Production, purification and characterization of hemolysins from Listeria ivanovii and Listeria monocytogenes Sv4b

Listeria contamination in chevon and mutton from retail meat shops and slaughter house environment of Punjab, India

Listeria monocytogenes is recognized as an emerging pathogen, with limited information or statistics on this pathogen. Moreover, poor hygienic practices in the unorganized meat sector favor its growth and survival. Therefore, the present study was carried out in Punjab, India to assess the prevalence of Listeria spp. in chevon and mutton samples, followed by its characterization. The prevalence of Listeria in chevon, mutton and swab samples collected from butchers' shops and slaughter houses was 1.82%, 3.21% and 6.66%, respectively. The 18 (2.67%) L. monocytogenes strains isolated from 786 samples carried one or more virulence-associated gene. Based on virulence genes, 6/18 isolates were potentially pathogenic. Serovar 4b (44.4%) was predominant, followed by serovars 1/2a (22.2%), 1/2b (22.2%) and 1/2c (11.1%). Detection of L. monocytogenes in meat and environment samples indicated poor hygiene, potential cross-contamination and risk of listeriosis to consumers and occupational workers. The findings of the study were also relevant to the fact that most of the slaughtering in Punjab, India is carried out in unorganized way, and this meat is not stored propery. Moreover, in the absence of a systematic foodborne disease surveillance and monitoring program, a database on foodborne diseases is absent, and needs to be established.

An acidic sphingomyelinase Type C activity from Mycobacterium tuberculosis

Sphingomyelinases (SMases) catalyze the hydrolysis of sphingomyelin to ceramide and phosphorylcholine. Sphingolipids are recognized as diverse and dynamic regulators of a multitude of cellular processes mediating cell cycle control, differentiation, stress response, cell migration, adhesion, and apoptosis. Bacterial SMases are virulence factors for several species of pathogens. Whole cell extracts of Mycobacterium tuberculosis strains H37Rv and CDC1551 were assayed using [N-methyl-(14)C]-sphingomyelin as substrate. Acidic Zn(2+)-dependent SMase activity was identified in both strains. Peak SMase activity was observed at pH 5.5. Interestingly, overall SMase activity levels from CDC1551 extracts are approximately 1/3 of those of H37Rv. The presence of exogenous SMase produced by M. tuberculosis during infection may interfere with the normal host inflammatory response thus allowing the establishment of infection and disease development. This Type C activity is different from previously identified M. tuberculosis SMases. Defining the biochemical characteristics of M. tuberculosis SMases helps to elucidate the roles that these enzymes play during infection and disease.

Invasive pathway of Listeria ivanovii in human amnion-derived WISH cells

Among Listeria genus, only two species, Listeria ivanovii and Listeria monocytogenes, are pathogenic. L. ivanovii is almost only associated with infections in animals, mainly sheep and cattle, and has rarely been associated with human infections, whereas L. monocytogenes causes severe illnesses in both humans and animals. To further investigate the pathogenetic features of L. ivanovii in humans, we undertook a study in which the intracellular behaviour of this pathogen was analysed in WISH cells, a cell line derived from human amniotic tissue, and compared to that of L. monocytogenes. Using microbiological, biochemical, and ultrastructural approaches, we demonstrate that L. ivanovii can adhere to and invade human amniotic cells, lyse the phagosomal membrane, polymerize host cell actin, and spread from cell to cell more efficiently than L. monocytogenes. However, although L. ivanovii is capable of specifically infecting and replicating in human amnion cells, its survival in cytoplasm is limited compared to that of L. monocytogenes.

Differences in gamma interferon production induced by listeriolysin O and ivanolysin O result in different levels of protective immunity in mice infected with Listeria monocytogenes and Listeria ivanovii

Two pathogenic species in the genus Listeria, Listeria monocytogenes and Listeria ivanovii, are characterized by the production of hemolysins belonging to cholesterol-dependent cytolysins, listeriolysin O (LLO) and ivanolysin O (ILO), respectively. LLO, produced by L. monocytogenes, is able to induce gamma interferon (IFN-gamma) production and contributes to the generation of Th1-dependent protective immunity. On the other hand, nothing is known about the role of ILO, produced by L. ivanovii, in this regard. In this study, we immunized mice with 0.1 50% lethal dose (LD(50)) of L. monocytogenes and L. ivanovii. Protective immunity against a challenge with 10 LD(50) was generated in mice infected with L. monocytogenes, whereas L. ivanovii infection did not induce protection. After immunization, the level of IFN-gamma in serum samples was increased in mice given L. monocytogenes but not in those given L. ivanovii. To determine the IFN-gamma-inducing activity of cytolysins, recombinant protein was constructed. Recombinant ILO exhibited significantly lower IFN-gamma-inducing activity than LLO. By comparing the IFN-gamma-inducing activity of a chimera incorporating LLO and ILO, it was found that domains 1 to 3 of LLO were critical for IFN-gamma-inducing activity while the counterpart in ILO was unable to induce cytokine production. These results suggested that the weak ability of ILO to induce IFN-gamma production is responsible for the failure of L. ivanovii to generate effective protective immunity.

Modification of the signal sequence cleavage site of listeriolysin O does not affect protein secretion but impairs the virulence of Listeria monocytogenes

Listeriolysin O (LLO, hly-encoded), a major virulence factor secreted by the bacterial pathogen Listeria monocytogenes, is synthesized as a precursor of 529 residues. To impair LLO secretion, the four residues of the predicted signal sequence cleavage site (EA-KD) were deleted and the mutant LLO protein was expressed in a hly-negative derivative of L. monocytogenes. Unexpectedly, the mutant protein was secreted in normal amounts in the culture supernatant and was fully haemolytic. N-terminal sequencing of the secreted LLO molecule revealed that N-terminal processing of the preprotein occurred three residues downstream of the natural cleavage site. L. monocytogenes expressing this truncated LLO showed a reduced capacity to disrupt the phagosomal membranes of bone marrow macrophages and of hepatocytes; and the mutant strain showed a 100-fold decrease in virulence in the mouse model. These results suggest that the first N-terminal residues of mature LLO participate directly in phagosomal escape and bacterial infection.

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

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

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal individuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research.

A method for purification of listeriolysin O from a hypersecretor strain of Listeria monocytogenes

A simple and convenient method for the purification of the hemolytic toxin listeriolysin O (LLO) from Listeria monocytogenes is described. Supernatants from bacteria cultures were purified by application to a CH2 spiral cartridge concentrator (Amicon) and ion exchange chromatography. A critical step is removal of contaminating RNA. The purified proteins had characteristics described for bacterial thiol-activated hemolysins: activation by a reducing agent (DTT) and inactivation by cholesterol. In addition, the molecular weight of 58, 000 and pH-dependent hemolytic activity of this purified protein are consistent with the previously published characteristics of LLO.

A novel PrfA-regulated chromosomal locus, which is specific for Listeria ivanovii, encodes two small, secreted internalins and contributes to virulence in mice

Several large, cell wall-associated internalins and one small, secreted internalin (InlC) have been described previously in Listeria monocytogenes. Using degenerate primers derived from sequenced peptides of an L. ivanovii major secreted protein, we identified a new 4.25 kb internalin locus of L. ivanovii, termed i-inlFE. The two proteins encoded by this locus, i-InlE and i-InlF, belong to the group of small, secreted internalins. Southern blot analyses show that the i-inlFE locus does not occur in L. monocytogenes. These data also indicate that six genes encoding small, secreted internalins are present in L. ivanovii, in contrast to L. monocytogenes, in which inlC encodes the only small internalin. The mature i-InlE protein (198 amino acids) is secreted in large amounts into the brain-heart infusion (BHI) culture medium in the stationary growth phase. In minimum essential medium (MEM), which has been used previously to induce PrfA-dependent gene transcription, i-inlE mRNA and i-InlE protein are expressed at high levels. As shown by Northern blot analysis and primer extension, transcription of the tandemly arranged i-inlF and i-inlE genes is dependent on the virulence regulator PrfA, and characteristic palindromic sequences ('PrfA-boxes') were identified in the promoter regions of i-inlF and i-inlE. Non-polar i-inlE and i-inlF deletion mutants and an i-inlFE double deletion mutant were constructed and tested in the mouse infection model. After intravenous infection, all three mutants entirely failed to kill C57BL/6 mice even at high infectious doses of 109 bacteria per mouse, whereas the LD50 for the parental strain was determined as 4 x 107 bacteria per mouse. These data suggest an important role for i-InlE and i-InlF in L. ivanovii virulence.

Dermatophilus congolensis: strain differences in expression of phospholipase activities

Interactions between Dermatophilus congolensis strains and with other bacteria of known haemolytic activities were used to elucidate the complex nature of haemolytic activities present in various D. congolensis strains. This was further analysed by measuring their specific phospholipase activities against defined substrates by thin layer chromatography. D. congolensis strains demonstrated haemolytic interactions (synergistic or antagonistic) with other D. congolensis strains and also other species of bacteria. Most isolates expressed lyso-phospholipase-D activity, while various strains also expressed sphingomyelinase-D activity, phospholipase-A versus phosphatidylcholines and/or cephalins, phospholipase-D versus phosphatidylcholines or all these activities, under the culture conditions used.

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.

The virulence gene cluster of Listeria monocytogenes is also present in Listeria ivanovii, an animal pathogen, and Listeria seeligeri, a nonpathogenic species

Most known Listeria monocytogenes virulence genes cluster within a 9.6-kb chromosomal region. This region is flanked on one end by two uncharacterized open reading frames (ORF A and ORF B) and ldh, an ORF presumably encoding the L. monocytogenes lactate dehydrogenase (J.-A. Vazquez-Boland, C. Kocks, S. Dramsi, H. Ohayon, C. Geoffroy, J. Mengaud, and P. Cossart, Infect. Immun. 60:219-230, 1992). We report here that the other end is flanked by prs, and ORF homologous to phosphoribosyl PPi synthetase genes. ORF B and prs were detected in all Listeria species and thus delimit the virulence region. This virulence gene cluster was detected exclusively in hemolytic Listeria species, Listeria ivanovii, an animal pathogen, and Listeria seeligeri, a nonpathogenic species.

Immunogold labelling of Listeria monocytogenes virulence-related factors within Caco-2 cells

We demonstrated by immunoelectron microscopy that listeriolysin O (LLO), phospholipases and other putative virulence-related proteins produced by Listeria monocytogenes were primarily cell-wall-associated when the bacterium infected Caco-2 tissue culture cell monolayers. Antibodies made to LLO, serogroup 1/2a reacted poorly with serogroup 4b cells and vice-versa, indicating fundamental structural differences in the two proteins. Finally, comet-tail pseudopod structures shown to be involved in cell-to-cell passage of Listeria in Caco-2 cells did not possess detectable Listeria antigens on their anterior surface or within their structure, suggesting that the phagocytic process is primarily host-cell-dependent once it is initiated by the bacterial cell.

Listeria ivanovii is capable of cell-to-cell spread involving actin polymerization

Listeria ivanovii has been considered to be pathogenic to animals but has rarely been found associated with human infections. It has been claimed that L. ivanovii lacks the actA gene, which in L. monocytogenes encodes a protein required for interaction with host cell actin. Using fluorescence microscopy and electron microscopy, we demonstrate that L. ivanovii can invade mammalian cells, lyse the phagosomal membrane, polymerize host cell actin, reorganize actin to form tails, and spread from cell to cell. However, no DNA homologous to the actA gene could be detected by polymerase chain reaction. Further, L. ivanovii lacks the 90-kDa surface protein which in L. monocytogenes is encoded by actA. Despite the ability to spread from cell to cell, L. ivanovii differed significantly from L. monocytogenes in being unable to form plaques on monolayers of 3T3 fibroblast cells.

Identification and enumeration of Listeria monocytogenes by nonradioactive DNA probe colony hybridization

A plasmid containing the cloned listeriolysin gene of Listeria monocytogenes was used as a probe to identify Listeria strains by DNA colony hybridization. The probe DNA was labeled with horseradish peroxidase in the presence of glutaraldehyde. After the hybridization and wash procedures, the hybrid molecules were detected by luminescence, which resulted from the oxidation of luminol by a horseradish peroxidase-hydrogen peroxide-coupled reaction. Of the 150 Listeria strains and 16 non-Listeria strains examined, the probe hybridized only with L. monocytogenes. The technique was also used to enumerate L. monocytogenes in artificially contaminated foods.

Purification of listeriolysin O and development of an immunoassay for diagnosis of listeric infections in sheep

A protein of 58,000-Da molecular mass was purified from the supernatant fluid of a dialysis sac culture of Listeria monocytogenes by cation-exchange chromatography. The purified protein, homogeneous by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and possessing the characteristics of listeriolysin O (LLO), was used to develop an indirect enzyme-linked immunosorbent assay. Anti-LLO antibodies were shown to be consistently produced in sheep after experimental challenge with L. monocytogenes serovar 4b. The assay also successfully detected and measured specific anti-LLO antibodies in the sera of silage-fed sheep among which listeric enteritis and abortions had occurred.

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.

Listeriolysin genes: complete sequence of ilo from Listeria ivanovii and of lso from Listeria seeligeri

The complete DNA sequences coding for the thiol-activated cytolysins from Listeria ivanovii, ivanolysin O (ILO) and for seeligerolysin O (LSO) from Listeria seeligeri have been determined. The deduced amino acid sequences revealed that: (i) the primary translation products comprise 528 (ILO) and 530 (LSO) amino acids, respectively, (ii) ILO contains two cysteines, LSO has a substitution in the conserved cysteine motif.

Listeria monocytogenes, a food-borne pathogen

The gram-positive bacterium Listeria monocytogenes is an ubiquitous, intracellular pathogen which has been implicated within the past decade as the causative organism in several outbreaks of foodborne disease. Listeriosis, with a mortality rate of about 24%, is found mainly among pregnant women, their fetuses, and immunocompromised persons, with symptoms of abortion, neonatal death, septicemia, and meningitis. Epidemiological investigations can make use of strain-typing procedures such as DNA restriction enzyme analysis or electrophoretic enzyme typing. The organism has a multifactorial virulence system, with the thiol-activated hemolysin, listeriolysin O, being identified as playing a crucial role in the organism's ability to multiply within host phagocytic cells and to spread from cell to cell. The organism occurs widely in food, with the highest incidences being found in meat, poultry, and seafood products. Improved methods for detecting and enumerating the organism in foodstuffs are now available, including those based on the use of monoclonal antibodies, DNA probes, or the polymerase chain reaction. As knowledge of the molecular and applied biology of L. monocytogenes increases, progress can be made in the prevention and control of human infection.

Purification and characterization of an extracellular 29-kilodalton phospholipase C from Listeria monocytogenes

We purified and characterized an extracellular phospholipase produced by Listeria monocytogenes. This enzyme was separated as a homogeneous protein of 29 kDa by chromatography on DEAE-52 cellulose and Bio-Gel P100 columns. It is a zinc-dependent phospholipase C (PLC) that is mainly active at pH 6 to 7 and expresses lecithinase activity and a weaker sphingomyelinase activity. The exoenzyme also hydrolyzed phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin but not phosphatidylinositol. It was distinct from the 36-kDa phosphatidylinositol PLC produced by L. monocytogenes and from the L. ivanovii sphingomyelinase. The pure protein expressed a weak, calcium-independent hemolytic activity and was not toxic in mice. Western immunoblot analysis using a rabbit immune serum raised against the enzyme showed that all virulent strains of L. monocytogenes tested produced in the culture supernatant a 29-kDa PLC. In contrast, no proteins antigenically related to the 29-kDa PLC were detected in supernatants of L. ivanovii, L. seeligeri, L. innocua, or L. welshimeri. The role in virulence of the 29-kDa PLC specifically produced by L. monocytogenes remains to be established.

Detection and identification of Listeria monocytogenes in cooked sausage products and in milk by in vitro amplification of haemolysin gene fragments

Recent outbreaks of listeriosis have emphasized the urgent need for rapid and reliable detection methods for Listeria spp., especially in food. Haemolysin production is a major factor in the pathogenesis of listeriosis and the polymerase chain reaction (PCR) was used to amplify two specific DNA fragments of the alpha- and the beta-haemolysin genes. The amplification system specifically recognized L. monocytogenes strains. The detection limit determined with pure cultures was 10 bacteria when estimated with alpha-haemolysin primers. In the analysis of 50 samples of cooked sausage products, bacterial colonies suspected to be Listeria spp. were isolated by conventional methods from six samples. PCR analysis identified three of six as L. monocytogenes. Subsequent serotyping showed perfect agreement with the PCR results. Since enrichment is the most time consuming step in conventional methods a PCR procedure which allows the direct detection of L. monocytogenes in milk was developed. Pasteurized milk was artificially contaminated with various levels of L. monocytogenes. The detection limit was determined to be 10 bacteria/10 ml milk and direct detection and identification of L. monocytogenes took less than two working days. These results show that this haemolysin gene amplification system is very rapid and reliable and therefore avoids cumbersome and lengthy cultivation steps.

Pore-forming bacterial protein hemolysins (cytolysins)

Protein toxins forming pores in biological membranes occur frequently in Gram-positive and Gram-negative bacteria. They kill either bacteria or eukaryotic cells (at most, a few seem to act on both groups of organisms). Most of the toxins affecting eukaryotes have clearly been shown to be related to the pathogenicity of the producing organisms. Toxin formation frequently involves a number of genes which encode the toxin polypeptide as well as proteins for toxin activation and secretion. Regulation of toxin production is usually coupled with that of the synthesis of a number of other virulence factors. Iron is the only known environmental factor that regulates transcription of a number of toxin genes by a Fur repressor-type mechanism, as has been originally described in Escherichia coli. Interestingly, the thiol-activated hemolysins (cytolysins) of Gram-positive bacteria contain a single cysteine which can be replaced by alanine without affecting the cytolytic activity. The Gram-negative hemolysins (cytolysins) are usually synthesized as precursor proteins, then covalently modified to yield an active hemolysin and secreted via specific export systems, which differ for various types of hemolysins.

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.

Functional similarity of Listeria ivanovii and Staphylococcus aureus in CAMP test

On the basis of synergistic haemolysis of Listeria ivanovii and Rhodococcus equi, we suspected CAMP positiveness of R. equi and complete CAMP negativeness of L. ivanovii and confirmed that the latter, showing a double-zone haemolysis like Staphylococcus aureus, could produce the same phenomenon in the CAMP test if used instead of staphylococci. This functional similarity of L. ivanovii and CAMP Staphylococcus could be used as an additional diagnostic test for L. ivanovii.

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.

Purification and characterization of two Listeria ivanovii cytolysins, a sphingomyelinase C and a thiol-activated toxin (ivanolysin O)

The strong bizonal hemolysis on blood agar and the positive CAMP reaction with Rhodococcus equi denotes the production of two different cytolytic factors by Listeria ivanovii. One was characterized as a thiol-activated (SH) cytolysin of 61 kilodaltons and was termed ivanolysin O (ILO) since data suggested that it is different from listeriolysin O, the SH-cytolysin produced by Listeria monocytogenes. The other is a 27-kilodalton hemolytic sphingomyelinase C that was found to be the cytolytic factor responsible for the halo of incomplete hemolysis synergistically enhanced by R. equi exosubstances. When thiol-disulfide exchange affinity chromatography and gel filtration were applied to the purification of ILO from concentrated L. ivanovii culture supernatants, the copurification of the two cytolysins was observed. This phenomenon seems to be due to the formation of intermolecular disulfide bonds between ILO and the sphingomyelinase, since the latter was found to contain free SH groups, not essential for the activity. These SH groups could react with the single cysteine residue characteristically present in the SH-cytolysins, forming a dimeric cytolytic complex. The purification of ILO was achieved by a further gel filtration with a reducing agent (dithiothreitol) in the eluent. A method for the purification of the sphingomyelinase based on selective sequestration of ILO from the L. ivanovii concentrated culture supernatant by the SH cytolysin target molecule cholesterol and thiol-disulfide affinity chromatography is described.

Detection of listeriolysin, the thiol-dependent hemolysin in Listeria monocytogenes, Listeria ivanovii, and Listeria seeligeri

The listeriolysin gene from a weakly hemolytic but virulent strain of Listeria monocytogenes serotype 1/2a was cloned in Escherichia coli K-12. Recombinants were identified on the basis of their cross-reactivities to hyperimmune antisera raised against streptolysin O and listeriolysin. Low levels of hemolytic activity were detected in crude lysates of strains harboring the listeriolysin gene. In DNA hybridization studies with five DNA probes that encoded the listeriolysin gene and surrounding sequences, highly homologous listeriolysin genes were found to be present in the species L. monocytogenes, Listeria ivanovii, and Listeria seeligeri. Immunoblotting performed with affinity-purified antibody to listeriolysin allowed the detection of this protein in supernatants of all three species. This study demonstrates for the first time that listeriolysin is produced by L. seeligeri and documents the genetic homology between the various listeriolysins produced by Listeria spp. Sequences unique to the species L. monocytogenes were found to be located downstream of the listeriolysin gene. Furthermore, the restriction fragment length polymorphisms detected with probes flanking the hlyA gene may be useful epidemiological markers in identifying and distinguishing virulent Listeria strains from each other.