 3a). Our protocol provides systematic tools to gain comparable data for investigating strain-specific variation of bacterial growth under stress.IMPORTANCE The pathogen Listeria monocytogenes causes the foodborne disease listeriosis, which can be fatal in immunocompromised individuals. L. monocytogenes tolerates several environmental stressors and can persist in food-processing environments and grow in foodstuffs despite traditional control measures such as high salt content. Nonetheless, L. monocytogenes strains differ in their ability to withstand stressors. Elucidating the intraspecies strain variability of L. monocytogenes stress tolerance is crucial for the identification of particularly tolerant strains. To enhance reliable identification of variability in bacterial stress tolerance phenotypes, we compiled a large-scale protocol for the entire data assembly and analysis of microbial growth experiments, providing a systematic approach and checklist for experiments on strain-specific growth ability. Our study illustrated the diversity and strain-specific variation of L. monocytogenes stress tolerance with an unprecedented scope and discovered biologically relevant serovar- and lineage-dependent phenotypes of NaCl tolerance.

Validation of Predicted Virulence Factors in Listeria monocytogenes Identified Using Comparative Genomics

Listeria monocytogenes is an intracellular facultative pathogen that causes listeriosis, a foodborne zoonotic infection. There are differences in the pathogenic potential of L. monocytogenes subtypes and strains. Comparison of the genome sequences among L. monocytogenes pathogenic strains EGD-e and F2365 with nonpathogenic L. innocua CLIP1182 and L. monocytogenes strain HCC23 revealed a set of proteins that were present in pathogenic strains and had no orthologs among the nonpathogenic strains. Among the candidate virulence factors are five proteins: putrescine carbamoyltransferase; InlH/InlC2 family class 1 internalin; phosphotransferase system (PTS) fructose transporter subunit EIIC; putative transketolase; and transcription antiterminator BglG family. To determine if these proteins have a role in adherence and invasion of intestinal epithelial Caco-2 cells and/or contribute to virulence, five mutant strains were constructed. F2365ΔinlC2, F2365Δeiic, and F2365Δtkt exhibited a significant (p < 0.05) reduction in adhesion to Caco-2 cells compared to parent F2365 strain. The invasion of F2365ΔaguB, F2365ΔinlC2, and F2365ΔbglG decreased significantly (p < 0.05) compared with the parent strain. Bacterial loads in mouse liver and spleen infected by F2365 was significantly (p < 0.05) higher than it was for F2365ΔaguB, F2365ΔinlC2, F2365Δeiic, F2365Δtkt, and F2365ΔbglG strains. This study demonstrates that aguB, inlC2, eiic, tkt, and bglG play a role in L. monocytogenes pathogenicity.

Comparative Analysis of Tools and Approaches for Source Tracking Listeria monocytogenes in a Food Facility Using Whole-Genome Sequence Data

As WGS is increasingly used by food industry to characterize pathogen isolates, users are challenged by the variety of analysis approaches available, ranging from methods that require extensive bioinformatics expertise to commercial software packages. This study aimed to assess the impact of analysis pipelines (i.e., different hqSNP pipelines, a cg/wgMLST pipeline) and the reference genome selection on analysis results (i.e., hqSNP and allelic differences as well as tree topologies) and conclusion drawn. For these comparisons, whole genome sequences were obtained for 40 Listeria monocytogenes isolates collected over 18 years from a cold-smoked salmon facility and 2 other isolates obtained from different facilities as part of academic research activities; WGS data were analyzed with three hqSNP pipelines and two MLST pipelines. After initial clustering using a k-mer based approach, hqSNP pipelines were run using two types of reference genomes: (i) closely related closed genomes (“closed references”) and (ii) high-quality de novo assemblies of the dataset isolates (“draft references”). All hqSNP pipelines identified similar hqSNP difference ranges among isolates in a given cluster; use of different reference genomes showed minimal impacts on hqSNP differences identified between isolate pairs. Allelic differences obtained by wgMLST showed similar ranges as hqSNP differences among isolates in a given cluster; cgMLST consistently showed fewer differences than wgMLST. However, phylogenetic trees and dendrograms, obtained based on hqSNP and cg/wgMLST data, did show some incongruences, typically linked to clades supported by low bootstrap values in the trees. When a hqSNP cutoff was used to classify isolates as “related” or “unrelated,” use of different pipelines yielded a considerable number of discordances; this finding supports that cut-off values are valuable to provide a starting point for an investigation, but supporting and epidemiological evidence should be used to interpret WGS data. Overall, our data suggest that cgMLST-based data analyses provide for appropriate subtype differentiation and can be used without the need for preliminary data analyses (e.g., k-mer based clustering) or external closed reference genomes, simplifying data analyses needs. hqSNP or wgMLST analyses can be performed on the isolate clusters identified by cgMLST to increase the precision on determining the genomic similarity between isolates.

Genes significantly associated with lineage II food isolates of Listeria monocytogenes

Background

Listeria monocytogenes is a widespread foodborne pathogen that can cause listeriosis, a potentially fatal infection. L. monocytogenes is subdivided into four phylogenetic lineages, with the highest incidence of listeriosis occurring within lineage I followed by lineage II. Strains of L. monocytogenes differ in their phenotypic characteristics, including virulence. However, the genetic bases for these observed differences are not well understood, and current efforts to monitor L. monocytogenes in food consider all strains to be equally virulent. We use a comparative genomics approach to identify genes and single nucleotide polymorphisms (SNPs) in 174 clinical and food isolates of L. monocytogenes that potentially contribute to virulence or the capacity to adapt to food environments.

Results

No SNPs are significantly associated with food or clinical isolates. No genes are significantly associated with food or clinical isolates from lineage I, but eight genes consisting of multiple homologues are associated with lineage II food isolates. These include three genes which encode hypothetical proteins, the cadmium resistance genes cadA and cadC, the multi-drug resistance gene ebrB, a quaternary ammonium compound resistance gene qac, and a regulatory gene. All eight genes are plasmid-borne, and most closed L. monocytogenes plasmids carry at least five of the genes (24/27). In addition, plasmids are more frequently associated with lineage II food isolates than with lineage II clinical isolates.

Conclusions

We identify eight genes that are significantly associated with food isolates in lineage II. Interestingly, the eight genes are virtually absent in lineage II outbreak isolates, are composed of homologues which show a nonrandom distribution among lineage I serotypes, and the sequences are highly conserved across 27 closed Listeria plasmids. The functions of these genes should be explored further and will contribute to our understanding of how L. monocytogenes adapts to the host and food environments. Moreover, these genes may also be useful as markers for risk assessment models of either pathogenicity or the ability to proliferate in food and the food processing environment.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5074-2) contains supplementary material, which is available to authorized users.

Virulence Gene Sequencing Highlights Similarities and Differences in Sequences in Listeria monocytogenes Serotype 1/2a and 4b Strains of Clinical and Food Origin From 3 Different Geographic Locations

The prfA-virulence gene cluster (pVGC) is the main pathogenicity island in Listeria monocytogenes, comprising the prfA, plcA, hly, mpl, actA, and plcB genes. In this study, the pVGC of 36 L. monocytogenes isolates with respect to different serotypes (1/2a or 4b), geographical origin (Australia, Greece or Ireland) and isolation source (food-associated or clinical) was characterized. The most conserved genes were prfA and hly, with the lowest nucleotide diversity (π) among all genes (P < 0.05), and the lowest number of alleles, substitutions and non-synonymous substitutions for prfA. Conversely, the most diverse gene was actA, which presented the highest number of alleles (n = 20) and showed the highest nucleotide diversity. Grouping by serotype had a significantly lower π value (P < 0.0001) compared to isolation source or geographical origin, suggesting a distinct and well-defined unit compared to other groupings. Among all tested genes, only hly and mpl were those with lower nucleotide diversity in 1/2a serotype than 4b serotype, reflecting a high within-1/2a serotype divergence compared to 4b serotype. Geographical divergence was noted with respect to the hly gene, where serotype 4b Irish strains were distinct from Greek and Australian strains. Australian strains showed less diversity in plcB and mpl relative to Irish or Greek strains. Notable differences regarding sequence mutations were identified between food-associated and clinical isolates in prfA, actA, and plcB sequences. Overall, these results indicate that virulence genes follow different evolutionary pathways, which are affected by a strain's origin and serotype and may influence virulence and/or epidemiological dominance of certain subgroups.

Small molecules targeting LapB protein prevent Listeria attachment to catfish muscle

Listeria monocytogenes is a Gram-positive foodborne pathogen and the causative agent of listeriosis. L. monocytogenes lapB gene encodes a cell wall surface anchor protein, and mutation of this gene causes Listeria attenuation in mice. In this work, the potential role of Listeria LapB protein in catfish fillet attachment was investigated. To achieve this, boron-based small molecules designed to interfere with the active site of the L. monocytogenes LapB protein were developed, and their ability to prevent L. monocytogenes attachment to fish fillet was tested. Results indicated that seven out of nine different small molecules were effective in reducing the Listeria attachment to catfish fillets. Of these, three small molecules (SM3, SM5, and SM7) were highly effective in blocking Listeria attachment to catfish fillets. This study suggests an alternative strategy for reduction of L. monocytogenes contamination in fresh and frozen fish products.

Listeria monocytogenes in Smoked Salmon and Other Smoked Fish at Retail in Italy: Frequency of Contamination and Strain Characterization in Products from Different Manufacturers

Seven hundred seventy-eight samples of packaged smoked fish (774 smoked salmon and 4 smoked swordfish) on sale in Italy, from 50 different manufacturers located in 12 European Union countries, were purchased from the Italian market between May and December 2011. The surface temperatures of the samples on sale ranged from 0 to 13°C (3.4 ± 1.5°C, mean ± SD). Six hundred eighty (87.4%) of 778 samples were stored at ≤4°C. One hundred fifty-seven samples (20.2%, 95% confidence interval 17.5 to 23.1%) were contaminated by Listeria monocytogenes , with 26 samples (3.3%, 95% confidence interval 2.3 to 4.8%) at levels >100 CFU/g. The maximum level of contamination was 1.3 ×10⁶ CFU/g. The differences in the level of contamination of smoked fish between countries (χ² = 91.54, P < 0.05) and manufacturers (χ² = 193.22, P < 0.05) were significant. The frequency of detection for products from different manufacturing premises ranged from 0 to 76.9%. Serotyping by serological agglutination revealed that the main serotypes detected were 1/2a (65.3%) and 1/2b (22.4%). Pulsed-field gel electrophoresis typing with restriction enzymes AscI and ApaI yielded 36 pulsotypes from 144 isolates, clustering into 17 groups. Eight main pulsotypes accounted for 70.8% of the isolates. Three of the main pulsotypes were exclusively from products of a single manufacturer. In general, products from the same manufacturer showed genetic homogeneity, with one strongly prevalent pulsotype. Different manufacturers usually showed very different levels of contamination of the final product, confirming the importance of the management of process hygiene for controlling L. monocytogenes contamination.

Listeria monocytogenes Isolates Carrying Virulence-Attenuating Mutations in Internalin A Are Commonly Isolated from Ready-to-Eat Food Processing Plant and Retail Environments

Listeria monocytogenes is a human foodborne pathogen that may cause an invasive disease known as listeriosis in susceptible individuals. Internalin A (InlA; encoded by inlA) is a virulence factor that facilitates crossing of host cell barriers by L. monocytogenes . At least 19 single nucleotide polymorphisms (SNPs) in inlA that result in a premature stop codon (PMSC) have been described worldwide. SNPs leading to a PMSC in inlA have been shown to be causally associated with attenuated virulence. L. monocytogenes pathogens carrying virulence-attenuating (VA) mutations in inlA have been commonly isolated from ready-to-eat (RTE) foods but rarely have been associated with human disease. This study was conducted to determine the prevalence of VA SNPs in inlA among L. monocytogenes from environments associated with RTE food production and handling. More than 700 L. monocytogenes isolates from RTE food processing plant (n = 409) and retail (n = 319) environments were screened for the presence of VA SNPs in inlA. Overall, 26.4% of isolates from RTE food processing plant and 32.6% of isolates from retail environments carried a VA mutation in inlA. Food contact surfaces sampled at retail establishments were significantly (P < 0.0001) more likely to be contaminated by a L. monocytogenes isolate carrying a VA mutation in inlA (56% of 55 isolates) compared with nonfood contact surfaces (28% of 264 isolates). Overall, a significant proportion of L. monocytogenes isolated from RTE food production and handling environments have reduced virulence. These data will be useful in the revision of current and the development of future risk assessments that incorporate strain-specific virulence parameters.

Risk Factors for Sporadic Non-Pregnancy Associated Listeriosis in Germany-Immunocompromised Patients and Frequently Consumed Ready-To-Eat Products

Non-pregnancy associated (N-PA) listeriosis, caused by Listeria monocytogenes, is a rare but severe disease, and is predominantly food-borne. Most cases appear sporadic and their infection vehicle remains unknown. Incidence has increased since 2008 in Germany. We aimed to identify underlying conditions and foods associated with sporadic N-PA listeriosis in Germany. We performed a nationwide case-control study from March 2012-December 2013. Cases were sporadic N-PA listeriosis patients notified to public health. Control subjects were age (40-65 years, 66-75 years, ≥ 76 years) frequency-matched persons from a nationwide random telephone sample. A structured questionnaire collected information on underlying diseases, therapies and >60 food items. We conducted multivariable logistic regression analysis, adjusting for host factors identified by causal diagram theory, and calculated population attributable fractions. We enrolled 109 cases and 1982 controls. Cases' median age was 69 years, 55% were male, 44% received immunosuppressive therapy within 3 months prior to illness onset; a further 28% had at least one immunocompromising disease. In multivariable analysis, immunosuppressive therapy (OR 8.8, 95%CI 4.9-15.6), immunocompromising disease (OR 2.7; 95%CI 1.4-5.2), gastric acid suppression (OR 3.0; 95%CI 1.4-6.3), the consumption of cold cooked sausages (OR 2.6; 95%CI 1.6-4.4), the preferred consumption of packaged cheese (OR 2.1; 95%CI 1.3-3.5) and pre-sliced cheese (OR 2.2; 95%CI 1.3-3.7) were significantly associated with N-PA listeriosis. These foods accounted for 59% of all cases. Typical high risk foods, e.g. cold seafood, certain types of cheeses, tended to be negatively associated with disease. In conclusion, immunosuppressive therapy and frequently consumed ready-to-eat foods are the main risk factors for sporadic N-PA listeriosis in Germany. To reduce their risk, immunocompromised persons should consume the identified foods well before the 'use-by' date. The microbiological criteria for Listeria monocytogenes in ready-to-eat foods may insufficiently protect persons who are markedly immunocompromised.

Twenty Years of Listeria in Brazil: Occurrence of Listeria Species and Listeria monocytogenes Serovars in Food Samples in Brazil between 1990 and 2012

Listeria spp. isolated from different food products and collected from 12 Brazilian states were sent to the Laboratory of Bacterial Zoonoses (Oswaldo Cruz Institute, Brazil) for identification. The aims of this study were to characterize these isolates, from 1990 to 2012, by using biochemical, morphological, and serotyping tests, and to analyze the distribution of L. monocytogenes serotypes on different food products and geographical locations. Serotyping was performed using polyclonal somatic and flagellar antisera. Of 5953 isolates, 5770 were identified as Listeria spp., from which 3429 (59.4%) were L. innocua, 2248 (38.9%) were L. monocytogenes, and 93 (1.6%) were other Listeria spp. L. innocua was predominantly isolated from 1990 to 2000, while L. monocytogenes was from 2001 to 2012. Regarding the serotype distribution in the foods, serotypes 1/2a and 4b were most common in processed meat and ready-to-eat products, respectively; serotypes 1/2a, 1/2b, and 4b were the most common in nonprocessed meat. The results above confirm the presence of the main serotypes of L. monocytogenes in different parts of the food chain from three regions of the country and emphasize the importance of improving the control measures, as tolerance zero policy and microbiological surveillance in Brazil.

Prevalence and distribution of Listeria monocytogenes inlA alleles prone to phase variation and inlA alleles with premature stop codon mutations among human, food, animal, and environmental isolates

In Listeria monocytogenes, 18 mutations leading to premature stop codons (PMSCs) in the virulence gene inlA have been identified to date. While most of these mutations represent nucleotide substitutions, a frameshift deletion in a 5' seven-adenine homopolymeric tract (HT) in inlA has also been reported. This HT may play a role in phase variation and was first identified among L. monocytogenes lineage II ribotype DUP-1039C isolates. In order to better understand the distribution of different inlA mutations in this ribotype, a newly developed multiplex real-time PCR assay was used to screen 368 DUP-1039C isolates from human, animal, and food-associated sources for three known 5' inlA HT alleles: (i) wild-type (WT) (A7), (ii) frameshift (FS) (A6), and (iii) guanine interruption (A2GA4) alleles. Additionally, 228 DUP-1039C isolates were screened for all inlA PMSCs; data on the presence of all inlA PMSCs for the other 140 isolates were obtained from previous studies. The statistical analysis based on 191 epidemiologically unrelated strains showed that strains with inlA PMSC mutations (n = 41) were overrepresented among food-associated isolates, while strains encoding full-length InlA (n = 150) were overrepresented among isolates from farm animals and their environments. Furthermore, the A6 allele was overrepresented and the A7 allele was underrepresented among food isolates, while the A6 allele was underrepresented among farm and animal isolates. Our results indicate that genetic variation in inlA contributes to niche adaptation within the lineage II subtype DUP-1039C.

Genetic characteristics of Japanese clinical Listeria monocytogenes isolates

Listeria monocytogenes causes foodborne illnesses through consumption of ready-to-eat foods. Although 135-201annual listeriosis cases have been estimated in Japan, the details regarding the clinical isolates such as infection source, virulence level, and other genetic characteristics, are not known. In order to uncover the trends of listeriosis in Japan and use the knowledge for prevention measures to be taken, the genetic characteristics of the past human clinical isolates needs to be elucidated. For this purpose, multilocus tandem-repeat sequence analysis (MLTSA) and multi-virulence-locus sequence typing (MVLST) were used in this study. The clinical isolates showed a variety of genetically distant genotypes, indicating they were from sporadic cases. However, the MVLST profiles of 7 clinical isolates were identical to those of epidemic clone (EC) I isolates, which have caused several serious outbreaks in other countries, suggesting the possibility that they have strong virulence potential and originated from a single outbreak. Moreover, 6 Japanese food isolates shared their genotypes with ECI isolates, indicating that there may be risks for listeriosis outbreak in Japan. This is the first investigational study on genetic characteristics of Japanese listeriosis isolates. The listeriosis cases happened in the past are presumably sporadic, but it is still possible that some isolates with strong virulence potential have caused listeriosis outbreaks, and future listeriosis risks also exist.

Listeria monocytogenes strains encoding premature stop codons in inlA invade mice and guinea pig fetuses in orally dosed dams

Listeria monocytogenes is an important food-borne bacterial pathogen and listeriosis can result in abortions in pregnant women. The bacterium can colonize food-processing environments, where specific molecular subtypes can persist for years. The purpose of this study was to determine the virulence potential of a group of food-processing persistent L. monocytogenes strains encoding a premature stop codon in inlA (encoding internalin A) by using two orally dosed models, pregnant mice and pregnant guinea pigs. A food-processing persistent strain of L. monocytogenes invaded placentas (n = 58; 10 % positive) and fetuses (3 % positive) of pregnant mice (n = 9 animals per strain), similar to a genetically manipulated murinized strain, EGD-e InlA m* (n = 61; 3 and 2 %, respectively). In pregnant guinea pigs (n = 9 animals per bacterial strain), a maternofetal strain (from a human fetal clinical fatal case) was isolated from 34 % of placenta samples (n = 50), whereas both food-processing persistent strains were found in 5 % of placenta samples (n = 36 or 37). One of the food-processing persistent strains, N53-1, was found in up to 8 % of guinea pig fetal liver and brain samples, whereas the maternofetal control was found in 6 % of fetal tissue samples. As the food-processing persistent strains carry a premature stop codon in inlA but are invasive in orally dosed pregnant mice and guinea pigs, we hypothesize that listerial crossing of the placental barrier can occur by a mechanism that is independent of an interaction between E-cadherin and InlA.

Genetic characterization of Listeria monocytogenes isolates from food processing facilities before and after postcook chiller heat treatment

Possible selection for and establishment of stress-resistant Listeria monocytogenes variants as a consequence of heating interventions is of concern to the food industry. Lineage analysis and multilocus variable number tandem repeat analysis (MLVA) was performed on 20 L. monocytogenes isolates, of which 15 were obtained before and 5 were obtained after heat treatment of a postcook meat chiller. The ctsR gene (a class III heat shock gene regulator) from 14 isolates was amplified and sequenced because previous work has indicated that spontaneous mutations can occur in this gene during heat treatment. Heat treatment of the meat chiller did not significantly change the relative abundance of the various L. monocytogenes lineages; lineage II strains (less-heat-resistant isolates) dominated both before and after heat treatment. MLVA typing confirmed that some isolates of L. monocytogenes occur both before and after heat treatment of the chiller. No isolate of L. monocytogenes indicated any likely functionally significant mutations in ctsR. This study indicates the absence of any obvious difference in the profiles of L. monocytogenes strains obtained before and after heat treatment of a meat chiller, based on the characteristics examined. Although this finding supports the effectiveness of heat treatment, the limited number of strains used and characteristics examined mean that further study on a larger scale is required before firm conclusions can be drawn.

International Life Science Institute North America Cronobacter (Formerly Enterobacter sakazakii) isolate set

Foodborne pathogen isolate collections are important for the development of detection methods, for validation of intervention strategies, and to develop an understanding of pathogenesis and virulence. We have assembled a publicly available Cronobacter (formerly Enterobacter sakazakii) isolate set that consists of (i) 25 Cronobacter sakazakii isolates, (ii) two Cronobacter malonaticus isolates, (iii) one Cronobacter muytjensii isolate, which displays some atypical phenotypic characteristics, biochemical profiles, and colony color on selected differential media, and (iv) two nonclinical Enterobacter asburiae isolates, which show some phenotypic characteristics similar to those of Cronobacter spp. The set consists of human (n = 10), food (n = 11), and environmental (n = 9) isolates. Analysis of partial 16S rDNA sequence and seven-gene multilocus sequence typing data allowed for reliable identification of these isolates to species and identification of 14 isolates as sequence type 4, which had previously been shown to be the most common C. sakazakii sequence type associated with neonatal meningitis. Phenotypic characterization was carried out with API 20E and API 32E test strips and streaking on two selective chromogenic agars; isolates were also assessed for sorbitol fermentation and growth at 45°C. Although these strategies typically produced the same classification as sequence-based strategies, based on a panel of four biochemical tests, one C. sakazakii isolate yielded inconclusive data and one was classified as C. malonaticus. EcoRI automated ribotyping and pulsed-field gel electrophoresis (PFGE) with XbaI separated the set into 23 unique ribotypes and 30 unique PFGE types, respectively, indicating subtype diversity within the set. Subtype and source data for the collection are publicly available in the PathogenTracker database (www. pathogentracker. net), which allows for continuous updating of information on the set, including links to publications that include information on isolates from this collection.

Salt stress-induced transcription of σB- and CtsR-regulated genes in persistent and non-persistent Listeria monocytogenes strains from food processing plants

Listeria monocytogenes is a foodborne pathogen that can persist in food processing environments. Six persistent and six non-persistent strains from fish processing plants and one persistent strain from a meat plant were selected to determine if expression of genes in the regulons of two stress response regulators, σ(B) and CtsR, under salt stress conditions is associated with the ability of L. monocytogenes to persist in food processing environments. Subtype data were also used to categorize the strains into genetic lineages I or II. Quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) was used to measure transcript levels for two σ(B)-regulated genes, inlA and gadD3, and two CtsR-regulated genes, lmo1138 and clpB, before and after (t=10 min) salt shock (i.e., exposure of exponential phase cells to BHI+6% NaCl for 10 min at 37°C). Exposure to salt stress induced higher transcript levels relative to levels under non-stress conditions for all four stress and virulence genes across all wildtype strains tested. Analysis of variance (ANOVA) of induction data revealed that transcript levels for one gene (clpB) were induced at significantly higher levels in non-persistent strains compared to persistent strains (p=0.020; two-way ANOVA). Significantly higher transcript levels of gadD3 (p=0.024; two-way ANOVA) and clpB (p=0.053; two-way ANOVA) were observed after salt shock in lineage I strains compared to lineage II strains. No clear association between stress gene transcript levels and persistence was detected. Our data are consistent with an emerging model that proposes that establishment of L. monocytogenes persistence in a specific environment occurs as a random, stochastic event, rather than as a consequence of specific bacterial strain characteristics.

Recombination and positive selection contributed to the evolution of Listeria monocytogenes lineages III and IV, two distinct and well supported uncommon L. monocytogenes lineages

Listeriamonocytogenes lineages III and IV represent two uncommon lineages of the human and animal pathogen L. monocytogenes, characterized by occurrence of unusual phenotypic and genetic characteristics that differentiate them from the common lineages I and II. To gain further insights into the evolution of lineages III and IV, we amplified and sequenced housekeeping genes (i.e., gap, prs, purM, ribC, and sigB), internalin genes (i.e., inlA, inlB, inlC, inlG, inlC2, inlD, inlE, inlF, and inlH) and the virulence gene cluster containing prfA, plcA, hly, mpl, actA, and plcB for lineages III (n = 7) and IV (n = 4) isolates. Phylogenetic analyses of the sequences obtained along with previously reported sequence data for 40 isolates representing lineages I (n = 18), II (n = 21), and III (n = 1), showed that lineages III and IV represent divergent and monophyletic lineages. The virulence gene cluster as well as the inlAB operon were present in all isolates, with inlF absent from all lineages III and IV isolates. While all lineage IV isolates contained only inlC (in addition to inlAB), lineage III isolates showed considerable diversity with regard to internalin gene presence, including presence of (i) only inlC (n = 2), (ii) inlC and inlGC2DE (n = 3), (iii) only inlGC2DE (n = 2), and (iv) inlC and inlC2DE (n = 1). In addition to evidence for horizontal gene transfer events, among lineages III and IV isolates, in prs, actA, plcB, mpl, inlA, inlB, inlG, inlD, and inlE, we also found significant evidence for positive selection in the hly promoter region and, along the lineages III and IV branches, for actA (including in sites recognized for interactions with proteins involved in actin tail polymerization). In conclusion, lineages III and IV represent two distinct monophyletic groups with contributions of intragenic recombination to the evolution of their internalin genes as well as contributions of positive selection to evolution of the virulence genes island.

Characterization of Listeria monocytogenes isolates of food and human origins from Brazil using molecular typing procedures and in vitro cell culture assays

The spreading of diseases through foods is a worldwide concern. Here, molecular and in vitro cell-culture assays were employed to characterize 63 Brazilian Listeria monocytogenes isolates (food, 47; clinical, 16). Serotype 4b was the most predominant (49%) followed by (1/2)b (30%), (1/2)a (10%), (1/2)c (6%), 3c (3%) and 3b (2%). Ribotyping yielded 17 ribopatterns, which were grouped into four phylogenetic clusters. Cluster A comprised of 39/63 isolates primarily of food origin, and clusters B, C and D contained both food and clinical isolates. Isolates were positive for virulence determinants prfA, hlyA and inlA: clinical isolates were more invasive to Caco-2 cells and expressed high levels of inlA transcripts than the food isolates. Highly invasive isolates also provoked more Ped-2E9 cells to die by apoptosis than the weakly-invasive strains. These data demonstrate a strong genetic relatedness among clinical and food isolates and suggest transmission of a subset of L. monocytogenes strains from food to humans.

Comparative proteomic analysis of Listeria monocytogenes strains F2365 and EGD

Listeria monocytogenes is a gram-positive, food-borne pathogen that causes disease in both humans and animals. There are three major genetic lineages of L. monocytogenes and 13 serovars. To further our understanding of the differences that exist between different genetic lineages/serovars of L. monocytogenes, we analyzed the global protein expression of the serotype 1/2a strain EGD and the serotype 4b strain F2365 during early-stationary-phase growth at 37 degrees C. Using multidimensional protein identification technology with electrospray ionization tandem mass spectrometry, we identified 1,754 proteins from EGD and 1,427 proteins from F2365, of which 1,077 were common to both. Analysis of proteins that had significantly altered expression between strains revealed potential biological differences between these two L. monocytogenes strains. In particular, the strains differed in expression of proteins involved in cell wall physiology and flagellar biosynthesis, as well as DNA repair proteins and stress response proteins.

Genome-wide analyses reveal lineage specific contributions of positive selection and recombination to the evolution of Listeria monocytogenes

Background

The genus Listeria includes two closely related pathogenic and non-pathogenic species, L. monocytogenes and L. innocua. L. monocytogenes is an opportunistic human foodborne and animal pathogen that includes two common lineages. While lineage I is more commonly found among human listeriosis cases, lineage II appears to be overrepresented among isolates from foods and environmental sources. This study used the genome sequences for one L. innocua strain and four L. monocytogenes strains representing lineages I and II, to characterize the contributions of positive selection and recombination to the evolution of the L. innocua/L. monocytogenes core genome.

Results

Among the 2267 genes in the L. monocytogenes/L. innocua core genome, 1097 genes showed evidence for recombination and 36 genes showed evidence for positive selection. Positive selection was strongly associated with recombination. Specifically, 29 of the 36 genes under positive selection also showed evidence for recombination. Recombination was more common among isolates in lineage II than lineage I; this trend was confirmed by sequencing five genes in a larger isolate set. Positive selection was more abundant in the ancestral branch of lineage II (20 genes) as compared to the ancestral branch of lineage I (9 genes). Additional genes under positive selection were identified in the branch separating the two species; for this branch, genes in the role category "Cell wall and membrane biogenesis" were significantly more likely to have evidence for positive selection. Positive selection of three genes was confirmed in a larger isolate set, which also revealed occurrence of multiple premature stop codons in one positively selected gene involved in flagellar motility (flaR).

Conclusion

While recombination and positive selection both contribute to evolution of L. monocytogenes, the relative contributions of these evolutionary forces seem to differ by L. monocytogenes lineages and appear to be more important in the evolution of lineage II, which seems to be found in a broader range of environments, as compared to the apparently more host adapted lineage I. Diversification of cell wall and membrane biogenesis and motility-related genes may play a particularly important role in the evolution of L. monocytogenes.

Lineage specific recombination and positive selection in coding and intragenic regions contributed to evolution of the main Listeria monocytogenes virulence gene cluster

The major virulence cluster of Listeria monocytogenes harbors six virulence genes that encode proteins critical for the intracellular life cycle of this human and animal pathogen. In this study, we determined the sequence (8709nt) of the virulence gene cluster (including the six main virulence genes) in 40 L. monocytogenes isolates from different source populations (human clinical cases, animal clinical cases, foods, and natural environments). An alignment of the full length cluster as well as individual gene alignments and alignments of intragenic regions were used for phylogenetic, recombination, and positive selection analyses. Initial phylogenetic analyses showed that the sequences represented two main clusters, consistent with previously defined L. monocytogenes phylogenetic lineages. The 40 sequences represented 25 distinct allelic types and the overall alignment included 592 polymorphic sites. Overall, our data show that (i) virulence genes in the main L. monocytogenes virulence gene cluster include highly conserved genes (i.e., hly and prfA) as well as diverse genes that appear to have evolved by positive selection (mpl, actA, and plcA), (ii) recombination has played an important role in the evolution of the virulence gene cluster, but is limited to lineage II isolates, and (iii) the promoter region driving the transcription of virulence genes transcribed early in intracellular infection (i.e., hly and plcA) has evolved by positive selection. The genes and intragenic regions in the L. monocytogenes virulence gene cluster thus have evolved independently, despite their close physical linkage, likely reflecting distinct selective pressures associated with expression and function of the proteins encoded in this region.

Distribution of internalin gene profiles of Listeria monocytogenes isolates from different sources associated with phylogenetic lineages

Listeria monocytogenes is a human foodborne pathogen with a broad range of hosts. While the L. monocytogenes genome encodes a number of internalins, which are leucine-rich repeat bacterial surface proteins with putative or confirmed roles in host cell attachment and invasion, the specific function of many internalins remains unclear. The distribution of 7 internalin genes (inlC, inlC2, inlD, inlE, inlF, inlG, and inlH) in 120 L. monocytogenes isolates from humans, foods, and animals was investigated to determine if the distribution of these proteins differed with respect to source or phylogenetic lineage. Isolates were classified into 6 different profiles based on internalin gene presence or absence, and a phylogeny based on one stress response (sigB) and two housekeeping (gap and prs) genes was used to correlate these profiles with L. monocytogenes phylogenetic lineages. All 69 isolates classified into L. monocytogenes lineage I, which is overrepresented among human disease cases, had the same internalin profile (presence of inlC and the inlC2DE operon). Lineage II (48 isolates), which is common among food and environmental sources, represented 4 internalin gene profiles, with most isolates carrying inlG and inlF in addition to inlC and inlC2DE. Our data indicate that L. monocytogenes isolates show diverse and distinct patterns of internalin gene presence/absence and L. monocytogenes internalin profiles are associated with phylogenetic lineages.

Culture independent analysis of ileal mucosa reveals a selective increase in invasive Escherichia coli of novel phylogeny relative to depletion of Clostridiales in Crohn's disease involving the ileum

Intestinal bacteria are implicated increasingly as a pivotal factor in the development of Crohn's disease, but the specific components of the complex polymicrobial enteric environment driving the inflammatory response are unresolved. This study addresses the role of the ileal mucosa-associated microflora in Crohn's disease. A combination of culture-independent analysis of bacterial diversity (16S rDNA library analysis, quantitative PCR and fluorescence in situ hybridization) and molecular characterization of cultured bacteria was used to examine the ileal mucosa-associated flora of patients with Crohn's disease involving the ileum (13), Crohn's disease restricted to the colon (CCD) (8) and healthy individuals (7). Analysis of 16S rDNA libraries constructed from ileal mucosa yielded nine clades that segregated according to their origin (P<0.0001). 16S rDNA libraries of ileitis mucosa were enriched in sequences for Escherichia coli (P<0.001), but relatively depleted in a subset of Clostridiales (P<0.05). PCR of mucosal DNA was negative for Mycobacterium avium subspecies paratuberculosis, Shigella and Listeria. The number of E. coli in situ correlated with the severity of ileal disease (rho 0.621, P<0.001) and invasive E. coli was restricted to inflamed mucosa. E. coli strains isolated from the ileum were predominantly novel in phylogeny, displayed pathogen-like behavior in vitro and harbored chromosomal and episomal elements similar to those described in extraintestinal pathogenic E. coli and pathogenic Enterobacteriaceae. These data establish that dysbiosis of the ileal mucosa-associated flora correlates with an ileal Crohn's disease (ICD) phenotype, and raise the possibility that a selective increase in a novel group of invasive E. coli is involved in the etiopathogenesis to Crohn's disease involving the ileum.

Sodium chloride enhances adherence and aggregation and strain variation influences invasiveness of Listeria monocytogenes strains

Some subtypes of Listeria monocytogenes can persist in the food-processing industry, but the reasons for such persistence are not known. In the present study, 10 strains of L. monocytogenes representing known persistent randomly amplified polymorphic DNA (RAPD) types from fish processing plants were compared to eight strains of different RAPD type and origin (clinical, food, and animal). All 18 strains of L. monocytogenes had similar growth patterns at different temperatures (5 or 37 degrees C) or different salinities (0.5 or 5% NaCl), and all strains formed a thin layer of adhered cells on a plastic surface when cultured in tryptone soya broth (TSB) with a total of 1% glucose. Many ready-to-eat foods, such as cold-smoked fish, contain NaCl at concentrations of 2 to 5%, and NaCl is present in the processing environment. Adding NaCl to TSB changed the adhesion patterns of all strains, and all adhered better when NaCl was added. Also, the addition of NaCl caused a marked aggregation of 13 of the strains; however, 5 of the 18 strains did not aggregate in the presence of up to 5% NaCl. The aggregates stuck to the plastic surface, and this occurred in all but one of the persistent RAPD types. Four strains represented one particular RAPD type that has been isolated as a persistent RAPD type in several fish processing plants for up to 10 years. Because this RAPD type often can contaminate fish products, it is important to address its potential virulence. The 18 strains differed markedly in their ability to invade Caco-2 cells, and the four strains representing the universal persistent RAPD type were the least invasive (10(2) to 10(3) CFU/ml), whereas other strains invaded Caco-2 cells at levels of 10(4) to 10(5) CFU/ml. Five of the 18 strains belonged to the genetic lineage 1 and were the most invasive. Although the most commonly isolated persistent RAPD type was low invasive, it is important to understand why moderate salinity facilitates aggregation and biofilm formation, for this understanding can be beneficial in developing procedures to reduce processing plant contamination.

Pulsed-field gel electrophoresis (PFGE) analysis of temporally matched Listeria monocytogenes isolates from human clinical cases, foods, ruminant farms, and urban and natural environments reveals source-associated as well as widely distributed PFGE types

A total of 495 temporally and geographically matched Listeria monocytogenes isolates from human clinical cases, foods, ruminant farms, and urban and natural environments were used to investigate L. monocytogenes pulsed-field gel electrophoresis (PFGE) type diversity. Two-enzyme (AscI and ApaI) PFGE discriminated 310 PFGE types and exhibited higher overall discriminatory power (Simpson's index of discrimination [D] = 0.995) than either EcoRI ribotyping (D = 0.950) or AscI or ApaI single-enzyme PFGE (D = 0.992 for both). Seven PFGE types showed significant associations with specific sources, including one and four PFGE types, respectively, associated with human clinical cases and foods. Spatial analysis of 13 PFGE types occurring >5 times showed that two PFGE types were specific to a single processing facility each, where they appear to have persisted over time. Nine PFGE types were geographically widespread and occurred among isolates from multiple sources. For example, a PFGE type that matched isolates from listeriosis outbreaks in Los Angeles and Switzerland occurred among isolates from farms (n = 7), human clinical cases (n = 4), environmental sources (n = 3), and foods (n = 1). Our data indicate that (i) PFGE is highly discriminatory for the subtyping of L. monocytogenes, (ii) some L. monocytogenes PFGE types are associated with specific sources, and (iii) some L. monocytogenes PFGE types are widely distributed and appear to be stable and pandemic. Large PFGE type databases representing isolates from different sources are thus needed to appropriately interpret subtype data in epidemiological investigations and to identify common as well as source-specific PFGE types.

International Life Sciences Institute North America Listeria monocytogenes strain collection: development of standard Listeria monocytogenes strain sets for research and validation studies

Research and development efforts on bacterial foodborne pathogens, including the development of novel detection and subtyping methods, as well as validation studies for intervention strategies can greatly be enhanced through the availability and use of standardized strain collections. These types of strain collections are available for some foodborne pathogens, such as Salmonella and Escherichia coli. We have developed a standard Listeria monocytogenes strain collection that has not been previously available. The strain collection includes (i) a diversity set of 25 isolates chosen to represent a genetically diverse set of L. monocytogenes isolates as well as a single hemolytic Listeria innocua strain and (ii) an outbreak set, which includes 21 human and food isolates from nine major human listeriosis outbreaks that occurred between 1981 and 2002. The diversity set represents all three genetic L. monocytogenes lineages (I, n = 9; II, n = 9; and III, n = 6) as well as nine different serotypes. Molecular subtyping by EcoRI automated ribotyping and pulsed-field gel electrophoresis (PFGE) with AscI and ApaI separated the 25 isolates in the diversity set into 23 ribotypes and 25 PFGE types, confirming that this isolate set represents considerable genetic diversity. Molecular subtyping of isolates in the outbreak set confirmed that human and food isolates were identical by ribotype and PFGE, except for human and food isolates for two outbreaks, which displayed related but distinct PFGE patterns. Subtype and source data for all isolates in this strain collection are available on the Internet and are linked to the PathogenTracker database (www.pathogentracker.com), which allows the addition of new, relevant information on these isolates, including links to publications that have used isolates from this collection. We have thus developed a core L. monocytogenes strain collection, which will provide a resource for L. monocytogenes research and development efforts with centralized Internet-based data curation and integration.

Novel method to identify source-associated phylogenetic clustering shows that Listeria monocytogenes includes niche-adapted clonal groups with distinct ecological preferences

While phylogenetic and cluster analyses are often used to define clonal groups within bacterial species, the identification of clonal groups that are associated with specific ecological niches or host species remains a challenge. We used Listeria monocytogenes, which causes invasive disease in humans and different animal species and which can be isolated from a number of environments including food, as a model organism to develop and implement a two-step statistical approach to the identification of phylogenetic clades that are significantly associated with different source populations, including humans, animals, and food. If the null hypothesis that the genetic distances for isolates within and between source populations are identical can be rejected (SourceCluster test), then particular clades in the phylogenetic tree with significant overrepresentation of sequences from a given source population are identified (TreeStats test). Analysis of sequence data for 120 L. monocytogenes isolates revealed evidence of clustering between isolates from the same source, based on the phylogenies inferred from actA and inlA (P = 0.02 and P = 0.07, respectively; SourceCluster test). Overall, the TreeStats test identified 10 clades with significant (P < 0.05) or marginally significant (P < 0.10) associations with defined sources, including human-, animal-, and food-associated clusters. Epidemiological and virulence phenotype data supported the fact that the source-associated clonal groups identified here are biologically valid. Overall, our data show that (i) the SourceCluster and TreeStats tests can identify biologically meaningful source-associated phylogenetic clusters and (ii) L. monocytogenes includes clonal groups that have adapted to infect specific host species or colonize nonhost environments.

Virulence of Listeria monocytogenes isolates from humans and smoked salmon, peeled shrimp, and their processing environments

The virulence of 82 Listeria monocytogenes isolates from human cases and cold-smoked salmon, cooked peeled shrimp, and their production environments was assessed using the plaque-forming assay and a subcutaneous inoculation test in mice. These isolates were previously typed using serotyping and pulsed-field gel electrophoresis. The isolates from food-production environments were collected in several surveys over the period of 5 years. Sixty-eight (99.8%) of 69 isolates tested from food and food-processing environments were considered virulent while only one was avirulent. All clinical isolates (13) were highly virulent. The isolates were from raw materials, final products, and the production environment. This stresses the importance of hygiene in the processing environment as well as among personnel to avoid contamination of the final product.

Molecular epidemiology and cluster analysis of human listeriosis cases in three U.S. states

To better understand the transmission and epidemiology of human listeriosis, 647 Listeria monocytogenes isolates obtained from human listeriosis cases in four U.S. locations (Michigan, Ohio, New York State, and New York City) over 61 months (1998 to 2003) were characterized by automated EcoRI ribotyping. A total of 65 ribotypes were differentiated among the characterized isolates; 393, 227, and 24 isolates were classified into lineages I, II, and III, respectively, and 3 isolates were not classified to lineage. The three most common ribotypes (responsible for 39% of all cases) represented L. monocytogenes epidemic clones, each of which had previously been linked to at least two human listeriosis outbreaks. Categorical analyses revealed that ribotypes and lineages were nonrandomly distributed among the four locations. Temporal cluster analysis of cases identified 13 statistically significant temporal subtype clusters, which represented 26% of all cases. Three of these clusters matched previously described human listeriosis outbreaks. Isolates involved in clusters belonged to nine ribotypes. Four, eight, and one cluster were caused by lineages I, II, and III, respectively. The two largest clusters were both caused by the epidemic clone representing ribotype DUP-1044A. Categorical analyses revealed no significant associations between lineage or ribotype and clinical manifestation (central nervous system infection, septicemia, fetal infection, or other infection) or disease outcome (fatal or not fatal). Although human listeriosis cases are caused by isolates belonging to a diversity of EcoRI ribotypes, specific lineage I epidemic clones cause a large number of human listeriosis cases. Many human listeriosis cases can be grouped into statistically significant temporal clusters, including widely distributed and region-specific clusters associated with isolates of various ribotypes. L. monocytogenes lineages and EcoRI ribotypes do not appear to differ in their likelihood of causing different clinical manifestations or mortality.

Contamination of cooked peeled shrimp (Pandalus borealis) by Listeria monocytogenes during processing at two processing plants

Listeria spp. and Listeria monocytogenes contamination was evaluated in cooked peeled shrimp (final or semifinal product, 82 samples) and the shrimp-processing environment (two plants, 613 samples) in eight surveys conducted from 1998 through 2001. Listeria was detected in 12.5% (78) of the 695 samples (11.2% of the samples were positive for L. monocytogenes), but none of the samples of final product contained Listeria. One hundred seventy-two L. monocytogenes isolates were characterized by pulsed-field gel electrophoresis. Cleavage with macrorestriction enzymes AscI and ApaI yielded 14 different pulsotypes in the plants; two types were dominant, one in each plant. Sixty-three of the 106 isolates in plant A and 43 of the 66 isolates in plant B were of the dominant types. Certain strains, mainly of serotypes 1/2c and 4b and pulsotypes 1A and 2H, were persistent for long periods in both plants. Adaptation of good hygienic practices in the processing plants, including strict rules concerning traffic of staff and equipment, and existing hygienic requirements appeared to be effective in preventing contamination between areas within plants and in the final product. The persistence of Listeria strains in these two processing plants indicates the importance of detecting the places in the processing environment (e.g., transporters, equipment, floors, and drains) where L. monocytogenes can survive so that cleaning and disinfection efforts can be directed to such niches.

Bacteriocin activity by Lactobacillus curvatus CWBI-B28 to inactivate Listeria monocytogenes in cold-smoked salmon during 4 degrees C storage

The inhibition effectiveness of a bacteriocin produced by Lactobacillus curvatus CWBI-B28 against Listeria monocytogenes was investigated in cold-smoked salmon during storage at 4 degrees C. Three bacteriocin-based strategies for the control of L. monocytogenes in foods (i.e., producing bacteriocin in situ, spraying with partially purified bacteriocin, and packaging in bacteriocin-coated plastic film), plus a newly developed method that uses cell-adsorbed bacteriocin (i.e., a suspension of producer cells on which maximum bacteriocin has been immobilized by pH adjustments), were assessed. Although all the approaches inactivated L. monocytogenes in cold-smoked salmon, various efficacy levels were observed. The behavior of L. monocytogenes was similar in samples treated with either partially purified bacteriocin or in situ bacteriocin production. In both of these cases, the counts of the pathogen declined to below the detectable limit of 0.7 log CFU/cm2 within the first week, but a approximately 0.95- and 1.3-log increase, respectively, occurred after day 14. The bioactive packaging film resulted in a slower inactivation of the pathogen but prevented any subsequent increase in the CFU throughout 22 days of storage at 4 degrees C. Application of the cell-adsorbed bacteriocin was shown to be the most effective means, as it resulted in a complete inactivation of the pathogen within 3 days, and no increase in Listeria counts occurred up to 22 days.

Associations amongListeria monocytogenesgenotypes and distinct clinical manifestations of listeriosis in cattle

OBJECTIVE

To determine whether specific strains of Listeria monocytogenes, as determined by genetic characteristics and virulence phenotypes, were associated with distinct clinical manifestations of listeriosis in cattle and thus may potentially have tissue specificity.

ANIMALS

32 cattle.

PROCEDURE

DNA sequence data for the virulence genes actAand inlAwere used to infer the phylogeny of L. monocytogenes and to test for positive selection. Isolates were screened for the presence or absence of internalin genes and assigned an internalin profile. Plaquing assays were performed to determine the relative cytopathogenicity of each isolate. Categorical data analyses were performed to describe associations among L. monocytogenes genotypes, virulence phenotypes, and clinical manifestations of listeriosis.

RESULTS

Results confirmed that L. monocytogenes represents 2 deeply separated evolutionary lineages. Genes actA and inlA contained amino acid sites under positive selection, and specific residues at some sites were associated with lineage and manifestation of listeriosis. Whereas lineage I was clonal and predominantly composed of isolates from cases of encephalitis, lineage II was more genetically diverse and equally represented by isolates from cases of encephalitis versus septicemia and fetal infection. Lineage I isolates also had greater cytopathogenicity in vitro, compared with lineage II isolates.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that L. monocytogenes virulence genes underwent positive selection that is consistent with the diversification of 2 evolutionary lineages: lineage I is clonal and associated with encephalitis, and lineage II is more genetically diverse and equally likely to cause both major forms of listeriosis in cattle.