Cloning of a gene encoding a major secreted polypeptide of Listeria monocytogenes and its potential use as a species-specific probe

Hemolytic activity reevaluation of putative nonpathogenic Listeria monocytogenes strains

Identification of 12 strains originally characterized as nonpathogenic Listeria monocytogenes was reassured following the evaluation of their hemolytic capability with a newly developed horse blood agar plate. Seven of the strains were observed consistently to be hemolytic and confirmed as L. monocytogenes with the use of two commercial systems: the Gene-Trak L. monocytogenes-specific colorimetric DNA hybridization assay and the API Listeria system. Except for one strain that formed typical smooth colonies, these hemolytic strains formed rough colonies on a selective medium, lithium chloride-ceftazidime agar. The rest of the strains were nonhemolytic and did not hybridize with the DNA probe; they were identified as Listeria innocua on the basis of their API Listeria system biochemical profile. All but one of these nonhemolytic strains formed smooth colonies on lithium chloride-ceftazidime agar.

Cytotoxic-T-lymphocyte responses to epitopes of listeriolysin O and p60 following infection with Listeria monocytogenes

In order to test the influence of the cell surface density of a specific H2-Kd-presented epitope on the subsequent level of the cytotoxic-T-lymphocyte (CTL) response directed against the epitope, we investigated the CTL response to two secreted products of Listeria monocytogenes from mice immunized with viable L. monocytogenes. We determined the response to the H2-Kd-presented amino acid 91 to 99 (aa91-99) immunodominant peptide of listeriolysin O (LLO) and to the aa217-225 immunodominant peptide of p60. The p60-derived peptide appears at the cell surface as an H2-Kd-complexed peptide at a level sixfold higher than that of LLO aa91-99. CTL frequency analysis of anti-LLO- or anti-p60-specific CTLs from mice immunized with wild-type L. monocytogenes showed that the numbers of immune spleen cell-derived CTLs specific for the two peptides were essentially equivalent. We have also found that Listeria-specific CTL populations lyse target cells pulsed with the p60 aa217-225 peptide with a magnitude of the lytic response markedly less than that for targets pulsed with the LLO aa91-99 peptide. Additionally, immunization with mutants of L. monocytogenes which do not stimulate anti-LLO-specific CTLs does not alter the CTL frequency of anti-p60-specific effector cells, with levels of anti-p60-specific CTLs similar to those seen in mice immunized with wild-type L. monocytogenes. These results suggest that the relative cell surface density of major histocompatibility complex class I-presented L. monocytogenes-derived epitopes is but one of the criteria which determine the magnitude of the cytotoxic effector cell response that develops in antilisterial immunity.

Probes and polymerase chain reaction for detection of food-borne bacterial pathogens

DNA-hybridization and the polymerase chain reaction (PCR) are techniques commonly used to detect pathogenic bacteria. In this paper, the use of these techniques for detection of Salmonella, E. coli, V. cholerae, non-O1 Vibrio, Yersinia enterocolitica, Campylobacter, Listeria monocytogenes, Staphylococcus aureus, Bacillus cereus, Clostridium perfringens, and C. botulinum is reviewed with emphasis on application in food microbiology. In food control, DNA-techniques have most often been used in a 'culture confirmation' fashion, i.e. bacteria are enriched and sometimes even purified by traditional culture procedures and thereafter identified by the use of DNA-based methods. The most desirable approach is, however, to detect organisms directly in the food, but major problems remain to be solved before this can be routinely performed.

A ribosomal DNA fragment of Listeria monocytogenes and its use as a genus-specific probe in an aqueous-phase hybridization assay

cDNAs were prepared from the total RNA of Listeria monocytogenes ATCC 19118 and used as probes to screen a genomic library of the same strain. Four clones were identified which contained ribosomal DNA fragments. Recombinant DNA from one of them was fractionated and differentially hybridized with the cDNA probes to RNA of L. monocytogenes and Kurthia zopfii. The resulting hybridization pattern revealed an HpaII fragment of 0.8 kb that was specific for the L. monocytogenes strain. The nucleotide sequence of this fragment showed 159 bases of the 3' end of the 16S rRNA gene, 243 bases of the spacer region, and 382 bases of the 5' end of the 23S rRNA gene. In dot blot hybridization assays, the 32P-labeled 784-bp fragment was specific only for Listeria species. Dot blot assays revealed that the 32P-labeled fragment can easily detect > or = 10 pg of total nucleic acids from pure cultures of L. monocytogenes, which corresponds to approximately 300 bacteria. This fragment was also used as a probe in an assay named the heteroduplex nucleic acid (HNA) enzyme-linked immunosorbent assay. In this system, the biotinylated DNA probe is hybridized in the aqueous phase with target RNA molecules and then specific HNAs are captured by HNA-specific antibodies. Captured HNA molecules are revealed with an enzyme conjugate of streptavidin. In a preliminary HNA enzyme-linked immunosorbent assay, the 784-bp fragment maintained its specificity for Listeria spp. and could detect 5 x 10(2) cells in artificially contaminated meat homogenate.

A 16S rRNA-based DNA probe and PCR method specific for Listeria ivanovii

A 16S rRNA-based DNA probe and polymerase chain reaction (PCR) method was developed for identification and rapid detection of Listeria ivanovii. The probe (R-1) is 5'-GTAGTGACGCATGTCATCAC-3' corresponding to positions 185-204 in the L. ivanovii 16S rRNA sequence. DNA hybridization results indicated that R-1 probe only reacted with L. ivanovii, and not with six other species of Listeria or other bacteria tested. The PCR method using R-1 and a reverse primer, R-2, was positive with all eight strains of L. ivanovii tested but was negative with six other species of Listeria, including nine strains of L. monocytogenes, and 20 other taxonomically related bacteria tested. In our PCR method, starting with whole bacterial cells, only 3 h were required for the PCR assay and 1 h for electrophoresis without any additional time for DNA isolation and DNA hybridization. This PCR method detected as few as 4 cells of L. ivanovii in pure cultures and 4-40 cells of L. ivanovii in inoculated and diluted mouse feed, blood, or faeces samples.

The homologous and heterologous regions within the iap gene allow genus- and species-specific identification of Listeria spp. by polymerase chain reaction

The iap gene of Listeria species encodes protein p60. The comparison of iap-related genes from different Listeria species indicated common and variable regions within these genes which appeared to be specific for each Listeria species. On the basis of the iap gene sequences, pairs of polymerase chain reaction (PCR) primers which allowed the unambiguous identification of all members of the genus Listeria, of groups of related Listeria species, and of L. monocytogenes, exclusively, were selected. The PCR primers specific for L. monocytogenes yielded PCR products which represented essentially the repeat region of the iap gene. The size of these PCR products allowed an estimate of the number of the TN repeat units within the repeat region of the p60 protein of an L. monocytogenes strain. The data indicated that the number of repeat units differed among L. monocytogenes isolates.

Development of polymerase chain reaction assays for detection of Listeria monocytogenes in clinical cerebrospinal fluid samples

In order to improve the diagnosis of Listeria meningitis or meningoencephalitis, especially in patients who have received antibiotics before their cerebrospinal fluid (CSF) has been examined, two assays for the detection of Listeria monocytogenes based on the polymerase chain reaction (PCR) were evaluated. After a standard PCR, the amplified DNA was detected either by a second round of PCR with internal primers followed by gel electrophoresis and ethidium bromide staining (nested PCR) or by dot blot hybridization to an internal digoxigenin-labeled probe (PCR-dot blot). For PCR, two sets of primers within the invasion-associated protein gene (iap gene) were chosen. They allowed for the highly specific detection of all L. monocytogenes reference strains tested (serotypes 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4b, 4c, 4d, and 7). These primers did not detect amplification products from various other gram-positive or gram-negative bacterial DNAs or human DNA. The sensitivities of both assays were assessed on sterile CSF samples that were artificially seeded with serial dilutions of L. monocytogenes serotype 4b cells. By both methods the limit of detection was less than 10 cells in the initial reaction. Since the nested PCR is more prone to contamination because of manipulation of the amplified products, a standard PCR assay followed by dot blot hybridization was applied to 52 CSF samples in a retrospective study. Of 28 CSF samples which were sterile or positive for bacteria other than Listeria species, 24 were PCR negative. In contrast, from 17 patients with culture-proven Listeria meningitis, 14 of 17 initial CSF samples were PCR positive, as were 3 of 7 culture-negative followup CSF samples taken after patients received antibiotics. These results support the usefulness of this approach in the diagnosis of Listeria meningitis, in particular, when antibiotic administration precedes culture of CSF.

Development of cell surface protein associated gene probe specific for Listeria monocytogenes and detection of the bacteria in food by PCR

A genomic library of L. monocytogenes was constructed using lambda Zap II-Eco RI and screened with a monoclonal antibody which is specific for a Listeria cell surface protein. Three positive clones each contained a 6.5 kb insert which in E. coli could express the same Listeria protein. The 6.5 kb insert was further digested with Hin dIII and the smaller fragments were subcloned into a plasmid vector (pBluescript) and screened with 32P-labelled genomic DNA from L. monocytogenes or L. innocua. Three clones which were positive with L. monocytogenes and negative with L. innocua were screened and each contained a 2.1 kb insert. The 2.1 kb insert was partly sequenced and some candidate oligomer probes from the sequences were selected and compared with sequences in a Genbank computer search. One such oligomer probe (T7-list) was confirmed to be specific for L. monocytogenes. The probe hybridized with all 28 strains of L. monocytogenes tested, but not with any of six other Listeria species nor 11 other bacteria tested. Using this probe-primer, a PCR method was developed which could detect as few as 2 cfu of L. monocytogenes in pure cultures, and as few as 4-10 cfu of L. monocytogenes when inoculated into foods.

Use of specific oligonucleotides for direct enumeration of Listeria monocytogenes in food samples by colony hybridization and rapid detection by PCR

Two 18-mer oligonucleotides derived from the sequence of hly, the gene coding for listeriolysin O, were shown to be specific for Listeria monocytogenes in the genus Listeria in colony hybridization tests. The oligonucleotides did not hybridize with any of the bacterial species found in food and co-isolated with Listeria on selective media. They were used in colony hybridization tests for enumeration of L. monocytogenes present in food samples after direct plating on selective media plates. In addition, two 24-mer oligonucleotides, each including the sequence of one of the 18-mers, were successfully used for the PCR-based detection of L. monocytogenes bacilli present in food samples after 48-h enrichment period. Using this technique, as little as 10(2) bacteria per ml of enrichment broth can be detected.

Molecular methods for environmental monitoring and containment of genetically engineered microorganisms

Plans to introduce genetically engineered microorganisms into the environment has led to concerns over safety and has raised questions about how to detect and to contain such microorganisms. Specific gene sequences, such as lacZ, have been inserted into genetically engineered microorganisms to permit their phenotypic detection. Molecular methods have been developed based upon recovery of DNA from environmental samples and gene probe hybridization to specific diagnostic gene sequences for the specific detection of genetically engineered microorganisms. DNA amplification using the polymerase chain reaction has been applied to enhance detection sensitivity so that single gene targets can be detected. Detection of messenger RNA has permitted the monitoring of gene expression in the environment. The use of reporter genes, such as the lux gene for bioluminescence, likewise has permitted the observation of gene expression. Conditional lethal constructs have been developed as models for containment of genetically engineered microorganisms. Suicide vectors, based upon the hok gene have been developed as model containment systems.

Detection of genes coding for listeriolysin and Listeria monocytogenes antigen A (lmaA) in Listeria spp. by the polymerase chain reaction

Two pairs of synthetic oligonucleotide primers were used in a polymerase chain reaction (PCR) protocol to detect targeted sequences in genes coding for listeriolysin O and Listeria monocytogenes antigen A (ImaA). Strains of Listeria spp. used in this study were isolated from clinical specimens, contaminated foods, and environmental sources. Primers were targeted to internal regions of the genes coding for listeriolysin (hlyA) and Listeria antigen (ImaA) and amplification fragments were detected after the PCR by agarose gel electrophoresis. PCR was performed using nucleic acids extracted from a collection of 74 strains of Listeria spp. including 18 reference strains, 41 L. monocytogenes, nine L. innocua, five L. seeligeri and one L. ivanovii, encompassing representative sources, serovars, and enzyme electrophoretic types. Although the listeriolysin gene was found exclusively in L. monocytogenes, some strains of serovar 4c were negative. Simultaneous presence of both genes was restricted to L. monocytogenes strains of serovars 1/2, 3, and 4. The ImaA gene was identified in five of 10 L. innocua strains and one L. ivanovii isolated from pork. Strains of L. seeligeri, L. welshimeri, and L. grayi were negative for both genes. The detection limits in the PCR were found to be 10 pg of nucleic acids for the hlyA gene and 1 pg for the ImaA gene.

Sensitive and specific detection of Listeria monocytogenes in milk and ground beef with the polymerase chain reaction

A sensitive and specific method for detection of Listeria monocytogenes in milk and ground-beef samples is described. It consists of culturing samples in listeria enrichment broth (LEB) and subculturing them from LEB to listeria plating media, followed by DNA extraction and species-specific detection of the organism by using the polymerase chain reaction (PCR). In developing the L. monocytogenes PCR assay, five oligonucleotide primers complementary to the nucleotide sequence of the listeriolysin O gene were synthesized and used in amplification experiments. PCR products of the predicted size, based on nucleotide sequence information, were generated with DNA from all of 72 L. monocytogenes strains with five different primer pairs. DNA from Listeria ivanovii, Listeria innocua, Listeria seeligeri, Listeria welshimeri, Listeria grayi, and Listeia murrayi strains and a panel of 47 bacterial strains representing 17 genera did not generate PCR products with the primer pairs employed. As little as 1 pg of L. monocytogenes DNA could be detected with the assay. To determine the most sensitive culture protocol to use in conjunction with the PCR assay, milk (10 ml) and ground-beef (25 g) samples were inoculated with L. monocytogenes at concentrations ranging from 0 to 10(5) CFU ml-1 or g-1, as appropriate for the sample. PCR assays on DNA extracted from growth on listeria plating media, inoculated with 24-h LEB samples cultures, were most sensitive, allowing detection of as little as 0.1 CFU of L. monocytogenes ml-1 or g-1 of milk and ground beef, respectively.

Comparison of methods for discrimination between strains of Listeria monocytogenes from epidemiological surveys

Total cellular DNA from 28 strains of Listeria monocytogenes isolated from food implicated in food-borne illness and from patients with listeriosis was digested with the restriction endonucleases HindIII, HaeIII, and EcoRI. Following agarose gel electrophoresis, the fragments were subjected to Southern blot hybridization with a digoxigenin-labeled cDNA probe transcribed from Escherichia coli 16S and 23S rRNA. The patterns of bands from genomic (DNA fingerprints) and rDNA fingerprints (ribotypes) were used for classifying L. monocytogenes strains, and the resulting subtypes were compared with serotyping and multilocus enzyme electrophoresis classification schemes. A total of 15 distinct and identical groups were obtained when genomic DNA was digested with either HindIII or HaeIII. The most discriminating enzyme for ribotyping of strains was EcoRI, which divided the 28 strains of L. monocytogenes into 6 ribotype groups. DNA fingerprinting and ribotyping differentiated L. monocytogenes from other Listeria spp., including L. ivanovii, L. welshimeri, and L. innocua as well as the lactic acid bacteria Lactococcus lactis subsp. lactis and subsp. cremoris. L. monocytogenes strains isolated from four independent food-borne illness incidents were analyzed by all typing methods. Patient and product isolates were not distinguishable by serotyping, ribotyping, or multilocus enzyme electrophoresis. DNA fingerprinting was the only method capable of differentiating these strains, or conversely, of proving relatedness of patient-product pairs of isolates. This method was a relatively simple, sensitive, reproducible, and highly discriminating method for epidemiological tracking of L. monocytogenes implicated in food-borne illness.

Expression of the iap gene coding for protein p60 of Listeria monocytogenes is controlled on the posttranscriptional level

Expression of the iap gene of Listeria monocytogenes encoding a major extracellular protein (p60) was analyzed. Different start sites for transcription of the iap gene were identified by primer extension analysis in L. monocytogenes and in a recombinant Escherichia coli clone. The mutant RIII of L. monocytogenes represents a member of the frequently occurring L. monocytogenes R mutants, which form cell chains and produce greatly reduced amounts of p60. However, the concentrations of iap-specific mRNA were similar in mutant RIII and the wild-type strain. The introduction of additional copies of the iap gene from wild-type L. monocytogenes led to an equal increase of iap mRNA in both strains, but overexpression of protein p60 was only observed in the wild-type strain. The nucleotide sequences of both iap genes and their 5' noncoding regions were identical in all parts that are essential for efficient transcription of the iap gene, translation of the iap-specific mRNA, and transport of the p60 protein. These data suggest that the expression of the iap gene in L. monocytogenes is controlled on the posttranscriptional level by a specific factor that is defective in mutant RIII.

Rapid confirmation of Listeria monocytogenes isolated from foods by a colony blot assay using a digoxigenin-labeled synthetic oligonucleotide probe

An oligodeoxyribonucleotide probe based on the sequence of a 321-bp internal fragment of the msp gene encoding a major secreted polypeptide of Listeria monocytogenes was labeled with digoxigenin by using terminal deoxynucleotidyl transferase. The specificity of the digoxigenin-labeled probe was determined by dot blot assays. The probe reacted with all strains of L. monocytogenes tested (12 of 12 strains representing five serotypes). The probe did not react with any other Listeria species or with other gram-positive bacteria (Brochothrix, Erysipelothrix, Corynebacterium, Rhodococcus, Lactobacillus, Leuconostoc, Bacillus, Staphylococcus, and Streptococcus). The probe was used to develop a colony blot assay for the rapid confirmation of L. monocytogenes on Listeria-selective agars which had been streaked with food enrichment cultures. Forty-eight food samples were tested by conventional culture and DNA colony blot assay. The sensitivity and specificity of the DNA colony blot were 100 and 97%, respectively.

Epidemiology of human listeriosis

During the 1980s, investigation of several large epidemics of listeriosis confirmed that transmission of L. monocytogenes in food causes human disease. Progress in laboratory detection and subtyping of the organism has enhanced our ability to compare human and environmental isolates of L. monocytogenes. Transmission by foodborne organisms is now recognized as causing both epidemic and sporadic listeriosis. Continued study of dietary risk factors associated with listeriosis is needed in order to develop dietary recommendations for the expanding population at increased risk of disease. Current research application of new molecular methods to the study of L. monocytogenes may improve the ability to diagnose pregnancy-associated disease and permit the rapid detection and control of L. monocytogenes in the food supply.

Detection of Listeria species and Listeria monocytogenes using polymerase chain reaction

Five oligonucleotide sequences are described that were used as primers in the polymerase chain reaction (PCR) to amplify specific sequences from Listeria DNA. When all five primers were used in combination, three PCR products were possible; a Listeria specific product that occurs with DNA from any Listeria sp., a Listeria monocytogenes specific product that occurs only in the presence of DNA from this organism and a universal product that is found using DNA from any bacterial source. The occurrence of these PCR products was used as a diagnostic test on bacteria isolated from various food samples to detect Listeria sp. and L. monocytogenes.

Detection of Listeria monocytogenes by direct colony hybridization on hydrophobic grid-membrane filters by using a chromogen-labeled DNA probe

A DNA probe specific for Listeria monocytogenes was isolated from a beta-hemolytic recombinant clone of an L. monocytogenes gene bank. It was labeled with horseradish peroxidase and used in a direct colony hybridization method on hydrophobic grid-membrane filters for the detection of the organism. Following color development of the chromogen, a commercial counter (HGMF Interpreter) was able to detect and count the organisms electronically. The method gave a positive reaction with 70 L. monocytogenes strains, while showing a negative reaction with 10 strains of other Listeria spp. and with 20 organisms of other genera.

Evaluation of hybridization characteristics of a cloned pRF106 probe for Listeria monocytogenes detection and development of a nonisotopic colony hybridization assay

An internal fragment (pRF106 fragment, ca. 500 bp) of a gene (msp) coding for a 60-kDa protein of Listeria monocytogenes serotype 1/2a was used to develop a screening method to discriminate between L. monocytogenes and avirulent Listeria spp. on primary isolation plates. The L. monocytogenes-derived probe fragment of pRF106 hybridized to a 13-kb fragment of L. monocytogenes and a 3-kb fragment of one cheese isolate strain of Listeria seeligeri under stringent hybridization conditions (mean thermal denaturation temperature [Tm]-5 degrees C). The probe also hybridized to a 6-kb fragment of Listeria innocua, Listeria ivanovii, and L. seeligeri under less stringent hybridization conditions (Tm-17 degrees C). The pRF106 fragment was labeled with digoxigenin-11-dUTP and used to develop a colony hybridization assay. Colonies from lithium chloride-phenylethanol-moxalactam agar were blotted onto nylon membranes. The cells were pretreated with microwaves before lysis with sodium hydroxide. DNA-DNA hybridization and posthybridization washing were done at high stringency (Tm-7 degrees C). The nonisotopic colony hybridization procedure was specific for L. monocytogenes when evaluated against pure cultures of L. monocytogenes and other Listeria species, excluding the cheese isolate of L. seeligeri. Also, it was specific for L. monocytogenes when evaluated with Listeria-negative food enrichment cultures that were inoculated in the laboratory with Listeria species.

Cloning of the listeriolysin O gene and development of specific gene probes for Listeria monocytogenes

A clone containing 3.1 kb of Listeria DNA was selected from a gene library of Listeria monocytogenes Scott A strain. The Escherichia coli clone produced hemolysin on sheep blood agar and in sonicated extracts but very little in the culture supernatant. This 3.1-kb DNA fragment and a 650-bp HindIII fragment located within the listeriolysin gene were used as probes in a colony hybridization assay. Both probes were specific for L. monocytogenes and did not hybridize with any other Listeria strains at high stringency. Two synthetic probes, one from the 650-bp HindIII fragment and one from the carboxy-terminal region of the protein, were also specific for L. monocytogenes.

The gene coding for protein p60 of Listeria monocytogenes and its use as a specific probe for Listeria monocytogenes

The gene of Listeria monocytogenes that encodes a major extracellular protein (p60) was cloned in Escherichia coli. The gene was designated iap, as p60 was previously shown to represent an invasion-associated protein (M. Kuhn and W. Goebel, Infect. Immun. 57:55-61, 1989). The recombinant E. coli clone expressed p60, as shown by immunoblotting. The complete nucleotide sequence of iap was determined. The deduced amino acid sequence of p60 (484 amino acids) contains a putative N-terminal signal sequence of 27 amino acids and an extended repeat region consisting of 19 threonine-asparagine units. Hybridization with the entire iap gene revealed the presence of homologous sequences in most other Listeria species. In contrast, a 400-base-pair internal iap probe which contained the whole repeat region hybridized only with genomic DNA from L. monocytogenes. Four oligonucleotides previously described as specific probes for the detection of L. monocytogenes (A. R. Datta, B. A. Wentz, D. Shook, and M. W. Trucksess, Appl. Environ. Microbiol. 54:2933-2937, 1988) were shown to be part of the iap gene.