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

Ribotype analysis of strain distribution in Listeria monocytogenes

Changes in the temporal and spatial patterns of strain distribution for the foodborne pathogen Listeria monocytogenes were studied by ribotyping using the Qualicon Riboprinter system. Ribotype patterns were obtained by using the restriction enzymes EcoRI and PvuII for 72 isolates of L. monocytogenes recovered from smoked salmon samples over a period of 3 years. Each pattern was classified both by comparison to a pattern library and by comparison among the 72 isolate patterns. Eleven EcoRI-based ribogroups and 16 PvuII groups were identified. Eight of the 11 EcoRI ribogroups were found in isolates obtained over a period of >12 months, and 75% of the EcoRI ribogroups that were found in more than one food sample were distributed nationally. Within the set of isolates, there were 26 instances where more than one isolate was obtained from a single food sample. In 35% of these instances, the co-isolates produced different ribotype patterns, indicating that multiple strains of L. monocytogenes commonly coexist in the same environment. Overall, these data indicate that the population of L. monocytogenes consists of a number of widely dispersed strains with little geographic or temporal stratification.

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.

Physical and genetic map of the Listeria monocytogenes EGD serotype 1/2a chromosome

Listeria monocytogenes is a facultative intracellular pathogen responsible for both invasive and non-invasive food-borne illness in animals and humans. In this study, macrorestriction analysis following pulsed-field gel electrophoresis was used to show that Listeria monocytogenes serovar 1/2a strain EGD has a single chromosome containing eight NotI fragments of 1100, 850, 365, 320, 275, 40, 30 and 20 kb in size and 11 AscI fragments of 860, 470, 410, 360, 320, 250, 110, 80, 50, 30 and 20 kb. The total genome therefore comprises 3000 +/- 50 kb. The creation of a physical and genetic map of the Listeria genome was achieved by generating NotI linking clones and their use in subsequent hybridisation analysis. Using isogenic mutants harbouring additional artificial NotI restriction sites, we were able to precisely map the positions of all currently known virulence genes on the chromosome.

Occurrence and typing of Listeria monocytogenes strains in retail vacuum-packed fish products and in a production plant

One hundred and ten samples of ready-to-eat, vacuum-packed, smoked and cold-salted fish products were collected from retail outlets in southern Finland during 1996 for examination of the occurrence and level of Listeria monocytogenes. The samples originated from 12 producers. Positive samples with levels exceeding 100 CFU/g were encountered mainly in one of the producers (no. 8). Therefore, 200 samples from the plant and the products of this producer were studied during August-September 1996 and May-September 1997, as well as 55 samples from the six fish farms providing raw material fish to this plant, during September 1997-January 1998. The isolates were characterised by serotyping and pulsed-field gel electrophoresis (PFGE). L. monocytogenes was isolated in 20% (22/110) of the samples from the retail market, originating from 6 producers. Ten of these positive samples contained L. monocytogenes at > 100 CFU/g (maximum 1.37 X 10(4) CFU/g). Seventeen percent (5/30) of cold-smoked and 50% (16/32) of cold-salted rainbow trout samples were contaminated. Only one hot-smoked fish product (2%) was found to be positive by enrichment. Nineteen (86%) of the strains isolated from the retail samples belonged to serovar 1/2a and three (14%) to serovar 4b. In further studies the production line of plant no. 8 was found to be contaminated. All of isolates from up until autumn, 1997 both the products and the production plant were serovar 1/2a; thereafter one strain of 4b and one of 1/2 (H-antigen untypeable) were isolated from the plant. The samples from raw material fish were all negative for L. monocytogenes. The samples from retail market fell into seven PFGE types. Five and nine PFGE types, respectively, were found from the products and the plant of producer no. 8. PFGE type A was detected from the retail products of four producers and was also dominant among the isolates from production plant no. 8. PFGE type A was the only one found repeatedly from skinning, salting and slicing units as well as from products throughout the whole period. PFGE proved to be a powerful tool for studying contamination points and routes in the production plant. The measures based on hazard analysis critical control points (HACCP) program resulted in L. monocytogenes negative samples at production plant no. 8 from the beginning of January 1998.

Characterisation of Listeria ivanovii isolates from the UK using pulsed-field gel electrophoresis

Forty-three Listeria ivanovii isolates were collected in the UK between 1991 and 1997 from: 35 animal infections; two human infections; five foods; and one environmental source. A further two type strains of L. ivanovii (subsp. ivanovii and subsp. londoniensis) were obtained from a culture collection. These bacteria were characterised by conventional phenotypic methods and by pulsed-field gel electrophoresis (PFGE) using ApaI and SmaI. Forty-two of the isolates from the UK were identified as L. ivanovii subsp. ivanovii and the remaining culture as L. ivanovii subsp. londoniensis. Six and four PFGE profiles were obtained using ApaI and SmaI digestion respectively; six composite profiles were obtained combining the results for both enzymes. The PFGE profile of the UK L. ivanovii subsp. londoniensis (isolated from processed shrimps) was similar to the type strain of this subspecies and differed from all of the L. ivanovii subsp. ivanovii tested. The majority of isolates (38 out of 45) belonged to one profile showing that the UK population of this bacterium is much less genetically diverse than similar studies have shown for Listeria monocytogenes.

Typing of Listeria monocytogenes strains by repetitive element sequence-based PCR

Listeria monocytogenes strains possess short repetitive extragenic palindromic (REP) elements and enterobacterial repetitive intergenic consensus (ERIC) sequences. We used repetitive element sequence-based PCR (rep-PCR) to evaluate the potential of REP and ERIC elements for typing L. monocytogenes strains isolated from humans, animals, and foods. On the basis of rep-PCR fingerprints, L. monocytogenes strains were divided into four major clusters matching origin of isolation. rep-PCR fingerprints of human and animal isolates were different from those of food isolates. Computer evaluation of rep-PCR fingerprints allowed discrimination among the tested serotypes 1/2a, 1/2b, 1/2c, 3b, and 4b within each major cluster. The index of discrimination calculated for 52 epidemiologically unrelated isolates of L. monocytogenes was 0.98 for REP- and ERIC-PCR. Our results suggest that rep-PCR can provide an alternative method for L. monocytogenes typing.

Longitudinal study of Escherichia coli O157:H7 dissemination on four dairy farms in Wisconsin

A 14-month longitudinal study was conducted on four dairy farms (C, H, R, and X) in Wisconsin to ascertain the source(s) and dissemination of Escherichia coli O157:H7. A cohort of 15 heifer calves from each farm were sampled weekly by digital rectal retrieval from birth to a minimum of 7 months of age (range, 7 to 13 months). Over the 14 months of the study, the cohort heifers and other randomly selected cattle from farms C and H tested negative. Farm R had two separate periods of E. coli O157:H7 shedding lasting 4 months (November 1995 to February 1996) and 1 month (July to August 1996), while farm X had at least one positive cohort animal for a 5-month period (May to October 1996). Heifers shed O157:H7 strains in feces for 1 to 16 weeks at levels ranging from 2.0 x 10(2) to 8.7 x 10(4) CFU per g. E. coli O157:H7 was also isolated from other noncohort cattle, feed, flies, a pigeon, and water associated with the cohort heifers on farms R and/or X. When present in animal drinking water, E. coli O157:H7 disseminated through the cohort cattle and other cattle that used the water source. E. coli O157:H7 was found in water at < 1 to 23 CFU/ml. Genomic subtyping by pulsed-field gel electrophoresis demonstrated that a single O157:H7 strain comprised a majority of the isolates from cohort and noncohort cattle, water, and other positive samples (i.e., from feed, flies, and a pigeon, etc.) on a farm. The isolates from farm R displayed two predominant XbaI restriction endonuclease digestion profiles (REDP), REDP 3 and REDP 7, during the first and second periods of shedding, respectively. Six additional REDP that were > or = 89% similar to REDP 3 or REDP 7 were identified among the farm R isolates. Additionally, the REDP of an O157:H7 isolate from a heifer on farm R in 1994 was indistinguishable from REDP 3. Farm X had one O157:H7 strain that predominated (96% of positive samples had strains with REDP 9), and the REDP of an isolate from a heifer in 1994 was indistinguishable from REDP 9. These results suggest that E. coli O157:H7 is disseminated from a common source on farms and that strains can persist in a herd for a 2-year period.

Genomic comparisons and Shiga toxin production among Escherichia coli O157:H7 isolates from a day care center outbreak and sporadic cases in southeastern Wisconsin

Contour-clamped homogeneous electric field pulsed-field gel electrophoresis (CHEF-PFGE) was used to compare Wisconsin isolates of Escherichia coli O157:H7, including 39 isolates from a 1994 day care center outbreak, 28 isolates from 18 individuals from the surrounding geographic area with sporadic cases occurring during the 3 months before the outbreak, and 3 isolates, collected in 1995, from patients with hemolytic-uremic syndrome (HUS) who were from eastern Wisconsin counties other than those inhabited by the day care center and sporadic-case individuals. The technique of CHEF-PFGE using XbaI identified seven highly related restriction endonuclease digestion profiles (REDPs) (93 to 98% similarity) among the 39 day care center isolates and nine XbaI REDPs (63 to 93% similarity) among the 28 isolates from sporadic-case individuals, including REDP 33, which was exhibited by both day care and sporadic-case isolates. PFGE analyses of sequential E. coli O157:H7 isolates from symptomatic day care center attendees revealed that the REDPs of 25 isolates from eight patients were indistinguishable whereas the REDPs of 2 of 6 isolates from two patients differed slightly (93 to 95% similarity). The REDPs of the three isolates from 1995 HUS patients were 78 to 83% similar, with REDP 26 being exhibited by one HUS-associated isolate and an isolate from one day care attendee who did not develop HUS. The genes for both Shiga toxins I and II (stx1 and stx2, respectively) were detected in all but one isolate (sporadic case), and Shiga toxin production by the day care center isolates was not significantly different from that of the other isolates, including the three HUS-associated isolates. Analyses of E. coli O157:H7 isolates from both the day care center outbreak and sporadic cases by CHEF-PFGE permitted us to define the REDP variability of an outbreak and geographic region and demonstrated that the day care center outbreak and a HUS case in 1995 were caused by E. coli O157:H7 strains endemic to eastern Wisconsin.

Genomic analysis of Clostridium botulinum group II by pulsed-field gel electrophoresis

Pulsed-field gel electrophoresis (PFGE) was optimized for genomic analyses of Clostridium botulinum (non-proteolytic) group II. DNA degradation problems caused by extracellular DNases were overcome by fixation of cells with formaldehyde prior to isolation. A rapid (4-h) in situ DNA isolation method was also assessed and gave indistinguishable results. Genomic DNA from 21 strains of various geographical and temporal origins was digested with 15 rare-cutting restriction enzymes. Of these, ApaI, MluI, NruI, SmaI, and XhoI gave the most revealing PFGE patterns, enabling strain differentiation. Twenty strains yielded PFGE patterns containing 13 pulsotypes. From summation of MluI, SmaI, and XhoI restriction fragments, the genome size of C. botulinum group II was estimated to be 3.6 to 4.1 Mb (mean +/- standard deviation = 3,890 +/- 170 kb). The results substantiate that after problems due to DNases are overcome, PFGE analysis will be a reproducible and highly discriminating epidemiological method for studying C. botulinum group II at the molecular level.

DNA fingerprinting of Listeria monocytogenes using enterobacterial repetitive intergenic consensus (ERIC) motifs-polymerase chain reaction/capillary electrophoresis

The molecular technique, enterobacterial repetitive intergenic consensus (ERIC)-polymerase chain reaction (PCR) produces genomic DNA fingerprint that discriminate bacterial species and strains. This technique was applied to the characterization of Listeria monocytogenes, an important food-borne pathogen implicated in numerous cases of listeriosis. The ERIC-PCR resulted in distinct DNA fingerprinting patterns of all L. monocytogene serotypes and Listeria species. Analysis of the genomic DNA fingerprints was accomplished using capillary electrophoresis (CE), an alternative technique to the conventional agarose gel method. The optimization of CE conditions (electrokinetic injection, applied voltage) resulted in the resolution of amplified DNA fragments up to 1000 bp. Comparisons of electropherograms provided genomic fingerprint templates which could be further used for supplementary information. The ERIC-PCR method coupled to CE provides a rapid technique in differentiating bacterial spp., and may contribute relevant information in food-borne outbreak studies.

Single-strand conformation polymorphism (SSCP) analysis of Listeria monocytogenes iap gene as tool to detect different serogroups

PCR-single-strand conformation polymorphism (PCR-SSCP) analysis is a convenient technique for the detection of mutations. As the mobility of single-stranded DNA is sequence-dependent it could therefore be used to determine serotype-related sequence variations in Listeria monocytogenes. Sero-specific patterns were observed in different L. monocytogenes serogroups.

Construction of the temperature-sensitive vectors pLUCH80 and pLUCH88 for delivery of Tn917::NotI/SmaI and use of these vectors to derive a circular map of Listeria monocytogenes Scott A, a serotype 4b isolate

A physical map of Listeria monocytogenes Scott A was generated by the pulsed-field technique of contour-clamped-homogeneous-electric-field (CHEF) electrophoresis. The circular genome of this serotype 4b strain contains 12 AscI fragments (38 to 790 kb), 5 NotI fragments (55 to 1,400 kb), 3 SrfI fragments (110, 1,110, and 2,000 kb), and 2 SfiI fragments (1,320 and 1,920 kb). Summation of individually sized fragments derived by digestion of Scott A genomic DNA with each of these four enzymes provided an average estimated genome length of 3,210 +/- 60 kb. Efforts to assemble the macrorestriction map benefited greatly from the construction and use of pLUCH80 and pLUCH88, temperature-sensitive vectors for delivering transposon Tn917::NotI/SmaI to the chromosome of Scott A. As another component of this study, the positions of four known virulence genes (inlA, mpl, hly, and prf) and three L. monocytogenes-specific sequences (lisM44, lisM51, and lisM52) were localized on the physical map of Scott A by hybridization. Probes prepared from lisM44, lisM51, and the four virulence genes hybridized within a cluster on a 150-kb fragment of the Scott A genome that overlaps part of the NotI-B and AscI-D fragments. The lisM52 probe hybridized with the AscI-F2 (120-kb) fragment of Scott A, which is separated from the NotI-B-AscI-D region by about 300 kb. These results established the first physical and genetic map of a serotype 4b strain of L. monocytogenes and provided further insight on this important food-borne pathogen at the genome level.

Sporadic case of listeriosis associated with the consumption of a Listeria monocytogenes-contaminated 'Camembert' cheese

Listeria monocytogenes is an intracellular gram-positive organism responsible for severe infections in both humans and animals. Whereas the food-borne transmission of listeriosis was demonstrated in several outbreaks, most cases of listeriosis occur sporadically and are rarely linked with consumption of contaminated foods. In this paper a case of septicaemia with L. monocytogenes in a 73-year-old immunocompromised man is described. Evidence for the association of this case of listeriosis with the consumption of a contaminated 'Camembert' cheese is provided by serotyping, esterase typing, DNA macrorestriction patterns analysis and level of virulence of the isolated strains for mice.

Evidence that mortality from Vibrio vulnificus infection results from single strains among heterogeneous populations in shellfish

Vibrio vulnificus is the leading cause of food-related mortality reported in the state of Florida. It is normal microflora in marine environments, where seawater and molluscan shellfish are the primary vectors of V. vulnificus disease. Risk correlates with seasonally high numbers of V. vulnificus bacteria during the summer months. Currently, the infectious dose for humans, as well as whether the disease is caused by single or multiple strains found in molluscan shellfish, is unknown. In this work, we studied pulsed-field gel electrophoresis profiles of V. vulnificus strains isolated from blood and oysters associated with V. vulnificus disease. Results showed that ca. 10(3) V. vulnificus bacteria/gram of oyster and higher concentrations were associated with human infections and that a single V. vulnificus strain, evidenced by pulsed-field gel electrophoresis profiles, was isolated from human tissues.

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.

DNA fragments from regions involved in surface antigen expression specifically identify Listeria monocytogenes serovar 4 and a subset thereof: cluster IIB (serotypes 4b, 4d, and 4e)

Listeria monocytogenes serotype 4b has frequently been implicated in sporadic as well as epidemic listeriosis. On the basis of pulsed-field fingerprinting, serotype 4b strains, along with strains of serotypes 4d and 4e, constitute one genomic cluster (IIB). We have identified two genomic regions essential for the expression of surface antigens which previously were shown to be specific to cluster IIB strains. A DNA probe of 1.1 kb derived from one of the regions (probe 1) hybridized only with strains of serotypes 4b, 4d, and 4e in Southern blots and dot blots. A different DNA probe of 0.3 kb (probe 2), derived from the other region, hybridized with all serovar 4 strains (serotypes 4b, 4a, 4c, 4d, and 4e). All other L. monocytogenes serotypes were negative with probe 1 or 2. Use of probe 1 in Southern blots of EcoRI-digested genomic DNA revealed a restriction fragment length polymorphism in serotype 4b strains, with the hybridizing EcoRI fragments being 4.5 kb (strains of the epidemic clone) and either 4.5 or 5.0 kb (all other serotype 4b strains). Although the probes hybridized with a special group of Listeria innocua strains which also expressed the surface antigens, the latter could be readily distinguished by the size of the hybridizing EcoRI fragment with probe 1 (ca. 2.2 kb). These data suggest that the combined use of these probes with L. monocytogenes can readily and specifically identify cluster IIB strains as well as the entire serovar 4 complex.

Intraspecific Genomic Variation Within Xanthomonas albilineans, the Sugarcane Leaf Scald Pathogen

ABSTRACT To better understand the nature of recent outbreaks of leaf scald disease of sugarcane in a number of sugarcane production regions of the world including Florida, Guadeloupe, Louisiana, Mauritius, Taiwan, and Texas, a study of the worldwide genetic variation of the pathogen was undertaken. A total of 218 strains from 31 geographic locations were examined. Genomic DNA of each strain was digested with the rare cutting restriction enzyme SpeI, and the fragments were separated by pulsed-field gel electrophoresis (PFGE). A total of 102 bands were identified, and 54 different DNA banding patterns (haplotypes) were observed. Eight groups of banding patterns, designated PFGE groups A through H, were consistently detected by visual, principal component, and cluster analyses. Five groups were comprised of multiple haplotypes representing numerous strains, and three were comprised of single haplotypes representing one strain each. The leaf scald outbreaks in Florida, Louisiana, Texas, and possibly Guadeloupe and Taiwan could be attributed to the introduction of strains belonging to PFGE group B. When infection by two strains each of the newly introduced strains (PFGE group B) and those previously present in Florida (PFGE group A) was analyzed in 22 sugarcane cultivars by reisolation 24 weeks after inoculation, a significantly greater mean frequency was detected for PFGE group B strains and no cultivar by PFGE group interaction was observed. Inadvertent dispersal of the pathogen among plants, possibly by means of aerosols or splashing water, was detected in a subsequent experiment. Strains of PFGE group B were recovered from the internal tissues of some plants inoculated with PFGE group A strains and were also found to be epiphytic colonizers of nonsymptomatic, noninoculated plants adjacent to the inoculated plants; whereas strains of PFGE group A were recovered only from plants that had been inoculated with them. Thus, the possibility became more apparent that strain variation might be associated, at least in part, with factors governing plant-to-plant spread of the pathogen in nature.

An outbreak of gastroenteritis and fever due to Listeria monocytogenes in milk

BACKGROUND

After an outbreak of gastroenteritis and fever among persons who attended a picnic in Illinois, chocolate milk served at the picnic was found to be contaminated with Listeria monocytogenes.

METHODS

In investigating this outbreak, we interviewed the people who attended the picnic about what they ate and their symptoms. Surveillance for invasive listeriosis was initiated in the states that receive milk from the implicated dairy. Stool and milk samples were cultured for L. monocytogenes. Serum samples were tested for IgG antibody to listeriolysin O.

RESULTS

Forty-five persons had symptoms that met the case definition for illness due to L. monocytogenes, and cultures of stool from 11 persons yielded the organism. Illness in the week after the picnic was associated with the consumption of chocolate milk. The most common symptoms were diarrhea (present in 79 percent of the cases) and fever (72 percent). Four persons were hospitalized. The median incubation period for infection was 20 hours (range, 9 to 32), and persons who became ill had elevated levels of antibody to listeriolysin O. Isolates from stool specimens from patients who became ill after the picnic, from sterile sites in three additional patients identified by surveillance, from the implicated chocolate milk, and from a tank drain at the dairy were all serotype 1/2b and were indistinguishable on multilocus enzyme electrophoresis, ribotyping, and DNA macrorestriction analysis.

CONCLUSIONS

L. monocytogenes is a cause of gastroenteritis with fever, and sporadic cases of invasive listeriosis may be due to unrecognized outbreaks caused by contaminated food.

A review of Listeria monocytogenes and listeriosis

Following the initial isolation and description in 1926 Listeria monocytogenes has been shown to be of world-wide prevalence and is associated with serious disease in a wide variety of animals, including man. Our knowledge of this bacterial pathogen and the various forms of listeriosis that it causes has until recently been extremely limited, but recent advances in taxonomy, isolation methods, bacterial typing, molecular biology and cell biology have extended our knowledge. It is an exquisitely adaptable environmental bacterium capable of existing both as an animal pathogen and plant saprophyte with a powerful array of regulated virulence factors. Most cases of listeriosis arise from the ingestion of contaminated food and in the UK the disease is particularly common in ruminants fed on silage. Although a number of forms of listeriosis are easily recognized, such as encephalitis, abortion and septicaemia, the epidemiological aspects and pathogenesis of infection in ruminants remain poorly understood. The invasion of peripheral nerve cells and rapid entry into the brain is postulated as a unique characteristic of its virulence, but relevant and practical disease models are still required to investigate this phenomenon. This review offers an up to date introduction to the organism with a description of virulence determinants, typing systems and a detailed account of listeriosis in animals. Experimental and field papers are reviewed and further sections deal with the diagnosis, treatment and control of listeriosis in animals. A final part gives an overview of listeriosis in man.

WHO-sponsored international collaborative study to evaluate methods for subtyping Listeria monocytogenes: restriction fragment length polymorphism (RFLP) analysis using ribotyping and Southern hybridization with two probes derived from L. monocytogenes chromosome

Seven laboratories participated in a WHO-sponsored international collaborative study, to evaluate methods for subtyping Listeria monocytogenes, by performing restriction fragment length polymorph sm (RFLP) analysis-based subtyping of an international study set of 80 strains of L. monocytogenes that included 22 epidemiologically related groups. The RFLP analysis was done by Southern hybridization with one of two types of probes found in multiple copies on the chromosome of L. monocytogenes. Six laboratories performed ribotyping. These laboratories used EcoRI enzyme to restrict the L. monocytogenes DNA and ribosomal RNA or DNA as the probe for Southern hybridizations. The seventh laboratory used Ncil to restrict the DNA, and two probes, one randomly cloned and the other containing repeat sequences cloned from L. monocytogenes DNA. The overall discriminating power of ribotyping, as estimated by calculation of Simpson's index of diversity, ranged from 0.83 to 0.88 for the six laboratories. The discriminating power of the combination of two probes used by Laboratory 7 was 0.91. Ribotyping and the cloned probes used by Laboratory 7 discriminated poorly between serotype 4b strains. Neither method identified three atypical strains (identified by other subtyping methods) included in three apparently epidemiologically related groups. Ribotyping did not discriminate between strains of serotypes 4b and 4b(X) in one epidemiologically related group of strains; one cloned probe used by Laboratory 7 discriminated between these strains. Intra-laboratory reproducibilities for the seven laboratories ranged from 80.0 to 100%. as determined by their abilities to correctly identify 11 pairs of duplicate strains included in the study set. Inter-laboratory reproducibilities were generally very good considering that no attempt was made to standardize protocols used by the participants.

Pulsed-field gel electrophoresis and ribotype profiles of clinical and environmental Vibrio vulnificus isolates

Vibrio vulnificus belongs to the autochthonous bacterial flora of warm estuarine waters. It can cause life-threatening extraintestinal disease in persons who have underlying illness and who consume raw shellfish or contact wounds with estuarine water. Currently, very little is known about genetic diversity within this species. In this report, we describe high-level variation in restriction fragment length polymorphism profiles among 53 clinical and 78 environmental isolates, as determined by pulsed-field gel electrophoresis. In contrast, ribotype profiles showed greater similarity. When combined ribotype profiles of clinical and environmental isolates were analyzed, four predominant clusters were observed. Interestingly, a low number (16%) of clinical isolates were found in cluster C, compared with clusters A, B, and D (range, 50 to 83%). In addition, 83% of all Hawaiian isolates were located in a single cluster, indicating a possible relationship between geography and genotype. We also report that spontaneous translucent colonial morphotypes were distinct by both restriction fragment length polymorphism and biochemical profiles, compared with opaque parent strains.

Genomic fingerprinting of 80 strains from the WHO multicenter international typing study of listeria monocytogenes via pulsed-field gel electrophoresis (PFGE)

An international multicenter typing study of Listeria monocytogenes was initiated by the World Health Organization (Food Safety Unit, Geneva) in order to evaluate the usefulness of various phenotypic and genotypic typing methods for L. monocytogenes, to select and standardize the most appropriate methods to define common nomenclature of varieties and to select specific reference strains. Pulsed-field gel electrophoresis was used in four laboratories for molecular characterization of a set of 80 'coded' strains distributed to all participating laboratories. The endonucleases ApaI and SmaI, used in all four laboratories, yielded between 21 and 28 restriction endonuclease digestion profiles (REDP). AscI was used, in addition, in laboratory A and displayed 21 REDP. The combination of ApaI, SmaI or AscI REDP established 25 to 33 genomic groups. depending on the laboratory and the number of viable strains. Agreement of typing data among the four laboratories ranged from 79 to 90%. Forty-nine (69%) of the 71 strains viable in all four laboratories were placed into exactly the same genomic groups in all four laboratories. The epidemiological relevance of the strains became known after decoding and it was shown that most of the epidemiologically related strains were correctly identified by the four groups of investigators. i.e., most related strains were placed into the same genomic groups by all four laboratories. Similar results were obtained when 11 duplicate cultures were analyzed-on average 84% of the duplicates were identified. Comparison of REDP obtained by laboratory A with REDP from previously analyzed set of 176 L. monocytogenes strains allowed the prediction of the serovar-groups of the 80 strains. These predictions of serovar-groups were later confirmed by serotyping results obtained by laboratories involved in the WHO multicenter typing study of L. monocytogenes. In general this study reconfirmed that PFGE is a very accurate and reproducible method for fine structure comparison and molecular typing of L. monocytogenes.

Subtyping Listeria monocytogenes isolates genetically related to the Swiss epidemic clone

Macrorestriction analysis by pulsed-field gel electrophoresis was used to assess the diversity of strains within the epidemic-associated electrophoretic type 1 (ET1) clone of Listeria monocytogenes. For this purpose, a total of 144 isolates from Switzerland shown by multilocus enzyme electrophoresis to belong to the ET1 were examined. These isolates were subtyped by macrorestriction analysis using the enzymes ApaI and SmaI and field inversion gel electrophoresis. Among these 144 isolates, 45 were isolated in human listeriosis cases of the postepidemic period of 1988 to 1993 and 44 were isolated in animal listeriosis cases of the same period. Forty-seven isolates were from the epidemic period of 1983 to 1987, and eight additional isolates were from cattle from two different farms. Twenty-nine different subtypes could be identified among the 144 isolates tested. Five major subtypes were found more frequently than the others during the postepidemic period, both in humans and in animals. Two of these subtypes had been previously implicated in outbreaks of listeriosis, thus suggesting that particular pulsed-field gel electrophoresis subtypes may be frequently associated with disease in humans and animals. Two of these frequent subtypes were also suspected to be related to small clusters of listeriosis cases during the postepidemic period. The results obtained by typing epidemiologically related isolates from different animals within the same farms and from different body sites of a given patient confirmed the potential of macrorestriction analysis for epidemiological studies restricted to short periods of time and to small number of isolates. The analysis of 47 isolates related to the Swiss listeriosis epidemic period of 1983 to 1987 and the use of Southern blotting and hybridization experiments show that the interpretation of relatedness between isolates presenting slightly different macrorestriction patterns may be more complex than commonly accepted. In such cases, careful interpretation of the potential molecular mechanisms leading to the differences observed between patterns is necessary.

Typing of Listeria monocytogenes by monocin and phage receptors

One hundred strains of Listeria monocytogenes from both sporadic and epidemic cases were typed by monocin production combined with phage receptor and reverse phage receptor methods. The monocin-phage combination gave 72 types with 100% typability and 97% reproducibility. The results were compared to those of serotyping, phage typing, ribotyping, multilocus enzyme electrophoresis, restriction enzyme analysis and RAPD (random amplification of polymorphic DNA). The monocin/phage types were comparable in terms of discrimination with other methods for epidemiological investigations. The index of discrimination of using the monocin typing and phage receptor/reverse phage receptor method combination (0.99) for both the 87 epidemiologically unrelated strains and the epidemiologically important serogroup 4 strains was the highest of the seven different methods analysed. This combination of methods was simple, highly discriminatory and reproducible and can be carried out in a non-specialized laboratory. However, like most of the other Listeria typing methods, both the method and the indicator test strains need to be standardized.

Repetitive element sequence-based PCR for species and strain discrimination in the genus Listeria

Repetitive element sequence-based PCR (rep-PCR) was used to generate DNA fingerprints for Listeria spp. Two primer sets (REP 1R-I REP 2-I and ERIC 1R ERIC 2) used in respectively REP- and ERIC-PCR revealed that bacteria of the genus Listeria possess short repetitive extragenic palindromic elements and enterobacterial repetitive intergenic consensus sequences. Specific band profiles obtained by ERIC-PCR enabled the identification of Listeria species. With both REP- and ERIC-PCR the L. monocytogenes serotypes 1/2a, 1/2b, 1/2c, 3b and 4b could be clearly distinguished from each other. Within the serotype 1/2a, REP-PCR showed a higher discriminative potential than ERIC-PCR and a comparable discriminative potential as RAPD combining 3-4 primers.

Recovery of different Listeria ribotypes from naturally contaminated, raw refrigerated meat and poultry products with two primary enrichment media

Isolation rates for Listeria monocytogenes and the other Listeria spp. typically improve when samples are enriched in more than one primary enrichment medium. This study evaluated the abilities of two primary enrichment media, University of Vermont-modified Listeria enrichment broth (UVM) and Listeria repair broth (LRB), to recover different ribotypes of Listeria spp. from raw meat and poultry samples. Forty-five paired 25-g retail samples of ground beef, pork sausage, ground turkey, and chicken (160 samples) underwent primary enrichment in UVM and LRB (30 degrees C for 24 h) followed by secondary enrichment in Fraser broth (35 degrees C for 24 and 40 h) and plating on modified Oxford agar. After 24 h of incubation of 35 degrees C, 608 Listeria colonies from selected positive samples were biochemically confirmed as L. monocytogenes (245 isolates), L innocua (276 isolates), and L. welshimeri (89 isolates) and then ribotyped with the automated Riboprinter microbial characterization system (E. I. du Pont de Nemours & Co., Inc.). Thirty-six different Listeria strains comprising 16 L. monocytogenes (including four known clinical ribotypes), 12 L. innocua, and 8 L. welshimeri ribotypes were identified from selected positive samples (15 samples of each product type; two UVM and two LRB isolates per sample). Twenty-six of 36(13 L. monocytogenes) ribotypes were detected with both UVM and LRB, whereas 3 of 36 (1 L. monocytogenes) and 7 of 36 (3 L. monocytogenes) Listeria ribotypes were observed with only UVM or LRB, respectively. Ground beef, pork sausage, ground turkey, and chicken yielded 22 (8 L. monocytogenes), 21 (12 L. monocytogenes), 20 (9 L. monocytogenes), and 19 (11 L. monocytogenes) different Listeria ribotypes, respectively, with some Listeria ribotypes confined to a particular product. More importantly, major differences in both the number and distribution of Listeria ribotypes, including previously recognized clinical and nonclinical ribotypes of L. monocytogenes, were observed when 10 UVM and 10 LRB isolates from five samples of each product were ribotyped. When a third set of six samples per product type was examined from which two Listeria isolates were obtained by using only one of the two primary enrichment media, UVM and LRB failed to detect L. monocytogenes (both clinical and nonclinical ribotypes) in two and four samples, respectively. These findings stress the importance of using more than one primary enrichment medium and picking a sufficient number of colonies per sample when attempting to isolate specific L. monocytogenes strains during investigations of food-borne listeriosis.

Prevalence and clonal nature of Escherichia coli O157:H7 on dairy farms in Wisconsin

A survey was conducted between March and October of 1994 to determine the prevalence and identify the sources of serotype O157:H7 isolates of Escherichia coli in Wisconsin dairy herds. A stratified sample of 400 farms was identified, and 70 farms with weaned calves less than 4 months old were included in the study. During the prevalence study, 5 of the 70 farms (herd prevalence, 7.1 +/- 4.5%) and fecal samples from 10 of 560 calves (animal prevalence, 1.8%) tested positive for serotype O157:H7. In a follow-up study, the five O157:H7-positive farms and seven of the O157:H7-negative farms identified in the prevalence study were visited again. An additional 517 fecal samples from cattle of various ages were tested, and a total of 15 animals from four of the five herds that were previously positive and 4 animals from two of seven herds that were previously negative tested positive for E. coli O157:H7. Observations made during the follow-up study suggested that horizontal transmission was an important means of E. coli O157:H7 dissemination on the farms. A total of 302 environmental samples, were examined, and 2 animal drinking water samples from one previously negative farm and 1 animal drinking water sample from a previously positive farm contained E. coli O157:H7. Analyses by the pulsed-field gel electrophoresis technique of contour-clamped homogeneous electric field electrophoresis revealed that isolates from the same farm displayed identical or very similar XbaI restriction endonuclease digestion profiles (REDP), whereas isolates from different farms typically displayed different REDP. However, more than one REDP was usually observed for a given herd over the 8-month sampling period. Analyses of multiple isolates from an animal revealed that some animals harbored O157:H7 strains that had different REDP, although the REDP of isolates obtained from the same fecal sample were very similar. Collectively, 160 bovine isolates obtained from 29 different animals and three water isolates displayed 20 distinct XbaI REDP. Our data revealed that there are several clonal types of serotype O157:H7 isolates in Wisconsin and indicated that there is probably more than one source of this pathogen on the dairy farms studied. However, animal drinking water was identified as one source of E. coli O157:H7 on one farm.

The clones of Listeria monocytogenes detected in food depend on the method used

Restriction enzyme analysis (REA) with pulsed-field gel electrophoresis (PFGE) has been used to characterize and compare Listeria monocytogenes strains isolated from foods by two methods, an enrichment procedure and a direct plating procedure. In total 151 isolates from nine foods were investigated. In six of the foods (101 strains investigated) only one clone of L. monocytogenes was found irrespective of the method used. In three foods (50 strains investigated) the direct plating procedure yielded more clones than the enrichment procedure. At the most, five clones were detected in the same food. The results presented here indicate that direct plating from the food reveals more L. monocytogenes clones than revealed by an enrichment procedure.

Listeria monocytogenes in poultry and poultry products: epidemiological investigations in seven Danish abattoirs

Listeria monocytogenes was isolated from 11/236 (4 x 7%) caecal samples from parent flocks, providing broilers to the abattoirs investigated. Caecal samples from 2078 broilers representing 90 randomly selected broiler flocks were negative for L. monocytogenes. A total of 3080 samples from seven abattoirs including poultry processing line samples, and final products were also examined for L. monocytogenes. Listeria monocytogenes was isolated in 0 x 3% to 18 x 7% of the samples collected in the different abattoirs. Epidemiological typing of 247 L. monocytogenes isolates, including serotyping, phage typing, pulsed-field gel electrophoresis and ribotyping revealed 62 different clones. Based upon typability and discriminatory power, DNA typing methods used were found equally suitable as epidemiological markers. Serotyping and phage typing were not found useful as epidemiological markers for poultry isolates of L. monocytogenes since only 120/247 (48 x 6%) isolates were typable by phage typing and 230/247 (93 x 1%) L. monocytogenes belonged to serotype 01 while 6/247 (2 x 4%) belonged to 04. The discovery of a few dominating clones in each abattoir might indicate an endemic occurrence of L. monocytogenes. It is concluded that L. monocytogenes in the broiler production is primarily localized to the abattoirs. The incidence of L. monocytogenes may be reduced by improving the hygiene.

Use of molecular typing methods to trace the dissemination of Listeria monocytogenes in a shrimp processing plant

Molecular typing of bacteria has been widely used in epidemiological studies but not as extensively for tracing the transmission of pathogenic bacteria in food plants. This study was conducted to examine the potential use of two molecular typing methods, random amplified polymorphic DNA (RAPD) analysis and pulsed-field gel electrophoresis (PFGE), to trace Listeria monocytogenes contamination in a shrimp processing plant. Ribotyping and phase typing were also performed on a select number of strains. One hundred fifteen strains of L. monocytogenes collected in different areas of a shrimp processing plant were first serotyped and then subtyped by molecular typing. RAPD and PFGE showed great promise for typing L. monocytogenes isolates since distinguishable and reproducible DNA polymorphisms were obtained. When the composite profile from both (RAPD and PFGE) methods was generated, there was an increase in the discriminatory power to discern differences between strains of L. monocytogenes. The results indicated that environmental strains all fell into composite profile groupings unique to the environment, while strains from both water and utensils shared another composite profile group. L. monocytogenes fresh shrimp isolates belonging to one profile group were found in different areas of the processing line. This same profile group was also present in food handlers from the processing and packaging areas of the plant.

Comparison of ribotyping, arbitrarily primed PCR, and pulsed-field gel electrophoresis for molecular typing of Listeria monocytogenes

Fifty-one clinical isolates of Listeria monocytogenes (15 isolates from two outbreaks and 36 epidemiologically unrelated isolates) were typed by conventional serotyping, ribotyping (RT), pulsed-field gel electrophoresis (PFGE), and arbitrarily primed PCR (AP-PCR). Serotyping was unable to distinguish between related and unrelated strains of L. monocytogenes. Each of the three molecular methods showed excellent typeability and reproducibility. Restriction with EcoRI and PvuII gave 16 and 23 RT patterns, respectively. Restriction with ApaI or SmaI generated 22 and 26 PFGE profiles, respectively. ApaI profiles were easier to interpret, with 10 to 15 bands each, while SmaI profiles had 15 to 20 bands each. AP-PCR with two different primers yielded 29 and 31 randomly amplified polymorphic DNA patterns, respectively. Strains from the same outbreak shared concordant patterns by each of the three methods. Of the three techniques evaluated, RT was the least discriminating and could not distinguish between strains from the two outbreaks. The abilities of AP-PCR and PFGE to differentiate between strains were comparable. However, AP-PCR was more rapid and easier to perform. We conclude that the DNA profiles generated by either AP-PCR or PFGE can be used to differentiate outbreak strains from epidemiologically unrelated strains and to clearly identify unrelated strains as being distinct from one another. We recommend that at least two independent primers be used for AP-PCR typing in order to improve its discriminatory power.

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.

Use of pulsed-field gel electrophoresis to link sporadic cases of invasive listeriosis with recalled chocolate milk

Pulsed-field gel electrophoresis established the linkage between recalled chocolate milk and a multistate invasive listeriosis outbreak during a four-product recall period. Listeria monocytogenes isolates from four hospitalized patients and an environmental dairy sample displayed AscI restriction endonuclease digestion profiles identical to that of the chocolate milk isolate.

Investigations related to the epidemic strain involved in the French listeriosis outbreak in 1992

Two hundred seventy-nine cases of human listeriosis (92 pregnancy-related cases and 187 non-pregnancy-related cases) caused by a serovar 4b and phagovar 2389:2425:3274:2671:47:108:340 strain were identified in France between March and December 1992. Epidemiological investigations included a case-control study (not described here) and microbiological analyses of foods. Results of the case-control study and characterization of food isolates identified pork tongue in jelly, a ready-to-eat meat product, as the major vehicle of this outbreak, and to a lesser extent, delicatessen products contaminated secondarily during handling in food stores. As far as serotyping, phage typing, DNA macrorestriction pattern analysis (obtained by pulsed-field gel electrophoresis [PFGE]), and ribotyping are concerned, this epidemic strain is phenotypically and genomically closely related to strains responsible for major outbreaks of listeriosis previously observed in Europe and North America. The epidemic strain sensu stricto as defined by PFGE (2/1/3) displayed the same serovar, phagovar, ribovar, and ApaI and NotI PFGE patterns as the epidemic strains from outbreaks in Switzerland, California, and Denmark, but it consistently showed differences in the SmaI PFGE profile. This information greatly contributed to the identification of the major food vehicle (pork tongue in jelly) and further allowed exclusion of other foods (cheese) as possible sources of this major listeriosis epidemic.

Randomly amplified polymorphic DNA analysis of Xylella fastidiosa Pierce's disease and oak leaf scorch pathotypes

Randomly amplified polymorphic DNA analysis was conducted with 14 primers to 17 strains of Xylella fastidiosa. There was a high degree of similarity among the seven Pierce's disease (PD) strains (Sxy > 0.93) and the seven oak leaf scorch (OLS) strains (Sxy > 0.96). However, the two groups were different, with a similarity index of 0.67, confirming the presence of a PD DNA cluster and suggesting the presence of a new OLS cluster. The control plum leaf scald strains (two strains) together with the periwinkle wilt strain had a much smaller similarity index (0.44) compared with the PD and OLS clusters.

Characterization of five esterases from Listeria monocytogenes and use of their electrophoretic polymorphism for strain typing

Esterases from Listeria monocytogenes strains isolated from cheeses were analyzed by starch gel electrophoresis. Five esterases, numbered from EST 1 to EST 5 in order of decreasing anodal migration, were identified. The EST 1, EST 3, EST 4, and EST 5 set was most active toward alpha-naphthyl propionate, while EST 2 was most active toward alpha-naphthyl acetate. Results from inhibitor studies suggest that all of these esterases were EC class 3.1.1.1 carboxylesterases, except that EST 1 and EST 3 also showed some sensitivity to parahydroxymercuribenzoate. Polymorphism of these five esterases was observed in the population. Twelve esterase patterns were defined and used to subdivide serotypes.

Molecular characterization of Yersinia enterocolitica by pulsed-field gel electrophoresis and hybridization of DNA fragments to ail and pYV probes

Sixty strains of Yersinia enterocolitica from five serogroups (O:3; O:9; O:8; O:5; and O:5,27) and eight non-Y. enterocolitica strains, recovered from diverse sources (humans, animals, food, and the environment) in Europe, Argentina, and the United States, were examined by the pulsed-field gel electrophoresis (PFGE) technique of contour clamped homogeneous electric field electrophoresis (CHEF) by using NotI and XbaI as restriction enzymes. NotI and XbaI generated 36 and 33 restriction endonuclease digestion profiles (REDP), respectively. By combining the results of both enzymes, 42 unique genomic groups were differentiated. DNA fragments were transferred to nylon membranes and hybridized with digoxigenin-labelled oligonucleotide probes to the ail gene and virulence plasmid to determine hybridization patterns and the potential virulence of the strains. The strains were tested for the presence of the plasmid by PFGE-CHEF and phenotypic characteristics encoded for by the virulence plasmid. Thirty of the 60 Y. enterocolitica strains tested harbored the virulence plasmid. The specificity of the ail and pYV probes was 100% when tested with 68 Yersinia strains and 19 different non-Yersinia strains. Sixteen selected Y. enterocolitica strains were tested for their virulence by lethality in iron- and desferrioxamine-sensitized mice. No correlation between REDP and the virulence of the strains was observed. The observed REDP and the hybridization patterns were very homogeneous within a serogroup and independent of the source of isolation. In addition, PFGE-CHEF was shown to be valuable in identifying and confirming serogroups. Principal component analysis of Dice similarity indices from REDP was an excellent tool for determining genetic relatedness among strains.

Clamped homogenous electric fields (CHEF) gel-electrophoresis of DNA restriction fragments for comparing genomic variations among strains of yersinia enterocolitica and Yersinia spp

Yersinia enterocolitica gastroenteritis was first recognized in the early 1960s and has since been reported with increasing frequency. To determine if strains of Y. enterocolitica, within a restricted region isolated over 8 years (1985-1993), originated from a single or multiple clones, pulsed-field gel electrophoresis (PFGE) of large chromosomal DNA restriction fragments generated by XbaI or NotI was used. A total of 27 isolates of Y. enterocolitica were analyzed, 24 from Austria (Vienna and Graz) consisting of serogroups 0:3 (17 isolates), 0:9 (6 isolates), 0:5 (1 isolate); 2 from Germany of serogroups 0:3 and 0:9 (1 isolate each); 1 from the U.S.A. of serogroup 0:8. Genomic fingerprints of these strains were compared to those of 8 other Yersinia species to ascertain if their restriction endonuclease digestion profiles (REDP) were serogroup and/or species specific. The 27 Y. enterocolitica strains could be divided into 16 genomic varieties according to their restriction patterns with NotI and XbaI. PFGE was highly discriminatory as strains belonging to the same serogroup could be subdivided into different genomic groups. Furthermore, Y. enterocolitica strains isolated from the same region, over an 8 year period, belonged to a few closely related clones. The genomic fingerprints of Yersinia were found to be species and serogroup specific.

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.