Pathogenicity and production of virulence factors by Listeria monocytogenes isolates from channel catfish

Profiling and source tracking of the microbial populations and resistome present in fish products

Microorganisms in processing environments significantly impact the quality and safety of food products and can serve as potential reservoirs for antibiotic-resistant genes, contributing to public health concerns about antimicrobial resistance (AMR). Fish processing plants represent an understudied environment for microbiome mapping. This study investigated the microbial composition, prevalence of Listeria spp., and resistome structures in three catfish processing facilities in the southeastern United States. The 16S rRNA gene sequencing revealed that the observed richness and Shannon diversity index increased significantly from fish to fillet. Beta diversity analysis showed distinct clustering of microbial communities between fish, environment, and fillet samples. Fast expectation-maximization microbial source tracking (FEAST) algorithm demonstrated that the microbiota presents in the processing environment contributed 48.2 %, 62.4 %, and 53.7 % to the microbiota present on fillet in Facility 1 (F1), F2, and F3, respectively. Food contact surfaces made larger contributions compared to the non-food contact surfaces. The linear discriminant analysis of effect size (LEfSe) identified specific microbial genera (e.g., Plesiomohas, Brochothrix, Chryseobacterium and Cetobacterium) that significantly varied between Listeria spp. positive and negative samples in all three processing plants. The metagenomic sequencing results identified 212 antimicrobial resistance genes (ARGs) belonging to 72 groups from the raw fish and fish fillet samples collected from three processing plants. Although there was a significant decrease in the overall diversity of ARGs from fish to fillet samples, the total abundance of ARGs did not change significantly (P > 0.05). ARGs associated with resistance to macrolide-lincosamide-streptogramin (MLS), cationic antimicrobial peptides, aminoglycosides, and beta-lactams were found to be enriched in the fillet samples when compared to fish samples. Results of this study highlight the profound impact of processing environment on shaping the microbial populations present on the final fish product and the need for additional strategies to mitigate AMR in fish products.

The Production of Listeriolysin O and Subsequent Intracellular Infections by Listeria monocytogenes Are Regulated by Exogenous Short Chain Fatty Acid Mixtures

Listeria monocytogenes is a foodborne pathogen capable of secreting listeriolysin O (LLO), a pore-forming toxin encoded by the hly gene. While the functions of LLO have been studied extensively, how the production of LLO is modulated by the intestinal environment, devoid of oxygen and enriched in short chain fatty acids (SCFAs), is not completely understood. Using L. monocytogenes strain 10403s, we found that hly transcription was moderately decreased by aerobic SCFA exposures but significantly increased by anaerobic SCFA exposures. Moreover, aerobic, but not anaerobic, exposure to low levels of SCFAs resulted in a significantly higher LLO activity. These results demonstrated that transcriptional and post-transcriptional regulations of LLO production were separately modulated by SCFAs and were responsive to oxygen levels. Examining isogenic mutants revealed that PrfA and SigB play a role in regulating LLO production in response to SCFAs. Effects of SCFAs were also present in the cardiotropic strain 07PF0776 but distinctly different from those in strain 10403s. For both strains, prior exposures to SCFAs altered intracellular infections in Caco-2 and RAW264.7 cells and the plaque sizes in L fibroblasts, a result confirming the ability of L. monocytogenes to adapt to SCFAs in ways that impact its subsequent infection outcomes.

Influence of pH on bile sensitivity amongst various strains of Listeria monocytogenes under aerobic and anaerobic conditions

Listeria monocytogenes is a dangerous bacterium that causes the food-borne disease listeriosis and accounts for nearly 20 % of food-borne deaths. This organism can survive the body's natural defences within the digestive tract, including acidic conditions and bile. Although the bile response has been analysed, limited information is available concerning the ability of L. monocytogenes to resist bile under anaerobic conditions, especially at acidic pH, which mimics conditions within the duodenum. Additionally, it is not known how the bile response varies between serotypes. In this study, the survival of strains representing six serotypes was analysed under aerobic and anaerobic conditions following exposure to bile. Exposure to bile salts at acidic pH increased toxicity of bile, resulting in a significant reduction in survival for all strains tested. However, following this initial reduction, no significant reduction was observed for an additional 2 h except for strain 10403S (P = 0.002). Anaerobic cultivation increased bile resistance, but a significant increase was only observed in virulent strains when exposed to bile at pH 5.5. Exposure to pH 3.0 prior to bile decreased viability amongst avirulent strains in bile in acidic conditions; oxygen availability did not influence viability. Together, the data suggested that being able to sense and respond to oxygen availability may influence the expression of stress response mechanisms, and this response may correspond to disease outcome. Further research is needed on additional strains to determine how L. monocytogenes senses and responds to oxygen and how this varies between invasive and non-invasive strains.

Genotypic analyses and virulence characterization of Listeria monocytogenes isolates from crayfish (Procambarus clarkii)

Listeria monocytogenes is a foodborne pathogen that can cause invasive illness in humans and farm animals. It is frequently isolated from dairy products and poultry. However, there have been few literatures on the genetic diversity and virulence potential of L. monocytogenes from freshwater animal. Thirty-nine L. monocytogenes strains from crayfish were isolated and identified in this study. Molecular subtyping and polymorphism of each isolate were analyzed by multilocus sequence typing (MLST). MLST divided the isolates into eight sequence types (STs), six of which from crayfish were the same with the isolates from environment and clinic. PCR detection of the eight genes related to virulence and multiplex PCR for serotyping showed that the eight virulence factors were present in the isolates and all the isolates belonged to four major L. monocytogenes serotype groups (1/2a, 1/2b, 1/2c, and 4b) frequently isolated from patients. In vivo pathogenicity of isolates was also evaluated in murine model and survival curve of infected mice suggested that ST1, ST4, and ST9 isolates were as virulent as the reference strain EGDe. This study provides preliminary insights into the genetic diversity of L. monocytogenes from crayfish and the genetic correlation between crayfish and clinical L. monocytogenes isolates. The results indicate the contamination in aquaculture could be the source of Listeria contamination and the isolates are likely to cause human listeriosis.

Use of used vs. fresh cheese brines and the effect of pH and salt concentration on the survival of Listeria monocytogenes

The aim of the study was to investigate how the use of fresh cheese brines compared with used brines and various combinations of pH and NaCl concentrations affected the survival of Listeria monocytogenes. Cheese brines from five Norwegian small scale cheese producers were analysed and showed great variations in pH (4·54–6·01) and NaCl concentrations (14·1–26·9 %). The survival of five strains of List. monocytogenes (two clinical isolates, two food isolates and one animal isolate) in four different cheese brines (three used and one fresh) was investigated. Results showed significant differences in survival both depending on the strains and the brines. Strains of human outbreak listeriosis cases showed greater ability to survive in the brines compared with food isolates and a List. monocytogenes reference strain (1–2 log10 difference after 200 d). All strains showed highest survival in the freshly prepared brine compared with the used brines. Molecular typing by multiple locus variable number tandem repeats analysis (MLVA) showed that there were no detectable alterations in the examined variable number tandem repeats of the genome in five strains after 200 d storage in any of the salt brines. Combined effects of pH (4·5, 5·25 and 6·0) and NaCl (15, 20 and 25 %) in fresh, filter sterilised brines on the survival of List. monocytogenes were examined and results showed that pathogen populations decreased over time in all brines. Death rates at any given NaCl concentration were highest at low pH (4·5) and death rates at any given pH were highest at low NaCl concentrations (15 %). In conclusion, the use of used brines reduced the survival of List. monocytogenes and a combination of low pH (4·5) and low salt concentrations (15 %) decreased the risk of List. monocytogenes survival compared with higher pH (5·25 or 6·0) and higher NaCl concentrations (20 or 25 %).

Prevalence, pathogenic capability, virulence genes, biofilm formation, and antibiotic resistance of Listeria in goat and sheep milk confirms need of hygienic milking conditions

Goat and sheep milk is consumed by human populations throughout the world; as a result, it has been proposed as an alternative, nutrient-rich milk to feed infants allergic to cow's milk. Unfortunately, potentially harmful bacteria have not been thoroughly tested in goat or sheep milk. Listeria monocytogenes is a harmful bacterium that causes adverse health effects if ingested by humans. The purpose of this study was to estimate the prevalence and characterize the phenotype, genotype, virulence factors, biofilm formation, and antibiopotential of Listeria isolated from the milk of goat and sheep. Udder milk samples were collected from 107 goats and 102 sheep and screened for mastitis using the California mastitis test (CMT). Samples were then examined for the presence of pathogenic Listeria spp; if detected, the isolation of pathogenic Listeria (L. monocytogenes and Listeria ivanovii) was completed using isolation and identification techniques recommended by the International Organization for Standards (ISO 11290-1, 1996), in addition to serological, in vitro and in vivo pathogenicity tests. The isolates were subjected to PCR assay for virulence associated genes (hlyA, plcA, actA, and iap). Pathogenic Listeria spp. were isolated from 5·6% of goat and 3·9% sheep milk samples, with 33·3 and 25% of these selected samples respectively containing L. monocytogenes. The results of this study provide evidence of the low-likelihood of contamination leading to the presence of L. monocytogenes in raw goat and sheep milk; however, this study also confirmed a strong in vitro ability for biofilm formation and pathogenic capability of L. monocytogenes if discovered in the milk. L. monocytogenes may be present in goat and sheep milk and in order to reduce the exposure, hygienic milking conditions must be employed for the milk to be considered a safe alternative for human consumption.

Clinical, pathological, and genetic characterization of Listeria monocytogenes causing sepsis and necrotizing typhlocolitis and hepatitis in a foal

Listeria monocytogenes was isolated from the blood, lungs, and liver of a 5-week-old American Quarter Horse filly that presented with a 2-day history of fever, lethargy, ataxia, and seizure activity. The foal was born on a well-managed breeding facility to a multiparous mare with no periparturient complications. At 8 hr of age, the foal had an adequate passive transfer of immunity (immunoglobulin G > 2,000 mg/dl). Since the time of birth, the foal reportedly had mild, intermittent diarrhea that responded to gastrointestinal protectants and probiotics. Despite prompt and aggressive treatment after hospital referral, the foal's condition deteriorated, and the foal was humanely euthanized. Postmortem gross and histopathologic examination revealed severe hepatitis with necrosis and fibrinonecrotic typhlitis and colitis. In addition to a positive blood culture for L. monocytogenes, immunohistochemistry confirmed the presence of this bacterium in the liver, cecum, and colon. Furthermore, a multiplex polymerase chain reaction identified the etiologic organism as a virulent L. monocytogenes strain.

Animal models of listeriosis: a comparative review of the current state of the art and lessons learned

Listeriosis is a leading cause of hospitalization and death due to foodborne illness in the industrialized world. Animal models have played fundamental roles in elucidating the pathophysiology and immunology of listeriosis, and will almost certainly continue to be integral components of the research on listeriosis. Data derived from animal studies helped for example characterize the importance of cell-mediated immunity in controlling infection, allowed evaluation of chemotherapeutic treatments for listeriosis, and contributed to quantitative assessments of the public health risk associated with L. monocytogenes contaminated food commodities. Nonetheless, a number of pivotal questions remain unresolved, including dose-response relationships, which represent essential components of risk assessments. Newly emerging data about species-specific differences have recently raised concern about the validity of most traditional animal models of listeriosis. However, considerable uncertainty about the best choice of animal model remains. Here we review the available data on traditional and potential new animal models to summarize currently recognized strengths and limitations of each model. This knowledge is instrumental for devising future studies and for interpreting current data. We deliberately chose a historical, comparative and cross-disciplinary approach, striving to reveal clues that may help predict the ultimate value of each animal model in spite of incomplete data.

Genome sequence of lineage III Listeria monocytogenes strain HCC23

More than 98% of reported human listeriosis cases are caused by Listeria monocytogenes serotypes within lineages I and II. Serotypes within lineage III (4a and 4c) are commonly isolated from environmental and food specimens. We report the first complete genome sequence of a lineage III isolate, HCC23, which will be used for comparative analysis.

Listeriosis Outbreak in Dairy Cattle Caused by an Unusual Listeria Monocytogenes Serotype 4b Strain

A listeriosis outbreak, in dairy cattle, with a high case mortality and acute death after onset of symptoms was investigated using gross pathology and bacteriologic approaches, including molecular characterization of a clinical Listeria monocytogenes isolate. In a herd of 315 animals, 9 animals showed clinical symptoms consistent with listeriosis, including 3 animals that died within 2–4 days after acute onset of clinical signs, 4 animals that were euthanized, and 2 that survived. Initial EcoRI ribotyping and serotyping indicated that this outbreak was caused by an unusual L. monocytogenes serotype 4b strain, which was classified into lineage III. Further characterization of this isolate by DNA sequencing–based subtyping methods indicated that the strain responsible for this outbreak represented a unique genotype as supported by its classification into a new sigB allelic type, which has not been identified previously among >290 isolates, and by compelling phylogenetic evidence. While lineage III isolates are generally rare, they seem to be more common among L. monocytogenes isolates from animals with clinical signs of listeriosis. This is the first report of a particularly severe clinical course of disease associated with infection by a lineage III strain. The high prevalence of Listeria spp., including L. monocytogenes, in the farm environments may favor emergence and evolution of novel, and possibly more virulent, L. monocytogenes strains. Continued monitoring of animal listeriosis cases and outbreaks may not only improve animal health but also aid in the early discovery of newly emerging L. monocytogenes strains.

Comparative genomics of the bacterial genus Listeria: Genome evolution is characterized by limited gene acquisition and limited gene loss

Background

The bacterial genus Listeria contains pathogenic and non-pathogenic species, including the pathogens L. monocytogenes and L. ivanovii, both of which carry homologous virulence gene clusters such as the prfA cluster and clusters of internalin genes. Initial evidence for multiple deletions of the prfA cluster during the evolution of Listeria indicates that this genus provides an interesting model for studying the evolution of virulence and also presents practical challenges with regard to definition of pathogenic strains.

Results

To better understand genome evolution and evolution of virulence characteristics in Listeria, we used a next generation sequencing approach to generate draft genomes for seven strains representing Listeria species or clades for which genome sequences were not available. Comparative analyses of these draft genomes and six publicly available genomes, which together represent the main Listeria species, showed evidence for (i) a pangenome with 2,032 core and 2,918 accessory genes identified to date, (ii) a critical role of gene loss events in transition of Listeria species from facultative pathogen to saprotroph, even though a consistent pattern of gene loss seemed to be absent, and a number of isolates representing non-pathogenic species still carried some virulence associated genes, and (iii) divergence of modern pathogenic and non-pathogenic Listeria species and strains, most likely circa 47 million years ago, from a pathogenic common ancestor that contained key virulence genes.

Conclusions

Genome evolution in Listeria involved limited gene loss and acquisition as supported by (i) a relatively high coverage of the predicted pan-genome by the observed pan-genome, (ii) conserved genome size (between 2.8 and 3.2 Mb), and (iii) a highly syntenic genome. Limited gene loss in Listeria did include loss of virulence associated genes, likely associated with multiple transitions to a saprotrophic lifestyle. The genus Listeria thus provides an example of a group of bacteria that appears to evolve through a loss of virulence rather than acquisition of virulence characteristics. While Listeria includes a number of species-like clades, many of these putative species include clades or strains with atypical virulence associated characteristics. This information will allow for the development of genetic and genomic criteria for pathogenic strains, including development of assays that specifically detect pathogenic Listeria strains.

Incidence and persistence of Listeria monocytogenes in the catfish processing environment and fresh fillets

Incidence of Listeria spp. in whole raw catfish, catfish fillets, and processing environments from two catfish processing facilities was determined in August 2008 and August 2009. Thirty-nine (18.4%) of 212 samples collected in August 2008 were positive for Listeria monocytogenes. Prevalences of Listeria species L. innocua and L. seeligeri-L. welshimeri-L. ivanovii were 11.3 and 23.6%, respectively. Of 209 samples collected in August 2009, 12.4% were positive for L. monocytogenes, 11% for L. innocua, and 19.6% for L. seeligeri-L. welshimeri-L. ivanovii. No Listeria grayi was detected in any of the samples. L. monocytogenes was not found in catfish skins and intestines, but was detected in catfish fillets, on food contact surfaces, and on non-food contact surfaces with frequencies of 45.0, 12.0, and 11.1%, respectively. In August 2008 isolates, serotypes 1/2b (62.2%) and 3b (15.6%) were frequently isolated, whereas the majority of the August 2009 isolates (92.3%) were serotype 1/2b. Genotyping analyses revealed that some genotypes of L. monocytogenes isolates were detected in one facility even after a year, but no persistence of L. monocytogenes was observed in the other facility. In addition, some L. monocytogenes isolates from fresh fillets showed genotypes that were either identical, or more than 90% similar, to those of L. monocytogenes isolates from food contact surfaces in the processing lines. The results of this study suggest that processing environment rather than whole raw catfish is an important source of L. monocytogenes contamination in the catfish fillets. These results should assist the catfish industry to develop better control and prevention strategies for L. monocytogenes.

Prevalence of Listeria spp including Listeria monocytogenes from apparently healthy sheep of Gujarat State, India

A total of 1002 samples comprising blood (n = 248), faecal swabs (n = 248), nasal swabs (n = 248) and deep vaginal swabs (n = 248) collected from the 248 sheep and 10 environmental samples of 10 sheep flocks were examined for the presence of pathogenic Listeria spp. Confirmation of the isolates was based on biochemical tests followed by phenotypic characterization by haemolysis on sheep blood agar, Christie Atkins Munch-Petersen (CAMP) test phosphatidylinositol-specific phospholipase C (PI-PLC) assay and phosphatidylcholine-specific phospholipase C (PI-PLC) assay. The isolates were subjected to genotypic characterization with the help of PCR assay for five virulence-associated genes, plcA, prfA, hlyA, actA and iap. The L. monocytogenes isolates were further subjected for multiplex-PCR-based serotyping. From 1002 samples screened, 16 (1.60%) were found positive for Listeria spp. Of these, seven samples (0.7%) were confirmed as L. monocytogenes and nine (0.9%) as L. innocua. All the seven isolates of L. monocytogenes were haemolytic, CAMP-positive, PI-PLC-positive, hlyA, pclA and prfA-positive by PCR, while only four isolates turned out to be PC-PLC-positive (opaque zone surrounding the growth). All the seven isolates of L. monocytogenes were serotyped as 4b. In conclusion, the PI-PLC assay and the virulence genes targeted PCR (plcA, prfA and hlyA plcA, prfA and actA genes for L. monocytogenes) hold a promise for rapid and reliable in vitro alternatives to in vivo pathogenicity tests.

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.

Toward an improved laboratory definition of Listeria monocytogenes virulence

Listeria monocytogenes is an opportunistic foodborne pathogen that encompasses a diversity of strains with varied virulence. The ability to rapidly determine the pathogenic potential of L. monocytogenes strains is integral to the control and prevention campaign against listeriosis. Early methods for assessing L. monocytogenes virulence include in vivo bioassays and in vitro cell assays. While in vivo bioassays provide a measurement of all virulence determinants of L. monocytogenes, they are not applied routinely due to their reliance on experimental animals whose costs have become increasingly prohibitive. As a low cost alternative, in vitro cell assays are useful for estimating the virulence of L. monocytogenes strains. However, these assays are often slow, and at times variable. Prior attempts to ascertain L. monocytogenes virulence by targeting virulence-associated proteins and genes have been largely unsuccessful, since many of the assay targets are present in both virulent and avirulent strains. Recent identification of novel virulence-specific genes (particularly internalin gene inlJ) has opened a new avenue for rapid, sensitive, and precise differentiation of virulent L. monocytogenes strains from avirulent strains. The application of DNA sequencing technique also offers an additional tool for assessing L. monocytogenes virulence potential. By providing an update on the laboratory methods that have been reported for the determination of L. monocytogenes pathogenicity, this review discusses future research needs that may help achieve an improved laboratory definition of L. monocytogenes virulence.

Incidence of Listeria spp. in raw milk in Shahrekord, Iran

Listerosis may be transmitted by direct contact with infected animals or by consumption of contaminated vegetables, meat, or milk products. Raw milk samples obtained from the Milk Industry Foundation, five private dairy companies, and individual dairy farms in Shahrekord, Iran, were tested for the presence of Listeria species. A total of 500 raw milk samples were analyzed by two-stage enrichment techniques. Identification of isolated Listeria was done using the Micro-ID Listeria Kit (Remel, Lenexa, KS). The overall incidence of Listeria species in raw milk was 2.2%. L. monocytogenes was found in 1.6% of the raw milk samples, while L. innocua was found in 0.6% of the samples. Additional studies are needed to assess the public health impact of contaminated milk in Shahrekord.

Characteristics of cell-mediated, anti-listerial immunity induced by a naturally avirulent Listeria monocytogenes serotype 4a strain HCC23

The characteristics of cell-mediated, anti-listerial immune response initiated by an avirulent Listeria monocytogenes serotype 4a strain HCC23 was assessed. Similar to virulent strain EGD, avirulent strain HCC23 grew readily within macrophage-like J774 cells, but nonhemolytic strain ATCC 15313 did not. Compared with EGD, HCC23 induced a relatively low level of gamma interferon (IFN-gamma) in mice, and ATCC 15313 stimulated no detectable IFN-gamma. The percentages of gated CD4 T cells from mice immunized with EGD and HCC23 showed a notable drop (to 30%) at 21 days post exposure in comparison with that (about 50%) from ATCC 15313-injected or untreated mice; and the percentage of gated NK cells from EGD-immunized group was markedly higher than those from other treatment groups. Mice immunized with HCC23 and EGD developed an equally strong protective immunity against listeriosis that was effective in both short and long terms, but those injected with ATCC 15313 or saline succumbed to listeriosis within 6 days of challenge.

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.

Comparative assessment of acid, alkali and salt tolerance inListeria monocytogenesvirulent and avirulent strains

Listeria monocytogenes is an opportunistic bacterial pathogen of man and animals that has the capacity to survive under extreme environmental conditions. While our knowledge on L. monocytogenes and its ability to sustain within wide pH and temperature ranges and salt concentrations has been largely built on the virulent strains of this species, relatively little is known about avirulent strains in this regard. In this study, we extend our analysis on avirulent L. monocytogenes strains. By subjecting three virulent (EGD, 874 and ATCC 19196) and three avirulent (ATCC 19114, HCC23 and HCC25) strains to various pH and salt concentrations, it was found that L. monocytogenes recovered well after treatment with 100 mM Tris at pH 12.0, and to a lesser extent at pH 3.0. Interestingly, avirulent L. monocytogenes strains showed a somewhat higher tolerance to alkali than virulent strains. This unique feature of avirulent L. monocytogenes strains may potentially be exploited for the development of a rapid technique for differentiation between avirulent and virulent strains. Furthermore, all L. monocytogenes strains tested were resistant to saturated NaCl (about 7 M, or 40% w/v) for a long period of time (20 h and possibly longer). Together, these results highlight that acid, alkali, and/or salt treatments commonly used in food product processing may not be sufficient to eliminate L. monocytogenes, and therefore stringent quality control measures at the beginning and end of the food manufacturing process is essential to ensure that such food products are free of listerial contamination.

Species-specific PCR determination of Listeria seeligeri

Listeria seeligeri is a non-pathogenic bacterium coming under the genus Listeria. As this bacterium resembles other Listeria species such as L. monocytogenes and L. ivanovii that are pathogenic to man and animals, it is important that rapid and precise identification techniques be available for L. seeligeri in cases where such determination is desirable. A specific molecular test on the basis of a uniquely present gene region in L. seeligeri will be of particular value under the circumstances. In this report, after comparative screening of genomic DNA from six Listeria species by dot blot hybridization, we isolated one L. seeligeri-specific clone (lse24-315) that contains an insert of 1538 bp. Using primers (lse24-315F and lse24-315R) derived from this clone, we showed that a specific PCR product of 375 bp was generated from genomic DNA of L. seeligeri strains only, but not of other Listeria species or common bacteria. Therefore, the PCR employing primers lse24-315F and lse24-315R provides a rapid, sensitive and specific method for distinguishing L. seeligeri from other Listeria and common bacteria.

PCR detection of a putative N-acetylmuramidase gene from Listeria ivanovii facilitates its rapid identification

Listeria ivanovii is a Gram-positive bacterial pathogen that is capable of causing abortions and stillbirths in farm animals, particularly sheep and cattle. In terms of morphological, biochemical and molecular characteristics, L. ivanovii resembles other Listeria species such as L. monocytogenes, a pathogen of both man and animals. In this study, through comparative analysis of genomic DNA from the six Listeria species, a L. ivanovii specific clone (liv22-228) containing a 946 bp insert was isolated. This clone contained the 5' ends of two divergently transcribed L. ivanovii genes and an intergenic spacer region, similar in organization to homologous regions from the L. innocua and L. monocytogenes genomes. Regions of low homology in the clone were identified by comparing to the L. innocua and L. monocytogenes genomes, and oligonucleotide primers (liv22-228F and liv22-228R) were designed. These primers amplified a 463 bp band from genomic DNA of L. ivanovii strains only, but not from other Listeria species or common bacteria. Thus, PCR employing L. ivanovii specific primers (liv22-228F and liv22-228R) provides a useful and straightforward method for rapid and precise determination of L. ivanovii.

Use of PCR primers derived from a putative transcriptional regulator gene for species-specific determination of Listeria monocytogenes

Listeria monocytogenes is an opportunistic bacterial pathogen that has accounted for an important portion of human foodborne diseases worldwide. In this study, through comparative analysis of L. innocua and L. monocytogenes genomic sequences, we selected a L. monocytogenes specific gene (lmo0733) that has the potential for specific detection of L. monocytogenes. Using PCR primers (lmo0733F and lmo0733R) derived from this gene, a specific fragment of 453 bp was amplified only from genomic DNA of L. monocytogenes strains. PCR products from other Listeria species as well as other Gram-positive and -negative species were not detectable, confirming the specificity of this assay. Thus, the PCR test employing primers lmo0733F and lmo0733R represents an additional tool in the diagnostic arsenal for rapid, sensitive and specific detection and identification of human infections due to L. monocytogenes.

Listeria monocytogenes: comparative interpretation of mouse virulence assay

Being an opportunistic bacterial pathogen, Listeria monocytogenes demonstrates significant strain variations in virulence and pathogenicity. The availability of laboratory procedures to ascertain the pathogenic potential of L. monocytogenes bacteria would greatly enhance the control and prevention of listerial infections. As a method that measures all virulent determinants, mouse virulence assay has been frequently used for assessing L. monocytogenes virulence. The pathogenic potential of a given L. monocytogenes strain as determined by mouse virulence assay is often calculated from mouse mortality data in combination with colony forming units (CFUs) derived from plate counts, and expressed by medium lethal dose (LD(50)). In this report, we describe an alternative method [i.e., relative virulence (%)] that does not involve CFU estimation, and is comparable to LD(50) for interpretation of mouse virulence assay for L. monocytogenes. The relative virulence (%) is obtained by dividing the number of dead mice with the total number of mice tested for a particular strain using a known virulent strain (e.g., L. monocytogenes EGD) as reference. Besides providing a more direct interpretation in comparison with LD(50) values for mouse virulence assay, this method requires fewer dosage groups per L. monocytogenes strain, and eliminates CFU estimation that is step subject to variations between runs and also between laboratories.

Characterization of virulent and avirulent Listeria monocytogenes strains by PCR amplification of putative transcriptional regulator and internalin genes

Listeria monocytogenes is an opportunistic bacterial pathogen that is an important cause of human food-borne illness worldwide. However, L. monocytogenes strains demonstrate considerable variation in pathogenic potential. In this report, virulent and avirulent L. monocytogenes isolates were compared by using a comparative screening strategy. Two clones were identified that contained DNA that was only present in virulent L. monocytogenes strains. PCR primers were designed for three genes from these clones and for five other selected L. monocytogenes genes. All eight primer sets predominantly detected virulent L. monocytogenes isolates, as determined by a mouse virulence assay; one of the putative internalin genes, lmo2821, was detected in all strains that were considered to be virulent. Primers from these eight genes were then tested by PCR against a larger panel of bacterial strains; each of the genes was detected predominantly in clinical or food L. monocytogenes isolates, rather than environmental isolates. The findings from this study suggest that virulent L. monocytogenes strains may possess genes that are not present in avirulent isolates, which could serve as markers for PCR assessment of L. monocytogenes virulence.

A predictive model to determine the effects of temperature, sodium pyrophosphate, and sodium chloride on thermal inactivation of starved Listeria monocytogenes in pork slurry

The effects and interactions of 27 combinations of heating temperature (57.5 to 62.5 degrees C), sodium pyrophosphate (SPP) level (0 to 0.5%, wt/vol), and salt (NaCl) level (0 to 6%, wt/vol) on the thermal inactivation of starved Listeria monocytogenes ATCC 19116 in pork slurry were investigated. A split-split plot experimental design was used to compare all 27 combinations. L. monocytogenes survivors were enumerated on tryptic soy agar supplemented with 0.6% yeast extract. The natural logarithm (loge) of the means of decimal reduction times (D-values) were modeled as a function of temperature, SPP level, and NaCl level. Increasing concentrations of SPP or NaCl protected starved L. monocytogenes from the destructive effect of heat. For example, D-values for the pathogen at 57.5 degrees C in pork slurry with 0, 3, and 6% NaCl were 2.79, 7.75, and 14.59 min, respectively. All three variables interacted to affect the thermal inactivation of L. monocytogenes. A mathematical model describing the combined effect of temperature, SPP level, and NaCl level on the thermal inactivation of starved L. monocytogenes was developed. There was strong correlation (R2 = 0.97) between loge D-values predicted by the model and those observed experimentally. The model can predict D-values for any combination of variables that falls within the range of those tested. This predictive model can be used to assist food processors in designing thermal processes that include an adequate margin of safety for the control of L. monocytogenes in processed meats.

Identification of Listeria innocua by PCR targeting a putative transcriptional regulator gene

Listeria innocua is a common, non-pathogenic bacterial species that shares morphological, biochemical and molecular characteristics with the pathogenic species L. monocytogenes. The presence of L. innocua may cause difficulty or confusion in the laboratory identification of L. monocytogenes or other Listeria spp. In this report, through examining the recently published genome sequence of L. innocua strain CLIP 11262 (serovar 6a), we identified a L. innocua-specific gene (lin0464) encoding a putative transcriptional regulator and evaluated its efficacy for species-specific detection by polymerase chain reaction (PCR). The specificity of the oligonucleotide primers (lin0464F and lin0464R) derived from this gene was confirmed with the formation of a 749-bp fragment in PCR from genomic DNA of L. innocua strains only. We expect that this assay will be useful in confirming identification of L. innocua or in studies where rapid detection of L. innocua is necessary.

Dietary oligofructose and inulin protect mice from enteric and systemic pathogens and tumor inducers

Prebiotics induce changes in the population and metabolic characteristics of the gastrointestinal bacteria, modulate enteric and systemic immune functions, and provide laboratory rodents with resistance to carcinogens that promote colorectal cancer. There is less known about protection from other challenges. Therefore, mice of the B6C3F1 strain were fed for 6 wk a control diet with 100 g/kg cellulose or one of two experimental diets with the cellulose replaced entirely by the nondigestible oligosaccharides (NDO) oligofructose and inulin. From each diet, 25 mice were challenged by a promoter of colorectal cancer (1,2-dimethylhydrazine), B16F10 tumor cells, the enteric pathogen Candida albicans (enterically), or were infected systemically with Listeria monocytogenes or Salmonella typhimurium. The incidences of aberrant crypt foci in the distal colon after exposure to dimethylhdrazine for mice fed inulin (53%) and oligofructose (54%) were lower than in control mice (76%; P < 0.05), but the fructans did not reduce the incidence of lung tumors after injection of the B16F10 tumor cells. Mice fed the diets with fructans had 50% lower densities of C. albicans in the small intestine (P < 0.05). A systemic infection with L. monocytogenes caused nearly 30% mortality among control mice, but none of the mice fed inulin died, with survival intermediate for mice fed oligofructose. Mortality was higher for the systemic infection of S. typhimurium (>80% for control mice), but fewer of the mice fed inulin died (60%; P < 0.05), with mice fed oligofructose again intermediate. The mechanistic basis for the increased resistance provided by dietary NDO was not elucidated, but the findings are consistent with enhanced immune functions in response to changes in the composition and metabolic characteristics of the bacteria resident in the gastrointestinal tract.

Risk factors for L. monocytogenes contamination of dairy products in Switzerland, 1990-1999

Our purpose was to identify the main hazards associated with the spread of Listeria monocytogenes in dairy products in Switzerland and to determine the changes in predominant serotypes of the isolates, using databases on dairy-processing and environments from the Swiss Dairy Research Station during the years 1990-1999. Overall, of 76,271 samples collected, 3722 (4.9%) were positive for the presence of L. monocytogenes. Cheese-ripening facilities had the highest proportion of positive samples (7.6%), followed by small-scale local dairies (4.4%). By sample type, the highest proportion of positive samples (9.5%) was observed in water samples used for cheese-washing, followed by cheese-surface swabs (5.0%). During the 10-year period, no positive samples were obtained from cream, ice cream, milk powder, yogurt, or fresh cheese. Of 3722 L. monocytogenes isolates, 1328 (35.7%) were serologically typeable. Serotypes 1/2a, 1/2b, and 4b accounted for 92.7% of the 1328 isolates. Until 1995, the most-prevalent serotype was 1/2b (annual proportional prevalence 39.3-72.2%)--whereas since 1996, 1/2a was the most prevalent (34.7-54.7%). During 1996-1999, serotype 1/2a increased by 88%, compared to the average of 1990-1995. In the final random-effect multivariable logistic model, the strongest predictor of a positive culture was samples from cheese-ripening plant (OR=1.54; 95% CI: 1.14, 2.08) and the second-strongest predictor was samples collected by someone who was employed by the plant (OR=1.48; 1.29, 1.71). Hard and semi-hard cheeses were more likely to be associated with serotype 1/2b and soft cheeses with serotype 1/2a.