Listeria monocytogenes isolates can be classified into two major types according to the sequence of the listeriolysin gene

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.

Mpl-Gene-Based Loop-Mediated Isothermal Amplification Assay for Specific and Rapid Detection of Listeria monocytogenes in Various Food Samples

Listeria monocytogenes represents a high risk in food and can trigger potentially fatal listeriosis. The objective of this study was to detect L. monocytogenes in food using the LAMP method in a fast, specific, sensitive manner and thus to preventively test food for the presence of the target species. The reaction was performed and established using the portable real-time fluorometer Genie^® II (OptiGene Ltd., Horsham, United Kingdom). In this new assay, six LAMP primers targeted the mpl-gene sequence of L. monocytogenes. A total of 148 different isolates, including 105 L. monocytogenes and 43 non-L. monocytogenes strains, were tested. Analytical sensitivity was determined based on different DNA- and cell concentrations. The detection limit with a detection rate of 100% was 5 pg of DNA or 275 colony-forming units (CFU) per reaction. Artificially contaminated minced beef and grated mozzarella were also tested. The assay was 100% successful to detect an initial bacterial contamination of 0.4-4 CFU g^-1 food after 24 h enrichment in half-Fraser broth. Finally, natively contaminated samples were tested in comparison to the microbiological reference method and real-time polymerase chain reaction. Native sample testing revealed 100% consistent findings between LAMP and the standard culture method after first enrichment for 24 h. In addition, a rapid colony-confirmation method was established that enabled reliable identification of L. monocytogenes isolates on different selective culture media using a simplified DNA extraction by boiling. This study showed that the developed assay was able to determine whether a food is safe with respect to the food-safety criteria of 100 CFU per gram, according to standards of the European Union, for L. monocytogenes and provided faster results than the cultural reference method.

Listeria monocytogenes isolates from Western Cape, South Africa exhibit resistance to multiple antibiotics and contradicts certain global resistance patterns

Food-borne disease outbreaks are common and offer valuable insights into the causes, impacts, and mechanisms underlying food pathogens. This also serves as a good foundation to validate the performance of current best practice control methods, for example antibiotics, that are used in the fight against food pathogens. Listeriosis outbreaks, caused by Listeria monocytogenes, is no exception. In 2018, South Africa experienced the largest global listeriosis outbreak recorded to date. However, despite the scale of this outbreak, information on the bacterium and its resistance towards antibiotics is still severely lacking. Furthermore, until now it remained to be determined whether L. monocytogenes antibiotic resistance patterns in South Africa mirror resistance patterns elsewhere in the world. The aim of this study was therefore to evaluate the efficacy of antibiotics that are currently used against L. monocytogenes. Using the European Committee on Antimicrobial Susceptibility Testing (EUCAST) disc diffusion method, L. monocytogenes isolates (n = 177) from diverse origins in the Western Cape, South Africa (clinical, food, and environment) were tested for susceptibility against five different antibiotics, namely ampicillin, erythromycin, chloramphenicol, gentamicin, and tetracycline. Isolates were collected over a period of two years (2017-2019). All isolates were susceptible to ampicillin, the currently recommended antibiotic, while a large number of isolates were resistant to chloramphenicol, erythromycin, and tetracycline. Also, patterns of resistance observed here are different to patterns observed elsewhere. The findings of this study demonstrate that it is imperative to continuously monitor the efficacy of currently recommended antibiotics, since resistance patterns can quickly develop when such antibiotics are overutilized, and secondly, that it is crucial to assess local antibiotic resistance patterns in conjunction with global patterns, since the latter is not necessarily generalizable to local scales.

Listeria monocytogenes in five Sardinian swine slaughterhouses: prevalence, serotype, and genotype characterization

In a 3-year study (2008 to 2011) to estimate the prevalence and the contamination sources of Listeria monocytogenes in pork meat in Sardinia, Italy, 211 samples were collected from five Sardinian swine slaughterhouses: 171 samples from slaughtered pigs and 40 from the slaughterhouse environment. Fifty L. monocytogenes isolates were characterized by PCR-based serotyping, presence of virulence-associated genes, and pulsed-field gel electrophoresis restriction analysis. The overall prevalence of L. monocytogenes was 33% in swine carcasses, 7% in cecal material, 23% on meat contact surfaces, and 25% on noncontact surfaces. Only two serotypes were detected: 1/2c (78%) and 1/2a (22%). In all, based on the presence of virulence-associated genes, eight pathogenic profiles were detected. Only 42% of all isolates carried the full complement of virulence-associated genes and were allotted to profile 1. Six pulsed-field gel electrophoresis profiles persisted in the slaughterhouses; restriction profiles appeared to be specific to each plant.

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

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

Gene Scanning of an Internalin B Gene Fragment Using High-Resolution Melting Curve Analysis as a Tool for Rapid Typing of Listeria monocytogenes

The ability to accurately track Listeria monocytogenes strains involved in outbreaks is essential for control and prevention of listeriosis. Because current typing techniques are time-consuming, cost-intensive, technically demanding, and difficult to standardize, we developed a rapid and cost-effective method for typing of L. monocytogenes. In all, 172 clinical L. monocytogenes isolates and 20 isolates from culture collections were typed by high-resolution melting (HRM) curve analysis of a specific locus of the internalin B gene (inlB). All obtained HRM curve profiles were verified by sequence analysis. The 192 tested L. monocytogenes isolates yielded 15 specific HRM curve profiles. Sequence analysis revealed that these 15 HRM curve profiles correspond to 18 distinct inlB sequence types. The HRM curve profiles obtained correlated with the five phylogenetic groups I.1, I.2, II.1, II.2, and III. Thus, HRM curve analysis constitutes an inexpensive assay and represents an improvement in typing relative to classical serotyping or multiplex PCR typing protocols. This method provides a rapid and powerful screening tool for simultaneous preliminary typing of up to 384 samples in approximately 2 hours.

Relative transcription of Listeria monocytogenes virulence genes in liver pâtés with varying NaCl content

Quantitative real time polymerase chain reaction (qRT PCR) was used to compare the relative transcription of prfA, inlA, sigB and clpC for three Listeria monocytogenes strains after incubation in i) a standard liver pâté versus brain heart infusion (BHI) broth and ii) the standard liver pâté versus three liver pâtés with reduced NaCl content of which one also has been supplied with organic acids (Ca-acetate and Ca-lactate). The three strains (EGD-e: reference strain; O57: more NaCl sensitive; 6896: more NaCl tolerant) were selected out of twelve strains based on their growth in BHI broth adjusted to 6%, 8%, 10% (w/v) NaCl. The three strains were spiked into the liver pâtés (10(9) cfu/g) and the BHI (10(9) cfu/ml) and incubated for 48 h at 7 degrees C; all incubation conditions supported growth of the strains. Extraction of intact listerial RNA from the liver pâtés was complicated by the complexity of the liver pâté matrix. However, a method has been optimized and described, and the quality of RNA extracted from liver pâtés was equal to the quality of RNA extracted from BHI. The amplification efficiencies of the six genes used for the transcription analyses (the four target genes and two reference genes, gap and rpoB) were within the acceptable range from 90% to 110% for all three strains in both liver pâté and BHI. Comparison of the three strains after incubation in the standard liver pâté and BHI showed that the relative transcription of prfA for O57 and the relative transcription of inlA and sigB for both O57 and 6896 were significantly higher when the strains were grown in BHI compared to the standard liver pâté. Reducing the NaCl content of the standard liver pâté did not change relative transcription levels of prfA, inlA, sigB or clpC (except for prfA in O57 and sigB in 6896). However, the presence of Ca-acetate and Ca-lactate induced relative transcription of the stress response gene, clpC, for all three strains. This study demonstrates that relative microbial gene transcription can be measured in complex food matrices and points to the need for designing experimental set-ups in real food matrices to replace the laboratory model systems. With respect to L. monocytogenes, it seems that the NaCl content of liver pâté can be lowered within the investigated range without significant changes in relative virulence gene transcription while more caution should be taken when adding organic acids such as acetate and lactate.

A new perspective on Listeria monocytogenes evolution

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

Formation of biofilm at different nutrient levels by various genotypes of Listeria monocytogenes

Strains of Listeria monocytogenes differ in their ability to form biofilms. The objectives of this study were to determine whether genetically related strains have similar biofilm-forming capacities and what effect nutrient concentration has on the ability of different strains to produce biofilms. Biofilms of 30 strains of L. monocytogenes, obtained from a variety of sources were grown on stainless steel in tryptic soy broth (TSB) or in a 1:10 dilution of TSB (DTSB) for 24 h at 32 degrees C. The amount of biofilm formed was determined with image analysis after cells were stained with bisBenzimide H 33258 (Hoechst 33258). The strains were genetically subtyped by repetitive element sequence-based PCR (rep-PCR) with the primer set rep-PRODt and rep-PROG5. Data were analyzed with an analysis of variance and Duncan's multiple range test. Eleven strains produced the same amount of biofilm in both media. Fourteen strains produced more biofilm in TSB than in DTSB. Five strains produced more biofilm in DTSB than in TSB. Serotype 4b strains produced more biofilm in TSB than did serotype 1/2a strains, whereas serotype 1/2a strains produced more biofilm in DTSB than did serotype 4b strains. Growth in DTSB resulted in decreased biofilm accumulation for serotype 4b strains. There was no correlation between genetic subtype and the amount of biofilm accumulation. These results indicate that strains of serotype 1/2a and serotype 4b differ in the regulation of their biofilm phenotype. The poor biofilm accumulation of serotype 4b isolates when grown in DTSB could be a factor in the predominance of serogroup 1/2 strains in food processing plants, where nutrients may be limited.

Genetic and phenotypic characterization of Listeria monocytogenes lineage III

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

Methods for the isolation and identification of Listeria spp. and Listeria monocytogenes: a review

Listeria monocytogenes is an important food-borne pathogen and is widely tested for in food, environmental and clinical samples. Identification traditionally involved culture methods based on selective enrichment and plating followed by the characterization of Listeria spp. based on colony morphology, sugar fermentation and haemolytic properties. These methods are the gold standard; but they are lengthy and may not be suitable for testing of foods with short shelf lives. As a result more rapid tests were developed based on antibodies (ELISA) or molecular techniques (PCR or DNA hybridization). While these tests possess equal sensitivity, they are rapid and allow testing to be completed within 48 h. More recently, molecular methods were developed that target RNA rather than DNA, such as RT-PCR, real time PCR or nucleic acid based sequence amplification (NASBA). These tests not only provide a measure of cell viability but they can also be used for quantitative analysis. In addition, a variety of tests are available for sub-species characterization, which are particularly useful in epidemiological investigations. Early typing methods differentiated isolates based on phenotypic markers, such as multilocus enzyme electrophoresis, phage typing and serotyping. These phenotypic typing methods are being replaced by molecular tests, which reflect genetic relationships between isolates and are more accurate. These new methods are currently mainly used in research but their considerable potential for routine testing in the future cannot be overlooked.

Sequence and binding activity of the autolysin-adhesin Ami from epidemic Listeria monocytogenes 4b

Ami is an autolytic amidase from Listeria monocytogenes that is targeted to the bacterial surface via its C-terminal cell wall anchoring (CWA) domain. We recently showed that the CWA domain from Ami of L. monocytogenes EGD (serovar 1/2a) (Ami 1/2a) mediated bacterial binding to mammalian cells. Here we studied the sequence and binding properties of Ami from CHUT 82337 (serovar 4b) (Ami 4b). The Ami 4b polypeptide is predicted to be 770 amino acids long (compared with the 917 amino acids of Ami 1/2a from EGD). Ami 1/2a and Ami 4b are almost identical in the N-terminal enzymatic domain (approximately 98% amino acid identity), but the sequence is poorly conserved in the C-terminal CWA domain, with only approximately 54% amino acid identity and eight GW modules in Ami 1/2a compared with six GW modules in Ami 4b. The purified Ami 4b CWA domain efficiently bound serovar 4b bacterial cells and only poorly bound serovar 1/2a bacterial cells. The Ami 4b CWA domain was also significantly less able to bind Hep-G2 human hepatocytic cells than the Ami 1/2a CWA domain. We sequenced the ami regions encoding CWA domains of reference strains belonging to the 12 L. monocytogenes serovars. The phylogenic tree constructed from the sequences yielded a binary division into group I (serovars 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, and 7) and group II (serovars 4a, 4b, 4c, 4d, and 4e). This is the first direct evidence of divergence between serovars 1/2a and 4b in a gene involved in the adhesion of L. monocytogenes to mammalian cells, as well as the first demonstration of allelic polymorphism correlated with the somatic antigen in this species.

Heteroduplex mobility assay for the identification of Listeria sp and Listeria monocytogenes strains: application to characterisation of strains from sludge and food samples

One hundred and ten Listeria sp. isolates from sewage sludge were identified according to phenotypic and genotypic methods. The Listeria sp. strains isolated from five types of sludge from three sewage treatment plants in Angers (France) and the surrounding area included L. monocytogenes (55.5%), L. innocua (29.1%), L. seeligeri (13.6%) and L. welshimeri (1.8%). The majority of L. monocytogenes strains belonged to serotypes 4b, 1/2b and 1/2a. Moreover, a heteroduplex mobility assay based on the 16S rRNA sequences was tested for its ability to identify the six species of the genus Listeria. This study, performed on 283 Listeria sp. strains from human, food and sewage sludge samples, showed that all the species were distinguishable from one another. L. innocua and L. seeligeri showed respectively three and two distinct banding patterns. Within L. monocytogenes, four groups (I-IV) were defined. The majority of food and environmental isolates were clustered in group I and it is noteworthy that group IV clustered epidemiologic isolates and strains belonging to serotypes 4b, 1/2a and 1/2b.

Variation in biofilm formation among strains of Listeria monocytogenes

Contamination of food by Listeria monocytogenes is thought to occur most frequently in food-processing environments where cells persist due to their ability to attach to stainless steel and other surfaces. Once attached these cells may produce multicellular biofilms that are resistant to disinfection and from which cells can become detached and contaminate food products. Because there is a correlation between virulence and serotype (and thus phylogenetic division) of L. monocytogenes, it is important to determine if there is a link between biofilm formation and disease incidence for L. monocytogenes. Eighty L. monocytogenes isolates were screened for biofilm formation to determine if there is a robust relationship between biofilm formation, phylogenic division, and persistence in the environment. Statistically significant differences were detected between phylogenetic divisions. Increased biofilm formation was observed in Division II strains (serotypes 1/2a and 1/2c), which are not normally associated with food-borne outbreaks. Differences in biofilm formation were also detected between persistent and nonpersistent strains isolated from bulk milk samples, with persistent strains showing increased biofilm formation relative to nonpersistent strains. There were no significant differences detected among serotypes. Exopolysaccharide production correlated with cell adherence for high-biofilm-producing strains. Scanning electron microscopy showed that a high-biofilm-forming strain produced a dense, three-dimensional structure, whereas a low-biofilm-forming strain produced a thin, patchy biofilm. These data are consistent with data on persistent strains forming biofilms but do not support a consistent relationship between enhanced biofilm formation and disease incidence.

Genome diversification in phylogenetic lineages I and II of Listeria monocytogenes: identification of segments unique to lineage II populations

Thirteen different serotypes of Listeria monocytogenes can be distinguished on the basis of variation in somatic and flagellar antigens. Although the known virulence genes are present in all serotypes, greater than 90% of human cases of listeriosis are caused by serotypes 1/2a, 1/2b, and 4b and nearly all outbreaks of food-borne listeriosis have been caused by serotype 4b strains. Phylogenetic analysis of these three common clinical serotypes places them into two different lineages, with serotypes 1/2b and 4b belonging to lineage I and 1/2a belonging to lineage II. To begin examining evolution of the genome in these serotypes, DNA microarray analysis was used to identify lineage-specific and serotype-specific differences in genome content. A set of 44 strains representing serotypes 1/2a, 1/2b, and 4b was probed with a shotgun DNA microarray constructed from the serotype 1/2a strain 10403s. Clones spanning 47 different genes in 16 different contiguous segments relative to the lineage II 1/2a genome were found to be absent in all lineage I strains tested (serotype 4b and 1/2b) and an additional nine were altered exclusively in 4b strains. Southern hybridization confirmed that conserved alterations were, in all but two loci, due to absence of the segments from the genome. Genes within these contiguous segments comprise five functional categories, including genes involved in synthesis of cell surface molecules and regulation of virulence gene expression. Phylogenetic reconstruction and examination of compositional bias in the regions of difference are consistent with a model in which the ancestor of the two lineages had the 1/2 somatic serotype and the regions absent in the lineage I genome arose by loss of ancestral sequences.

Subtyping Listeria monocytogenes through the combined analyses of genotype and expression of the hlyA virulence determinant

AIMS

A major challenge for Listeria monocytogenes diagnostics is that this bacterium is ubiquitous in the environment, and that only a small fraction of the lineages are potential human pathogens. The aim of this work was to obtain a better subtyping of L. monocytogenes through utilization of combined analyses of genotype and the expression of the virulence determinant hlyA.

METHODS AND RESULTS

We investigated the effect of growth temperature and medium on the hlyA expression. The gene expression levels were determined by real-time quantitative reverse transcription PCR. The expression pattern of hlyA was highly diverse among the different strains tested. The expression ranged from repression to a 1000-fold induction for growth at 42 degrees C, as compared with 0 degrees C. The expression patterns were compared with the corresponding genotypes. There were surprisingly low correlations between the expression patterns and the genotype clusterings. This is exemplified for the virulent type strain NTNC 7973 and non-virulent type strain DSMZ 20600. These strains are genetically nearly identical, while the hlyA gene expression patterns are very different.

CONCLUSIONS

The hlyA gene expression was highly diverse even within genetically clustered subgroups of L. monocytogenes. Consequently, the gene expression patterns can be used to further differentiate the strains within these genetic subgroups.

SIGNIFICANCE AND IMPACT OF THE STUDY

A major limitation in the control of L. monocytogenes is that the current tools for subtyping are not accurate enough in determining the potential virulent strains. The impact of this study is that we have developed a subtyping approach that actually targets a virulence property.

ADSA Foundation Scholar Award--An integrated science-based approach to dairy food safety: Listeria monocytogenes as a model system

Transmission of food- and milkborne pathogens often involves complex interactions among the pathogen, the environment, and one or multiple host species. A complete understanding of these interactions is critical to allow the development of science-based, effective intervention strategies for foodborne infectious diseases. This article summarizes our studies on the transmission, ecology, pathogenesis and population genetics of Listeria monocytogenes, which we have used as model for a food- and milkborne pathogen that infects multiple hosts and also has considerable ability to survive and multiply in nonhost environments. Application of molecular subtyping tools in conjunction with phenotypic characterization of selected strains has allowed us to define distinct L. monocytogenes subtypes and clonal groups that appear to differ in relevant phenotypic characteristics that may affect their abilities to be transmitted through food systems. For example, a genetic group designated as lineage I has been shown to be not only more common among human listeriosis cases than among animal cases, but lineage I strains also appear to show an increased in vitro ability to spread intracellularly from host cell to host cell. These findings are consistent with the fact that while genetically diverse strains may be classified to one bacterial species, these strains often differ from one another in important genetic and phenotypic characteristics. I thus propose that evolutionary- and molecular subtyping-based definitions of bacterial subtypes and clonal groups will provide critical insight into the microbial ecology of dairy food systems, including not only foodborne pathogens, but also organisms important for dairy fermentation and spoilage.

Multi locus fingerprinting of Listeria monocytogenes by sequence-specific labeling of DNA probes combined with array hybridization

We have developed an alternative multi locus sequence typing (MLST) approach that targets the variable genetic changes directly in a DNA array format. Our approach is based on DNA array hybridization in combination with sequence-specific labeling of oligonucleotide probes. Listeria monocytogenes was chosen for the development and evaluation of the assay. The genes hlyA, iap, flaA, inlA and actA were targeted. Twenty-nine suitable probe regions were identified within these genes. The DNA array results from 32 different strains were compared to serotype and amplified fragment length polymorphism data. This comparison showed that our DNA array method gave good discrimination between the strains analyzed. In conclusion, the DNA array-based MLST method is a promising tool for fingerprint bacteria.

Amplified fragment length polymorphism (AFLP) analysis of Listeria monocytogenes

An agarose gel based single enzyme AFLP method using EcoR1 digestion of Listeria monocytogenes DNA was developed for epidemiological typing. The method was evaluated with 84 L. monocytogenes cultures, and results were compared with those obtained with serotyping, phage-typing and cadmium and arsenic resistance typing. The AFLP method was reproducible and 14 different banding patterns comprising between five and eight DNA fragments were produced. All except two of the AFLP patterns were serorype specific. Different AFLP patterns were recognised within serovar 4b (four patterns), 1/2a (two patterns), 1/2b (six patterns): single patterns were obtained from cultures of serovars 1/2c, 3a, 3b and 3c. There were associations with AFLP results and those from phage-typing and cadmium and arsenic resistance typing, although each method showed some independence. This preliminary evaluation suggests that this AFLP method will be useful for epidemiological typing of L. monocytogenes.

Similar Listeria monocytogenes pulsotypes detected in several foods originating from different sources

The purpose of the study was to obtain fingerprinting data of Listeria monocytogenes strains isolated in various foods to determine possible associations of strains with product type, producer, country or isolation time. Two hundred and ninety-five L. monocytogenes strains originating from food items of 41 producers of 10 countries were characterized by pulsed-field gel electrophoresis (PFGE) typing. Combination of AscI and ApaI macrorestriction patterns (MRP) yielded 66 different pulsotypes. Ten pulsotypes were common to two or more product types and 17 pulsotypes were detected in foods of more than one producer having no apparent association with each other. Similar pulsotypes of L. monocytogenes were recovered in products of different countries over several years. Some of the pulsotypes were recurrently recovered from the same product of the same producer, suggesting a possible persistence of these strains in the processing plant. However, some of the recurrently isolated L. monocytogenes pulsotypes were repeatedly found in products of several producers, which may indicate that persistent houseflora strains are not always producer-specific. Furthermore, the similarity of macrorestriction patterns expressed as clusters, based on the numerical analysis of macrorestriction patterns, was not found to correlate with product type, country, producer or year of isolation. Our data suggest a wide geographical and temporal distribution of a number of L. monocytogenes strains isolated in food products. The existence of similar L. monocytogenes strains in various food products of several producers should be considered if food strain fingerprint results are used to help trace the vehicles for infections.

Genetic homogeneity among Listeria monocytogenes strains from infected patients and meat products from two geographic locations determined by phenotyping, ribotyping and PCR analysis of virulence genes

Thirty Listeria monocytogenes isolates from human patients and foods originated from two different geographic locations without any epidemiological relations were analyzed for their genotypic and phenotypic virulence gene expressions and genetic relatedness. All strains contained virulence genes, inlA, inlB, actA, hlyA, plcA and plcB, with expected product size in PCR assay except for the actA gene. Some strains produced actA gene product of 268 and others 385 bp. Phenotypically, all were hemolytic but showed variable expressions of phospholipase activity. Ribotyping classified isolates into 12 different groups based on the similarity to DuPont Identification numbers (DID), which consisted primarily of clinical or food isolates or both. Cluster analysis also indicated possible existence of clones of L. monocytogenes that are found in food or human hosts or are evenly distributed between these two. Two isolates (F1 from food and CHL1250 from patient) had unique ribotype patterns that were not previously reported in the RiboPrinter database. This study indicates distribution of diverse L. monocytogenes strains in clinical and food environments. The isolates showed 92-99% genetic homogeneity, in spite of their origins from two different geographic locations and environments.

Combined ribotyping and random multiprimer DNA analysis to probe the population structure of Listeria monocytogenes

To improve our understanding of the genetic links between strains originating from food and strains responsible for human diseases, we studied the genetic diversity and population structure of 130 epidemiologically unrelated Listeria monocytogenes strains. Strains were isolated from different sources and ecosystems in which the bacterium is commonly found. We used rRNA gene restriction fragment length polymorphism analysis with two endonucleases and random multiprimer DNA analysis with seven oligonucleotide primers to study multiple genetic features of each strain. We used three clustering methods to identify genetic links between individual strains and to determine the precise genetic structure of the population. The combined results confirmed that L. monocytogenes strains can be divided into two major phylogenetic divisions. The method used allowed us to demonstrate that the genetic structure and diversity of the two phylogenetic divisions differ. Division I is the most homogeneous and can easily be divided into subgroups with dissimilarity distances of less than 0.30. Each of these subgroups mainly, or exclusively, contains a single serotype (1/2b, 4b, 3b, or 4a). The serotype 4a lineage appears to form a branch that is highly divergent from the phylogenetic group containing serotypes 1/2b, 4b, and 3b. Division II contains strains of serotypes 1/2a, 1/2c, and 3a. It exhibits more genetic diversity with no peculiar clustering. The fact that division II is more heterogeneous than division I suggests that division II evolved from a common ancestor earlier than division I. A significant association was found between division I and human strains, suggesting that strains from division I are better adapted to human hosts.

Expression of ActA, Ami, InlB, and listeriolysin O in Listeria monocytogenes of human and food origin

Expression of proteins involved in the adhesion of Listeria monocytogenes to mammalian cells or in the intracellular life cycle of this bacterium, including listeriolysin O (LLO), ActA, Ami, and InlB, was used to compare two populations of L. monocytogenes strains. One of the populations comprised 300 clinical strains, and the other comprised 150 food strains. All strains expressed LLO, InlB, and ActA. No polymorphism was observed for LLO and InlB. Ami was detected in 283 of 300 human strains and in 149 of 150 food strains. The strains in which Ami was not detected were serovar 4b strains. Based on the molecular weights of the proteins detected, the strains were divided into two groups with Ami (groups Ami1 [75% of the strains] and Ami2 [21%]) and into four groups with ActA (groups ActA1 [52% of the strains], ActA2 [18%], ActA3 [30%], and ActA4 [one strain isolated from food]). Logistic regression showed that food strains were more likely to belong to group ActA3 than human strains (odds ratio [OR] = 2.90; P = 1 x 10(-4)). Of the strains isolated from patients with non-pregnancy-related cases of listeriosis, bacteremia was predominantly associated with group Ami1 strains (OR = 1.89; P = 1 x 10(-2)) and central nervous system infections were associated with group ActA2 strains (OR = 3.04; P = 1 x 10(-3)) and group ActA3 strains (OR = 3.91; P = 1 x 10(-3)).

Comparative genetic characterization of Listeria monocytogenes isolates from human and animal listeriosis cases

Listeria monocytogenes isolates from human sporadic and epidemic cases (n=119) and from animal cases (n=76) were characterized by automated ribotyping and PCR-restriction fragment length polymorphism (PCR-RFLP) typing of the virulence genes actA and hly. This combination of typing methods differentiated 39 distinctive strains, each reflecting a unique combination of ribotypes, hly and actA alleles. Simpson's index of discrimination indicated a high discriminatory ability of ribotyping for both animal (0.867) and human isolates (0.857), which was further increased by the addition of hly and actA typing (0.916 and 0.904, respectively). Ribotype and hly allele data were further used to group isolates into three genetically distinct lineages. Each lineage is composed of several ribotype fragment subsets, each of which contains multiple ribotypes characterized by common ribotype fragments. To determine whether certain clones of L. monocytogenes show indications for unique pathogenic potential or host specificity, frequency distributions for five genetic characteristics (i.e. lineage, ribotype, ribotype fragment subset and hly and actA allele) were calculated for isolates from animal cases, human epidemic cases and human sporadic cases. Lineage III isolates were found less frequently in human cases (1 of 119 isolates) than in animal cases (8 of 76 isolates; P=0.003). These results suggest the possibility of host specificity for non-primate mammals among lineage III strains. In addition, lineage I strains were found more frequently among human cases than among animal cases (P<0.001). Among the eight hly alleles observed, hly allele 1 was more common among human isolates as compared to animal isolates (P=0.002). We also identified one ribotype (DUP-1030) which was significantly more common among animal isolates (P=0.005) and one ribotype (DUP-1038; lineage I) which was significantly more common among human epidemic isolates as compared to human sporadic isolates (P<0.001). These findings confirm the presence of clonal groups of L. monocytogenes, which appear to be characterized by unique virulence or host specificity patterns. This study also establishes baseline data describing the genetic diversity of human and animal L. monocytogenes isolates which can be utilized in future surveillance programmes to track the emergence of new strains.

Amplified fragment length polymorphism (AFLP) analysis of Listeria monocytogenes

AFLP analysis using four selective primers was performed on a set of 33 Listeria monocytogenes including strains from patients and foods implicated in outbreaks, human sporadic cases or foods. Strains were tested belonging to serovars 1/2a, 1/2b, 1/2c, 3b, and 4b. Using one of the primers, the AFLP technique generated 20 different sized DNA fragments. The 33 cultures segregated into 14 different patterns, each comprising 7-12 different fragments. Although the method was not sufficiently discriminatory for epidemiological typing, AFLP analysis reconfirmed the observation that L. monocytogenes comprises two major genetic groups: group 1 includes strains of serovars 1/2a and 1/2c, while group 2 serovars 1/2b, 3b and 4b.

Listeria monocytogenes in pork slaughtering and cutting plants. Use of RAPD, PFGE and PCR-REA for tracing and molecular epidemiology

In order to determine the origin of pork cuts contamination by Listeria monocytogenes, 287 isolates, collected from five French pork slaughtering and cutting plants, from live pigs to pork cuts, were characterised using three molecular typing methods: random amplification of polymorphic DNA (RAPD) carried out with five different primers, genomic macrorestriction using ApaI with pulsed-field gel electrophoresis (PFGE) and a PCR-restriction enzyme analysis (PCR-REA) based on the polymorphism existing within the inlA and inlB genes. Results obtained from RAPD and PFGE were closely related and distinguished respectively 17 RAPD types (r1-r17) and 17 PFGE types (a1-a17) among the 287 isolates, whereas the PCR-REA analysis only yielded two profiles (p1 and p2). Considering the combined results obtained with the three molecular typing methods, 19 Listeria monocytogenes genotypes (1-19) were distinguished. Serotyping led at least four serotypes being distinguished: 1/2a, 3a, 1/2c and 3c. The application of genotyping identified the predominance of a Listeria monocytogenes strain of type (1) and other very closely related ones (5, 9, 10, 12, 13, 14, 16 and 19) which were present on pork as well as in the environment within the five investigated plants. This study also pointed out the presence of these closely related Listeria monocytogenes strains over a 1-year period in the environments of two plants, even after cleaning and disinfection procedures. This highlights the possibility for some Listeria monocytogenes strains to persist in pork processing environments and raises the problem of the efficiency of cleaning and disinfection procedures used in pork slaughterhouses, chilling and cutting rooms.

Rapid detection of Listeria monocytogenes by PCR-ELISA

A rapid detection system specific for Listeria monocytogenes based upon the polymerase chain reaction was developed. The specificity of the primers and the probe annealing to the coding region of the mpl gene proved positive with the DNA from a total of 103 L. monocytogenes strains, while DNA from another 73 Listeria and non-Listeria strains tested negative. To facilitate detection with large numbers of samples, a microtitre plate assay was established with biotinylated probes. Use of a standard DNA prevented false-negative results when used as an internal amplification control in the PCR-ELISA. As the described method required approximately 5-6 h to be completed it may prove useful in the detection of L. monocytogenes in food.

Division of Listeria monocytogenes serovar 1/2a strains into two groups by PCR and restriction enzyme analysis

Altogether, 100 strains of Listeria monocytogenes serovar 1/2a isolated from humans, animals, food, and the environment were typed by a combination of PCR and restriction enzyme analysis (REA). A PCR product of 2,916 bp, containing the downstream end of the gene inlA (955 bp), the space between inlA and inlB (85 bp), and 1,876 bp of the gene inlB, was cleaved with the enzyme AluI, and the fragments generated were separated by gel electrophoresis. By this method two different cleavage patterns were obtained. Seventy of the 100 strains shared one restriction profile, and the remaining 30 strains shared the second one. No relation was found between the types differentiated by PCR-REA and the origins of the strains.

General stress transcription factor sigmaB and its role in acid tolerance and virulence of Listeria monocytogenes

The gene encoding the general stress transcription factor sigmaB in the gram-positive bacterium Listeria monocytogenes was isolated with degenerate PCR primers followed by inverse PCR amplification. Evidence for gene identification includes the following: (i) phylogenetic analyses of reported amino acid sequences for sigmaB and the closely related sigmaF proteins grouped L. monocytogenes sigmaB in the same cluster with the sigmaB proteins from Bacillus subtilis and Staphylococcus aureus, (ii) the gene order in the 2, 668-bp portion of the L. monocytogenes sigB operon is rsbU-rsbV-rsbW-sigB-rsbX and is therefore identical to the order of the last five genes of the B. subtilis sigB operon, and (iii) an L. monocytogenes sigmaB mutant had reduced resistance to acid stress in comparison with its isogenic parent strain. The sigB mutant was further characterized in mouse models of listeriosis by determining recovery rates of the wild-type and mutant strains from livers and spleens following intragastric or intraperitoneal infection. Our results suggest that sigmaB-directed genes do not appear to be essential for the spread of L. monocytogenes to mouse liver or spleen at 2 and 4 days following intragastric or intraperitoneal infection.

Ribotypes and virulence gene polymorphisms suggest three distinct Listeria monocytogenes lineages with differences in pathogenic potential

A total of 133 Listeria monocytogenes isolates were characterized by ribotyping and allelic analysis of the virulence genes hly, actA, and inlA to uncover linkages between independent phylogenetic and specific virulence markers. PCR-restriction fragment length polymorphisms revealed 8 hly, 11 inl4, and 2 actA alleles. The combination of these virulence gene alleles and ribotype patterns separated L. monocytogenes into three distinct lineages. While distinct hly and inlA alleles were generally found to cluster into these three lineages, actA alleles segregated independently. These three phylogenetic lineages were confirmed when 22 partial actA DNA sequences were analyzed. The clinical history of the L. monocytogenes strains showed evidence for differences in pathogenic potential among the three lineages. Lineage I contains all strains isolated during epidemic outbreaks of listeriosis, while no human isolates were found in lineage III. Animal isolates were found in all three lineages. We found evidence that isolates from lineages I and III have a higher plaquing efficiency than lineage II strains in a cell culture assay. Strains from lineage III also seem to form larger plaques than strains from lineage II. A distinctive ribotype fragment and unique 16S rRNA gene sequences furthermore suggest that lineage III might represent a L. monocytogenes subspecies. None of the 20 human isolates available but 11% of our animal isolates were grouped in this lineage, indicating that strains in this lineage might have reduced virulence for humans.

A RE-PCR method to distinguish Listeria monocytogenes serovars

Strains (107) of L. monocytogenes were tested with a PCR-restriction enzyme analysis with two new original primers. A segment of 1395 bp containing the entire iap gene in L. monocytogenes was amplified by the PCR technique. The PCR product was cleaved with the restriction enzymes HindIII and RsaI, and the fragments generated were separated by gel electrophoresis. Two groups of serovars were obtained: one group contained serovars 1/2a and 1/2c, the other group contained serovars 1/2b, 3b and 4b. The PCR-restriction enzyme analysis method described in this paper could be a useful tool for the unambiguous division of L. monocytogenes into two serovar groups, and it could be used to study the evolution of different serotypes and groups of serotypes in foods produced in the same processing plant and processed during the same month. The RE-PCR method used can give a rapid confirm at the subgroup level in the laboratory of an epidemiological association between human disease and suspected sources of contaminated food.

A Gly145Ser substitution in the transcriptional activator PrfA causes constitutive overexpression of virulence factors in Listeria monocytogenes

Virulence genes in Listeria monocytogenes are coordinately expressed under the control of the transcriptional activator PrfA, encoded by prfA, a member of the cyclic AMP (cAMP) receptor protein (CRP)/FNR family of bacterial regulators. Strain P14-A is a spontaneous mutant of L. monocytogenes serovar 4b which produces elevated levels of virulence factors (M. T. Ripio, G. Domínguez-Bernal, M. Suárez, K. Brehm, P. Berche, and J. A. Vázquez-Boland, Res. Microbiol. 147:371-384, 1996). Here we report that P14-A and other variant strains with the same phenotype carry a point mutation in codon 145 of prfA, leading to a Gly-->Ser substitution. trans-complementation experiments with PrfA-deficient mutants demonstrated that the Gly145Ser prfA allele causes overexpression of virulence factors in L. monocytogenes, to the levels found in the virulence factor-overexpressing variants. In strain P14-A with a chromosomal Glyl45Ser prfA background, transcription of prfA and of PrfA-dependent virulence genes remained constitutively high under culture conditions in which virulence factor expression is downregulated in wild-type L. monocytogenes. The Glyl45Ser substitution is located in a PrfA stretch (residues 141 to 151) showing high sequence similarity to the D alpha-helix of CRP. Interestingly, well-characterized crp* mutations, which make CRP functionally active in the absence of cAMP, map in this region (i.e., Gly141Ser and Ala144Thr substitutions). By analogy with the CRP model, the phenotype conferred to L. monocytogenes by the Gly145Ser substitution in PrfA could be due to the mutant regulatory protein being locked in a transcriptionally active, cofactor-independent conformational state. Our observations allow the construction of a model for PrfA-dependent virulence gene regulation in which the levels of virulence factor expression depend primarily on the conformational state of the PrfA protein, which alternates between active and inactive forms according to its interaction with an environmentally regulated signal molecule or cofactor.

Antibodies to Listeria monocytogenes

Listeria monocytogenes is one of the leading foodborne pathogens and has been implicated in numerous outbreaks in the last 2 decades. Immunocompromised populations are usually the most susceptible to Listeria infections. Although the pathogenic mechanism is a complex process, significant progress has been made in unravelling the mechanism in recent years. It is now clear that numerous extracellular and cell-associated proteins, such as internalin, listeriolysin, actin polymerization protein, phospholipase, metalloprotease, and possibly p60 proteins, are essential for L. monocytogenes entry into mammalian cells, survival inside the phagosome, escape into the cytoplasm, and cell-to-cell spread. Other proteins may be responsible for growth and physiology or to maintain the structural integrity of the bacteria. Monoclonal and polyclonal antibodies have been developed against many of those antigens or their synthetic derivatives that have helped greatly to determine the structure and function of these antigens. The antibodies were also used for the diagnosis and detection, immunocytochemical staining, and serotyping of Listeria. Humoral immune response to live L. monocytogenes cells was examined in naturally or experimentally infected hosts. Studies revealed that only extracellular antigens induced the humoral response, whereas cell-associated antigens had apparently no response. It is speculated that during the occasional bacteremic phase, L. monocytogenes releases extracellular antigens that are then processed by the immune system for antibody production. As L. monocytogenes is an intracellular pathogen, the cell-associated antigens are not persistent in the blood circulation and thus fail to stimulate the humoral immune response.

Identification of a CD4+ T cell-stimulating antigen of pathogenic bacteria by expression cloning

Identifying the immunogenic proteins that elicit pathogen-specific T cell responses is key to rational vaccine design. While several approaches have succeeded in identifying major histocompatibility complex (MHC) class I bound peptides that stimulate CD8+ T cells, these approaches have been difficult to extend to peptides presented by MHC class II molecules that stimulate CD4+ T cells. We describe here a novel strategy for identifying CD4+ T cell-stimulating antigen genes. Using Listeria monocytogenes-specific, lacZ-inducible T cells as single-cell probes, we screened a Listeria monocytogenes genomic library as recombinant Escherichia coli that were fed to macrophages. The antigen gene was isolated from the E. coli clone that, when ingested by the macrophages, allowed generation of the appropriate peptide/MHC class II complex and T cell activation. We show that the antigenic peptide is derived from a previously unknown listeria gene product with characteristics of a membrane-bound protein.

Differentiation of epidemic-associated strains of Listeria monocytogenes by restriction fragment length polymorphism in a gene region essential for growth at low temperatures (4 degrees C)

The growth of Listeria monocytogenes in food stored in the cold has often been implicated in outbreaks of listeriosis. Many subtyping schemes have suggested that epidemic-associated strains belong to a unique genetic group. It has not yet been possible, however, to identify molecular or bacteriologic markers unique to epidemic-associated strains. Recently we cloned three genes of L. monocytogenes, ltrA, ltrB, and ltrC, which are essential for growth at low temperatures (4 degrees C). The use of a 1.2-kb PstI fragment derived from ltrB as a probe in Southern blots of HindIII-digested DNA revealed three hybridization patterns: the first (a 5.0-kb band) was observed in strains of serotypes 4b, 1/2b, and 3b; the second (a 3.1-kb band) was seen in strains of serotypes 1/2a, 3a, 1/2c, and 3c; and the third (a 9.5-kb band) was characteristic of epidemic-associated serotype 4b strains. These and other data suggest that probes derived from this gene region that is essential for growth at low temperatures can be useful molecular tools for the subtyping of strains implicated in food-borne listeriosis.

Division of Listeria monocytogenes serovar 4b strains into two groups by PCR and restriction enzyme analysis

Altogether, 133 strains of Listeria monocytogenes serovar 4b were investigated. A segment of 2,916 bp containing parts of the two genes inlA and inlB in L. monocytogenes was amplified by the PCR technique. The PCR product obtained was cleaved with the restriction enzyme AluI, and the fragments generated were separated by gel electrophoresis, leading to two distinct groups: PCR-restriction enzyme analysis groups I and II, containing 37 and 96 strains, respectively. The PCR-restriction enzyme analysis method described in this paper could be a useful tool for the subtyping of L. monocytogenes serovar 4b strains.

The identification of bacterial gene expression differences using mRNA-based isothermal subtractive hybridization

We describe a method for isolating and determining differences in gene expression between related bacterial strains. The method is based upon differences in mRNA expression. To demonstrate this procedure, cDNA generated from total RNA of Listeria monocytogenes serotype 1/2a was hybridized to total RNA from a Tn916 mutant of serogroup 1/2a (M3) that was deficient in the production of listeriolysin O, the product of the hly gene. The single-stranded cDNA fragments remaining after hybridization represent the difference in expressed genes between the two strains. These subtraction products were used as hybridization probes to identify the corresponding hly gene in a Southern hybridization.

Identification and classification of Listeria by two-dimensional protein mapping

The cellular proteins of 29 Listeria strains belonging to different species and serotypes were analysed by two-dimensional (2-D) electrophoresis with the help of a computerized 2-D gel analysis system. The protein patterns were similar among strains within a Listeria species, but were different from one species to another. The comparative analysis of these protein maps enabled us to find specific proteins and to determine the genetic relatedness among Listeria spp. strains. The cluster analysis based on protein mapping showed a division between species and a clear separation of L. monocytogenes from other Listeria species. Inside L. monocytogenes the strains were divided into two main clusters in correlation with flagellar antigenic structures; this is in concordance with the results that have been found on the basis of multilocus enzyme electromorphs or DNA-restriction patterns. This technique enabled us to subtype the strains sharing the same serovar or the same lysovar. Because of its independence and high discriminating capacity, the 2-D protein mapping technique might provide a powerful tool for the identification and classification of Listeria strains.

Monoclonal antibodies with a high degree of specificity for Listeria monocytogenes serotype 4b

Strains of Listeria monocytogenes serotype 4b account for a large fraction of sporadic listeriosis cases, as well as all major food-borne epidemics attributed to this pathogen. We have identified a set of three monoclonal antibodies which showed a high degree of specificity for strains of L. monocytogenes serotype 4b. Two of these antibodies (c74.33 and c74.180, isotypes immunoglobulin M [IgM] and IgG3, respectively) recognized all serotype 4b strains, whereas antibody c74.22 (isotype IgG1) failed to recognize certain epidemic-associated strains. The corresponding antigens were located on the surface of the bacteria and were expressed following bacterial growth in different media and over a wide range of temperatures (4, 22, and 37 degrees C). Heating L. monocytogenes cells at 80,90, or 100 degrees C abolished reactivity for c74.22 but not for c74.33 MAb. These MAbs were negative for all of the non-Listeria strains tested, including representatives of several gram-negative and gram-positive species. The surface antigen recognized by c74.22 appeared to be associated with the ability of the bacteria to enter (invade) mammalian cells in culture.

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

Clamped homogeneous electric field (CHEF) electrophoresis was optimized for genomic analyses of Listeria monocytogenes. Various human, animal, food, and environmental isolates, as well as strains representing other Listeria species, were separately digested with rarely cutting endonucleases. Of 176 L. monocytogenes strains analyzed, the enzymes AscI and ApaI established 63 and 72 unique restriction endonuclease digestion profiles (REDP), respectively. The 22 non-L. monocytogenes strains exhibited 18 AscI and 19 ApaI unique REDP. Statistical analyses of REDP information using the Dice coincidence index and principal component analysis revealed two distinct genomic divisions of L. monocytogenes that also correlated with the flagellar (H) antigen type: division I contained serovar 1/2a, 1/2c, 3a, and 3c stains and division II contained serovar 1/2b, 3b, 4b, 4d, and 4e strains. Division I isolates digested with ApaI were further grouped into cluster IA (serovar 1/2c and 3c) and cluster IB (serovar 1/2a and 3a) strains. Likewise, division II isolates digested with ApaI were further grouped into cluster IIA (serovar 1/2b and 3b) and cluster IIB (serovar 4b, 4d, and 4e) strains. These data indicate that genotypic data generated by CHEF can be directly related to phenotypic data generated by serotyping for establishing the overall relatedness of isolates. Moreover, these data further substantiate that CHEF analysis is a reproducible and highly discriminating method for characterizing L. monocytogenes strains at the molecular level.

Analysis of fluorescent pseudomonads based on 23S ribosomal DNA sequences

The regions from positions 1391 to 1545 and 1620 to 1865 (Escherichia coli numbers) of the 23S ribosomal DNA sequences have been analyzed for a number of Pseudomonas fluorescens and P. putida isolates. Variability was observed only in three smaller regions from positions 1484 to 1508, 1531 to 1542, and 1714 to 1748, corresponding to helices 58, 59, and 63, respectively, where up to 53% dissimilarity was found. Sequence analysis did not allow clear distinctions among P. fluorescens biovars, P. chlororaphis, and P. putida biovar B.

Diagnosis and epidemiological association of Listeria monocytogenes strains in two outbreaks of listerial encephalitis in small ruminants

Two outbreaks of epizootic listerial encephalitis, one in sheep and one in goats, were investigated through pathology, microbiology, and DNA amplification-based techniques. Efforts were made to survey the diversity of Listeria monocytogenes strains in the silage consumed by affected animals and to verify the causal relationship between silage and disease outbreak. In both outbreaks, L. monocytogenes was isolated from silage and brain tissue samples. Random amplified polymorphic DNA patterns revealed two distinct L. monocytogenes strains, one of which was identical to the sheep brain isolate, in the silage associated with the outbreak in sheep. Three brain isolates and one silage isolate, all of which had different random amplified polymorphic DNA patterns, were found in the outbreak involving goats. All isolates from both outbreaks were indistinguishable in an in vitro assay for cell-to-cell spread and growth in macrophages. All brain isolates from the goat outbreak had identical intracellular ActA patterns, which were different from the pattern for the silage isolate. While the sheep brain isolate had an ActA pattern different from that of the corresponding silage isolate, the patterns for the brain isolates from the two outbreaks were not identical. This survey demonstrates the diversity of L. monocytogenes in silage and suggests the existence of one or more selective processes by which certain strains are more prone to give rise to disease.

Characterization of two putative Listeria monocytogenes genes encoding polypeptides homologous to the sensor protein CheA and the response regulator CheY of chemotaxis

The nucleotide sequence of a region located downstream of the Listeria monocytogenes flagellin gene, flaA, has been determined. DNA sequence analysis revealed the presence of two open reading frames with a potential to encode polypeptides of 13.1 and 68.7 kDa, respectively. The deduced polypeptides show a high degree of identity to the chemotactic proteins, CheY and CheA, respectively, from Bacillus subtilis and Escherichia coli. Moreover, significant features of CheY and CheA are conserved in the L. monocytogenes homologues. The high degree of conservation suggests that the polypeptides are involved in signal transduction controlling chemotaxis in L. monocytogenes and consequently the putative genes are named cheY and cheA. Northern blot and primer extension analysis suggested that cheY and cheA are transcribed as a bicistronic unit and that the transcription is thermoregulated.

Cloning and characterization of a gene encoding flagellin of Listeria monocytogenes

The gene, flaA, encoding the flagellin protein of Listeria monocytogenes (strain 12067) has been isolated from an expression library in Escherichia coli using a flagellin-specific monoclonal antibody. DNA sequence analysis of a positive clone revealed the presence of an open reading frame of 287 amino acid residues with a calculated molecular mass of 30.4 kDa. Comparison of this sequence with flagellins from other bacteria showed a significant degree of homology in both the N- and C-terminal parts of the protein. The flagellin mRNA was determined to be 1 kb in size, which is the expected size for a monocistronic mRNA, and the temperature-dependent expression of flagellin was found to be regulated at the transcriptional level. Southern blot analysis, using the flagellin gene as probe, indicated that L. monocytogenes can be divided into two groups. These groups correspond to the flagellar antigens AB and ABC, respectively, as well as to the two types of L. monocytogenes based on the DNA sequence of the listeriolysin gene.

Is any strain of Listeria monocytogenes detected in food a health risk?

Listeria spp. have been isolated from various food items. This fact does not mean in any case a true health risk. A balanced appraisal should be based on quantitative as well as qualitative aspects. Actually, there is still an open debate whether a limited number of Listeria has to be tolerated at least in certain food items. In addition, the pathogenic potency of an isolate may be put into account. Pathogenicity of various Listeria spp. definitely varies. Most Listeria spp., except Listeria monocytogenes, can be regarded as harmless to man. Also, not all strains of L. monocytogenes are pathogenic: rough variants possess only reduced virulence; non-hemolytic mutants have completely lost their pathogenic potency. Furthermore, several other virulence factors may be lost under natural conditions, so that among the majority of hemolytic, pathogenic isolates there may be others which are non-pathogenic or of low virulence only. Unfortunately, these strains actually cannot be recognized and characterized by common laboratory tests, so that animal pathogenicity seems to be the only way to get a final conclusion on the health risk of an isolate of L. monocytogenes from any food. The problem raised by this is which animal test is able to predict a true health risk either for normal hosts or for immunocompromised patients?

Immunoaffinity purification, stabilization and comparative characterization of listeriolysin O from Listeria monocytogenes serotypes 1/2a and 4b

We developed a simple and highly effective procedure for stabilizing the haemolytic activity of listeriolysin O (LLO) from Listeria monocytogenes after immunoaffinity purification. The haemolytic activity of LLO was stabilized by eluting it directly into tubes containing an alkaline buffer (5 mM lysine, 140 mM KCl, 50% ethylene glycol, pH 11.5). The purified LLO retained 100% of its haemolytic activity after 6 weeks of storage at -20 degrees C. LLO purified from a strain of L. monocytogenes serotype 1/2a (ATCC 43249) and LLO purified from a strain of L. monocytogenes serotype 4b (F 2365) isolated from a Mexican-style cheese, showed no significant differences in pH and temperature stability. When incubated in buffers at pH values from 4 to 12 at 4 degrees C and 25 degrees C, LLO from serotypes 1/2a and 4b retained maximal haemolytic activity at pH 8 after 4 h of incubation. LLO from both serotypes lost their haemolytic activity after incubation at 50 degrees C for 25 min.