Membrane damage by pore-forming bacterial cytolysins

Channel Formation by LktA of Mannheimia (Pasteurella) haemolytica in Lipid Bilayer Membranes and Comparison of Channel Properties with Other RTX-Cytolysins

Cytolysin LktA is one of the major pathogenicity factors of Mannheimia haemolytica (formerly Pasteurella haemolytica) that is the cause of pasteurellosis, also known as shipping fever pneumonia, causing substantial loss of sheep and cattle during transport. LktA belongs to the family of RTX-toxins (Repeats in ToXins) that are produced as pathogenicity factors by a variety of Gram-negative bacteria. Sublytic concentrations of LktA cause inflammatory responses of ovine leukocytes. Higher concentrations result in formation of transmembrane channels in target cells that may cause cell lysis and apoptosis. In this study we investigated channel formation by LktA in artificial lipid bilayer membranes made of different lipids. LktA purified from culture supernatants by polyethylene glycol 4000 precipitation and lyophilization had to be activated to frequently form channels by solution in 6 M urea. The LktA channels had a single-channel conductance of about 60 pS in 0.1 M KCl, which is about one tenth of the conductance of most RTX-toxins with the exception of adenylate cyclase toxin of Bordetella pertussis. The LktA channels are highly cation-selective caused by negative net charges. The theoretical treatment of the conductance of LktA as a function of the bulk aqueous concentration allowed a rough estimate of the channel diameter, which is around 1.5 nm. The size of the LktA channel is discussed with respect to channels formed by other RTX-toxins. We present here the first investigation of LktA in a reconstituted system.

Repair of a Bacterial Small β-Barrel Toxin Pore Depends on Channel Width

Membrane repair emerges as an innate defense protecting target cells against bacterial pore-forming toxins. Here, we report the first paradigm of Ca²⁺-dependent repair following attack by a small β-pore-forming toxin, namely, plasmid-encoded phobalysin of Photobacterium damselae subsp. damselae. In striking contrast, Vibrio cholerae cytolysin, the closest ortholog of phobalysin, subverted repair. Mutational analysis uncovered a role of channel width in toxicity and repair. Thus, the replacement of serine at phobalysin´s presumed channel narrow point with the bulkier tryptophan, the corresponding residue in Vibrio cholerae cytolysin (W318), modulated Ca²⁺ influx, lysosomal exocytosis, and membrane repair. And yet, replacing tryptophan (W318) with serine in Vibrio cholerae cytolysin enhanced toxicity. The data reveal divergent strategies evolved by two related small β-pore-forming toxins to manipulate target cells: phobalysin leads to fulminant perturbation of ion concentrations, closely followed by Ca²⁺ influx-dependent membrane repair. In contrast, V. cholerae cytolysin causes insidious perturbations and escapes control by the cellular wounded membrane repair-like response.

Previous studies demonstrated that large transmembrane pores, such as those formed by perforin or bacterial toxins of the cholesterol-dependent cytolysin family, trigger rapid, Ca²⁺ influx-dependent repair mechanisms. In contrast, recovery from attack by the small β-pore-forming Staphylococcus aureus alpha-toxin or aerolysin is slow in comparison and does not depend on extracellular Ca²⁺. To further elucidate the scope of Ca²⁺ influx-dependent repair and understand its limitations, we compared the cellular responses to phobalysin and V. cholerae cytolysin, two related small β-pore-forming toxins which create membrane pores of slightly different sizes. The data indicate that the channel width of a small β-pore-forming toxin is a critical determinant of both primary toxicity and susceptibility to Ca²⁺-dependent repair.

Phobalysin, a Small β-Pore-Forming Toxin of Photobacterium damselae subsp. damselae

Photobacterium damselae subsp. damselae, an important pathogen of marine animals, may also cause septicemia or hyperaggressive necrotizing fasciitis in humans. We previously showed that hemolysin genes are critical for virulence of this organism in mice and fish. In the present study, we characterized the hlyA gene product, a putative small β-pore-forming toxin, and termed it phobalysin P (PhlyP), for "photobacterial lysin encoded on a plasmid." PhlyP formed stable oligomers and small membrane pores, causing efflux of K(+), with no significant leakage of lactate dehydrogenase but entry of vital dyes. The latter feature distinguished PhlyP from the related Vibrio cholerae cytolysin. Attack by PhlyP provoked a loss of cellular ATP, attenuated translation, and caused profound morphological changes in epithelial cells. In coculture experiments with epithelial cells, Photobacterium damselae subsp. damselae led to rapid hemolysin-dependent membrane permeabilization. Unexpectedly, hemolysins also promoted the association of P. damselae subsp. damselae with epithelial cells. The collective observations of this study suggest that membrane-damaging toxins commonly enhance bacterial adherence.

eIF2α Confers Cellular Tolerance to S. aureus α-Toxin

We report on the role of conserved stress-response pathways for cellular tolerance to a pore forming toxin. First, we observed that small molecular weight inhibitors including of eIF2α-phosphatase, jun-N-terminal kinase (JNK), and PI3-kinase sensitized normal mouse embryonal fibroblasts (MEFs) to the small pore forming S. aureus α-toxin. Sensitization depended on expression of mADAM10, the murine ortholog of a proposed high-affinity receptor for α-toxin in human cells. Similarly, eIF2α (S51A/S51A) MEFs, which harbor an Ala knock-in mutation at the regulated Ser51 phosphorylation site of eukaryotic translation initiation factor 2α, were hyper-sensitive to α-toxin. Inhibition of translation with cycloheximide did not mimic the tolerogenic effect of eIF2α-phosphorylation. Notably, eIF2α-dependent tolerance of MEFs was toxin-selective, as wild-type MEFs and eIF2α (S51A/S51A) MEFs exhibited virtually equal sensitivity to Vibrio cholerae cytolysin. Binding of S. aureus α-toxin to eIF2α (S51A/S51A) MEFs and toxicity in these cells were enhanced as compared to wild-type cells. This led to the unexpected finding that the mutant cells carried more ADAM10. Because basal phosphorylation of eIF2α in MEFs required amino acid deprivation-activated eIF2α-kinase 4/GCN2, the data reveal that basal activity of this kinase mediates tolerance of MEFs to α-toxin. Further, they suggest that modulation of ADAM10 is involved. During infection, bacterial growth may cause nutrient shortage in tissues, which might activate this response. Tolerance to α-toxin was robust in macrophages and did not depend on GCN2. However, JNKs appeared to play a role, suggesting differential cell type and toxin selectivity of tolerogenic stress responses. Understanding their function or failure will be important to comprehend anti-bacterial immune responses.

Pneumolysin causes neuronal cell death through mitochondrial damage

Bacterial toxins such as pneumolysin are key mediators of cytotoxicity in infections. Pneumolysin is a pore-forming toxin released by Streptococcus pneumoniae, the major cause of bacterial meningitis. We found that pneumolysin is the pneumococcal factor that accounts for the cell death pathways induced by live bacteria in primary neurons. The pore-forming activity of pneumolysin is essential for the induction of mitochondrial damage and apoptosis. Pneumolysin colocalized with mitochondrial membranes, altered the mitochondrial membrane potential, and caused the release of apoptosis-inducing factor and cell death. Pneumolysin induced neuronal apoptosis without activating caspase-1, -3, or -8. Wild-type pneumococci also induced apoptosis without activation of caspase-3, whereas pneumolysin-negative pneumococci activated caspase-3 through the release of bacterial hydrogen peroxide. Pneumolysin caused upregulation of X-chromosome-linked inhibitor of apoptosis protein and inhibited staurosporine-induced caspase activation, suggesting the presence of actively suppressive mechanisms on caspases. In conclusion, our results indicate additional functions of pneumolysin as a mitochondrial toxin and as a determinant of caspase-independent apoptosis. Considering this, blocking of pneumolysin may be a promising cytoprotective strategy in pneumococcal meningitis and other infections.

Drug treatment of pneumococcal pneumonia in the elderly

Streptococcus pneumoniae has been recognised as a major cause of pneumonia since the time of Sir William Osler. Drug-resistant S. pneumoniae (DRSP), which have gradually become resistant to penicillins as well as more recently developed macrolides and fluoroquinolones, have emerged as a consequence of indiscriminate use of antibacterials coupled with the ability of the pneumococcus to adapt to a changing antibacterial milieu. Pneumococci use cell wall choline components to bind platelet-activating factor receptors, colonise mucosal surfaces and evade innate immune defenses. Numerous virulence factors that include hyaluronidase, neuraminidase, iron-binding proteins, pneumolysin and autolysin then facilitate cytolysis of host cells and allow tissue invasion and bloodstream dissemination. Changes in pneumococcal cell wall penicillin-binding proteins account for resistance to penicillins, mutations in the ermB gene cause high-level macrolide resistance and mutations in topoisomerase IV genes coupled with GyrA gene mutations alter DNA gyrase and lead to high-level fluoroquinolone resistance. Risk factors for lower respiratory tract infections in the elderly include age-associated changes in oral clearance, mucociliary clearance and immune function. Other risks for developing pneumonia include poor nutrition, hypoalbuminaemia, bedridden status, aspiration, recent viral infection, the presence of chronic organ dysfunction syndromes including parenchymal lung disease and recent antibacterial therapy. Although the incidence of infections caused by DRSP is rising, the effect of an increase in the prevalence of resistant pneumococci on mortality is not clear. When respiratory infections occur, rapid diagnosis and prompt, empirical administration of appropriate antibacterial therapy that ensures adequate coverage of DRSP is likely to increase the probability of a successful outcome when treating community-acquired pneumonia in elderly patients, particularly those with multiple risk factors for DRSP. A chest x-ray is recommended for all patients, but other testing such as obtaining a sputum Gram's smear is not necessary and should not prolong the time gap between clinical suspicion of pneumonia and antibacterial administration. The selection of antibacterials should be based upon local resistance patterns of suspected organisms and the bactericidal efficacy of the chosen drugs. If time-dependent agents are chosen and DRSP are possible pathogens, dosing should keep drug concentrations above the minimal inhibitory concentration that is effective for DRSP. Treatment guidelines and recent studies suggest that combination therapy with a beta-lactam and macrolide may be associated with a better outcome in hospitalised patients, and overuse of fluoroquinolones as a single agent may promote quinolone resistance. The ketolides represent a new class of macrolide-like antibacterials that are highly effective in vitro against macrolide- and azalide-resistant pneumococci. Pneumococcal vaccination with the currently available polysaccharide vaccine is thought to confer some preventive benefit (preventing invasive pneumococcal disease), but more effective vaccines, such as nonconjugate protein vaccines, need to be developed that provide broad protection against pneumococcal infection.

RTX toxin structure and function: a story of numerous anomalies and few analogies in toxin biology

It can be agreed that RTX toxins contribute to the pathogenesis of different diseases by causing dysfunction of the general cellular reactions of the immune response. The suggestion that RTX toxins induce cytokine production in nonimmune cells that would ultimately cause tissue damage is an expansion of their role in disease pathogenesis (Uhlen et al. 2000). Investigators in the RTX toxin field may not agree with me, but precise and satisfactory answers to the following questions are not yet available. How do RTX toxins mechanistically damage a cell? Do RTX toxins have receptors in the classic sense, in which there is a reversible ligand and receptor complex? What is responsible for the common Ca2+ ion influx in affected cells? The recent observation that an RTX toxin stimulates host-cell-mediated Ca2+ ion oscillation in part challenges the long held concept that these toxins damage cells by the direct formation of pores. Are the Ca2+ ion fluxes truly the noxious cellular insult? What is the final molecular structure of RTX toxins at the time they cause cellular death? How does the common requirement for acyl modification among RTX toxins fit into the toxin structure and mechanism of cellular killing, particularly when mixtures of unusual fatty acids are used by some toxins? There are a number of outstanding laboratories throughout the world that are seeking answers to these questions. We can reasonably expect that during the next decade research on the structure and function of RTX toxins will lead to new chemotherapeutic targets and reagents for basic cell biology and biotechnology.

Biological properties of pneumolysin

Pneumolysin is a thiol-activated membrane-damaging toxin produced by Streptococcus pneumoniae. The toxin plays a role in virulence of the pneumococcus in animal models of infection. Pneumolysin has a range of biological activity including the ability to lyse eukaryotic cells and to interfere with the function of cells and soluble molecules of the immune system. The use of purified native and mutant toxin and of isogenic mutants of the pneumococcus expressing altered versions of the toxin has allowed the contribution of the various activities of this multifunctional toxin to virulence to be defined.

Bacillus cereus causing fulminant sepsis and hemolysis in two patients with acute leukemia

PURPOSE

Hemolysis is so rarely associated with Bacillus cereus sepsis that only two very well documented cases have been reported. This article reports two unusual cases of Bacillus cereus sepsis with massive intravascular hemolysis in patients who had acute lymphoblastic leukemia (ALL).

PATIENTS AND METHODS

A 20-year-old woman who was 9 weeks pregnant experienced a relapse of ALL. A therapeutic abortion was performed. During week 4 of reinduction the patient had abdominal pain, nausea, and vomiting, with severe neutropenia but no fever. Her condition deteriorated rapidly with cardiovascular collapse, acute massive intravascular hemolysis, and death within hours of the onset of symptoms. Blood cultures were positive for Bacillus cereus. Postmortem histologic examination and cultures revealed Bacillus cereus and Candida albicans in multiple organs. The second patient, a 10-year-old girl, presented with relapsed T-cell ALL. In the second week of reinduction, she had abdominal pain followed by hypotension. Again, no fever was noted. Laboratory studies showed intravascular hemolysis 12 hours after admission. Aggressive support was promptly initiated. Despite disseminated intravascular coagulation; cardiovascular, hepatic, and renal failure; and multiple intracerebral hypodense lesions believed to be infarcts, the patient recovered fully and resumed reinduction therapy.

CONCLUSIONS

Bacillus cereus infection can have a fulminant clinical course that may be complicated by massive intravascular hemolysis. This pathogen should be suspected in immunosuppressed patients who experience gastrointestinal symptoms and should not be precluded by the absence of fever, especially if steroids such as dexamethasone are being given. Exchange transfusion may be lifesaving in Bacillus cereus septicemia associated with massive hemolysis.

Acanthamoeba castellanii metabolites increase the intracellular calcium level and cause cytotoxicity in wish cells

Previous studies have shown that trophozoites of the pathogenic free-living amoeba Acanthamoeba castellanii rapidly lyse a variety of cells in vitro. However, the role played by cytolitic molecules that may participate in Acanthamoebal cytopathogenicity has yet to be completely elucidated. The aim of this work was to study whether soluble molecules released by A. castellanii trophozoites could induce cytopathic effect in human epithelial cells in vitro. The results obtained indicate that A. castellanii trophozoites constitutively elaborate and release soluble factors that immediately elicit a cytosolic free-calcium increase in target cells. This phenomenon is induced by low molecular weight amoebic metabolites and depends on a transmembrane influx of extracellular calcium. Morphological changes, cytoskeletal damage, cell death and cytolysis followed the elevation of cytosolic free-calcium levels. Calcium ions are very important for cell homeostasis, in fact, they control the functions of a variety of cellular responses, including secretion, cell proliferation and apoptosis. Our results suggest that the substained elevation of the cytosolic free-calcium in response to A. castellanii metabolites might play a fundamental role in target cell damage during Acanthamoeba infections.

Prelytic and lytic conformations of erythrocyte-associated Escherichia coli hemolysin

Flow cytometry was developed as a method to assess the conformation of erythrocyte-bound Escherichia coli hemolysin polypeptide (HlyA). Topology of membrane-associated hemolysin (HlyA(E)) was investigated by testing surface accessibility of HlyA regions in lytic and nonlytic bound states, using a panel of 12 anti-HlyA monoclonal antibodies (MAbs). Hemolysin associates nonlytically with erythrocytes at 0 to 2 degrees C. To test the hypothesis that the nonlytic HlyA(E) conformation at 0 to 2 degrees C differs from the lytic conformation at 23 degrees C, MAb epitope reactivity profiles at the two temperatures were compared by flow cytometry. Four MAbs have distinctly increased reactivity at 0 to 2 degrees C compared to 23 degrees C. HlyA requires HlyC-dependent acylation at lysine residues 563 and 689 for lytic function. Toxin with cysteine substitution mutations at each lysine (HlyA(K563C) and HlyA(K689C)) as well as the nonacylated form of hemolysin made in a HlyC-deficient strain were examined by flow cytometry at 0 to 2 and 23 degrees C. The three mutants bind erythrocytes at wild-type toxin levels, but there are conformational changes reflected by altered MAb epitope accessibility for six of the MAbs. To test further the surface accessibility of regions in the vicinity of MAb-reactive epitopes, HlyA(E) was proteolytically treated prior to testing for MAb reactivity. Differences in protease susceptibility at 0 to 2 degrees and 23 degrees C for the reactivities of three of the MAbs further support the model of two distinct conformations of cell-associated toxin.

Biochemical and physiological characteristics of HlyA, a pore-forming cytolysin of Vibrio cholerae serogroup O1

Among the various toxins produced by the bacterial species Vibrio cholerae is HlyA, a cytolytic protein commonly called the E1 Tor hemolysin. HlyA is synthesized and processed in a complex manner involving various processed or degraded forms, that may co-purify and complicate the interpretation of biochemical and physiological experiments. In this study a single form of HlyA was purified by gel filtration and chromatofocusing using fast protein liquid chromatography in the presence of protease inhibitors. A 45-fold purification was obtained, with a final recovery of 17% of pure 60,000 mol. wt HlyA. A significant improvement in specific activity to 8.5 x 10(6) Chinese hamster ovary tissue culture units per mg protein was obtained. Physiological activity studies indicated that cytolysis of erythrocytes (hemolysis) was inhibited by oxygen: storage of HlyA under oil, and experimentation in N2-flushed buffers maintained activity. HlyA-mediated lysis of human erythrocytes was characterized by a significant lag phase, followed by a rapid induction of hemolysis. Hemolysis was inhibited by sucrose, an osmotic protectant, suggesting that the initial action of HlyA on erythrocytes is to raise the basal cation permeability of the cell membrane. The most likely cytolytic mechanism is thus the formation of transmembrane lesions such as homopolymer pores in target cells, as has been found for toxins from numerous other bacterial pathogens.

Phosphorylation of a 25 kDa protein is induced by thermostable direct hemolysin of Vibrio parahaemolyticus

Thermostable direct hemolysin (TDH) is a possible virulence factor produced by Vibrio parahaemolyticus. Although TDH has a variety of biological activities, including hemolytic activity, the biochemical mechanism of action remains uncertain. Here we analysed biochemical events, especially phosphorylation, caused by TDH in erythrocytes, and found that TDH caused significant phosphorylations of proteins on erythrocyte membrane. Phosphorylation of proteins was studies using [gamma-32P] ATP and SDS-PAGE. A number of protein kinase inhibitors were tested, to determine which types of kinases were involved in the phosphorylation events. TDH induced the phosphorylation of two proteins on membranes of human erythrocyte that are sensitive to TDH. The estimated molecular weight of these proteins was 25 and 22.5 kDa. Interestingly, the 22.5 kDa, but not the 25 kDa protein, was phosphorylated on the membrane of TDH-insensitive (resistant) horse erythrocytes. Moreover, a mutant TDH (R7), which retained binding ability but lost hemolytic activity, also phosphorylated only the 22.5 kDa protein on human erythrocyte membranes. Among the protein kinase inhibitors used the protein kinase C inhibitors, (staurosporine and calphostin C) showed marked inhibition of phosphorylation of 25kDa protein. In addition to phosphorylation, these protein kinase C inhibitors suppressed hemolysis by TDH. These results indicate that the phosphorylation of the 25 kDa protein seems to be essential for the hemolysis by TDH after it binds to erythrocyte membranes.

Systemic and mucosal protective immunity to pneumococcal surface protein A

To date our studies demonstrate that PspA is a highly immunogenic molecule in mice and that it can elicit immunity to otherwise fatal infections following iv, ip, in, and it challenge. Although the molecule is serologically variable, it is sufficiently cross-reactive so that immunization with a single PspA can protect against strains of highly diverse serotypes. It is anticipated that a vaccine composed of a mixture of carefully chosen PspA molecules will be able to elicit protective immunity to virtually all pneumococci. If this vaccine proved efficacious in man, it would provide a more simple and less costly means of immunizing against pneumococcal infection than using recombinant vaccines. This could be especially important in the developing world where the cost of successful vaccines must be no more than pennies per dose. If PspA is found to be less efficacious than capsular polysaccharides, it may be valuable as a protein component of a PS-protein conjugate vaccine. In this capacity, PspA might expand the breath of protection elicited by a vaccine composed of only a few polysaccharide-protein conjugates representing capsule types most commonly associated with infectious pneumococci.

Evidence that a region(s) of the Clostridium perfringens enterotoxin molecule remains exposed on the external surface of the mammalian plasma membrane when the toxin is sequestered in small or large complexes

In studies performed to investigate the topology of Clostridium perfringens enterotoxin (CPE) when this toxin is associated with intestinal brush border membrane (BBMs), it was shown that radiolabeled CPE antibodies react more strongly against intact CPE-treated BBMs than against control BBMs. Immunoprecipitation studies then demonstrated that CPE antibodies are able to react with both small and large CPE-containing complexes while these complexes are still present in intact BBMs. Therefore, at least a portion of the CPE molecule appears to remain surface exposed in BBMs throughout the action of this toxin.

Effect of defined point mutations in the pneumolysin gene on the virulence of Streptococcus pneumoniae

The thiol-activated toxin pneumolysin is a known pneumococcal virulence factor, with both cytotoxic (hemolytic) and complement activation properties. Copies of the pneumolysin gene carrying defined point mutations affecting either or both of these properties were introduced into the chromosome of Streptococcus pneumoniae D39 by insertion-duplication mutagenesis. The virulences of these otherwise isogenic strains were then compared. There was no significant difference in either the median survival time or overall survival rate between mice challenged with D39 derivatives producing the wild-type toxin and those expressing a pneumolysin gene with an Asp-385-->Asn mutation, which abolishes the complement activation property. However, mice challenged with strains carrying either His-367-->Arg or Trp-433-->Phe plus Cys-428-->Gly mutations, which reduce hemolytic activity to approximately 0.02 and 0.0001% of the wild-type level, respectively, had significantly greater median survival times and overall survival rates than mice challenged with D39 derivatives expressing a wild-type pneumolysin gene. No additional reduction in virulence was observed when mice were challenged with a D39 derivative carrying Trp-433-->Phe, Cys-428-->Gly, and Asp-385-->Asn, rather than Trp-433-->Phe and Cys-428-->Gly, mutations in the pneumolysin gene. Thus, it appears that in the intraperitoneal challenge model, the contribution of pneumolysin to virulence is largely attributable to its hemolytic (cytotoxic) properties rather than to its capacity to activate complement. Interestingly, however, the amount of pneumolysin required for full virulence may be very small, as D39 derivatives carrying the Trp-433-->Phe mutation (which reduces hemolytic activity to 0.1% of the wild-type level) had intermediate virulence.

Mammalian cells transfected with the listeriolysin gene exhibit enhanced proliferation and focus formation

Mouse 3T6 and 3T3 fibroblasts and rat epithelial L2 cells were transfected with recombinant plasmids containing the listeriolysin gene (hly) of Listeria monocytogenes. This bacterial gene (with and without the 5' signal sequence) was cloned under the control of a murine metallothionein promoter, resulting in elevated transcription of both forms of the hly gene after induction with ZnSO4. However, the gene product could be observed only when the listeriolysin gene lacking the 5' signal sequence was used. Intact listeriolysin could not be detected in the cytoplasm or in the supernatant of the hly-transfected cells. 3T6 and L2 cells transfected with the intact hly gene exhibited significantly increased cell proliferation and increased formation of actin microfilaments upon induction of hly expression with ZnSO4. Both cell types are not contact inhibited and formed large piles of spherical cells after transfection with hly. In contrast, contact-inhibited 3T3 cells transfected with the hly gene showed increased proliferation but no formation of such cell aggregates. When 3T6 fibroblasts were transfected with the hly gene without the 5' signal sequence, inhibition of growth, lack of cell layer confluency, and altered (spherical) cell morphology were observed.

Mechanism of action of equinatoxin II, a cytolysin from the sea anemone Actinia equina L. belonging to the family of actinoporins

Actinia equina equinatoxin II (EqT-II) is a representative of a family of pore-forming, basic, polypeptide toxins from sea anemones, now called actinoporins. This family comprises at least 27 members, which are all hemolytic at rather low concentrations. Red blood cell (RBC) hemolysis by EqT-II is the result of a colloid-osmotic shock caused by the opening of toxin-induced pores. Using osmotic protectants of different size the functional radius of the lesion was estimated to be approximately 1.1 nm. These pores are most probably constituted by oligomeric aggregates of cytolysin molecules, whose presence on the membrane of lysed RBC was directly demonstrated by polyacrylamide gel electrophoresis (PAGE) after covalent cross-linking. EqT-II is active also against a variety of mammalian cells including leukocytes, platelets and cardiomiocytes. An increased permeability of the plasma membrane after Eq-II attack is compatible with the notion that the toxin forms pores also on these cells. Eq-II permeabilises even purely lipidic model membranes, suggesting a protein receptor is not necessary. Using calcein-loaded unilamellar vesicles (UVs) comprised of phosphatydylcholine (PC) mixed with other lipids we observed that the rate and extent of permeabilization greatly increases when sphingomyelin (SM) or the ganglioside GM1 were introduced, particularly in the case of large UVs (which are more sensitive to the toxin than small UVs). PAGE indicated that the increased effect of Eq-II on SM containing vesicles is due to an increased level of toxin binding to such vesicles. The formation of cation-selective channels by EqT-II was directly demonstrated using planar lipid membranes where the toxin induced discrete increases of the film conductivity. The conductance of the channel was consistent with the estimated size of the lesion formed in RBC. Several factors can affect toxin activity: serum, low pH, low ionic strength and multivalent cations are potent inhibitors. pH Dependence is bell shaped, optimum activity being between pH 8 and 9. Similarly the action of Ca2+ is also bivalent: up to a concentration of approximately 2 mM it stimulates hemolysis, but above this concentration it inhibits (with 50% inhibition occurring at approximately 10 mM). When the known amino acid sequences of actinoporins are examined a common trait emerges; the presence of a well conserved, amphiphilic, putative alpha-helix at the N-terminus, which might be involved in the insertion of EqT-II in lipid membranes.

Clostridium perfringens enterotoxin acts by producing small molecule permeability alterations in plasma membranes

Clostridium perfringens enterotoxin (CPE) appears to utilize a unique mechanism of action to directly affect the plasma membrane permeability of mammalian cells. CPE action involves a multi-step action which culminates in cytotoxicity. Initially CPE binds to a protein receptor on mammalian plasma membranes. The membrane-bound CPE then becomes progressively more resistant to release by proteases (a phenomenon consistent with the insertion of CPE into membranes). This 'inserted' CPE then participates in the formation of a large complex in plasma membranes which contains one CPE: one 70 kDa membrane protein: one 50 kDa membrane protein. Upon formation of large complex, plasma membranes become freely permeable to small molecules such as ions and amino acids. This CPE-induced disruption of the cellular colloid-osmotic equilibrium then causes secondary cellular effects and cell death.

Bacillus cereus and related species

Bacillus cereus is a gram-positive aerobic or facultatively anaerobic spore-forming rod. It is a cause of food poisoning, which is frequently associated with the consumption of rice-based dishes. The organism produces an emetic or diarrheal syndrome induced by an emetic toxin and enterotoxin, respectively. Other toxins are produced during growth, including phospholipases, proteases, and hemolysins, one of which, cereolysin, is a thiol-activated hemolysin. These toxins may contribute to the pathogenicity of B. cereus in nongastrointestinal disease. B. cereus isolated from clinical material other than feces or vomitus was commonly dismissed as a contaminant, but increasingly it is being recognized as a species with pathogenic potential. It is now recognized as an infrequent cause of serious nongastrointestinal infection, particularly in drug addicts, the immunosuppressed, neonates, and postsurgical patients, especially when prosthetic implants such as ventricular shunts are inserted. Ocular infections are the commonest types of severe infection, including endophthalmitis, panophthalmitis, and keratitis, usually with the characteristic formation of corneal ring abscesses. Even with prompt surgical and antimicrobial agent treatment, enucleation of the eye and blindness are common sequelae. Septicemia, meningitis, endocarditis, osteomyelitis, and surgical and traumatic wound infections are other manifestations of severe disease. B. cereus produces beta-lactamases, unlike Bacillus anthracis, and so is resistant to beta-lactam antibiotics; it is usually susceptible to treatment with clindamycin, vancomycin, gentamicin, chloramphenicol, and erythromycin. Simultaneous therapy via multiple routes may be required.

A ring-shaped structure with a crown formed by streptolysin O on the erythrocyte membrane

Streptolysin O (SLO) is a membrane-damaging toxin produced by most strains of group A beta-hemolytic streptococci. We performed ultrastructural analysis of SLO-derived lesions on erythrocyte membranes by examining electron micrographs of negatively stained preparations. SLO formed numerous arc- and ring-shaped structures with or without holes on membranes. Rings formed on intact cell membranes had an inner diameter of ca. 24 nm and had distinct borders of ca. 4.9 nm in width, but the diameter of rings varied from 24 to 30 nm on membranes of erythrocyte ghosts. Image analysis of electron micrographs demonstrated that each ring was composed of an inner and an outer layer. Each layer contained an array of 22 to 24 SLO molecules. On the top of the ring, we found a characteristic crown that projected from the cell membrane. The crown was separated by an electron-dense layer from the basal part of the ring that was embedded in the lipid bilayer of the erythrocyte membrane. Heights of the three parts, namely, the crown (head), the space (neck), and the basal portion (base), were ca. 3.2, 1.6, and 5.0 nm, respectively, and we postulated that these parts are the constituents of a single SLO molecule. The volumes of SLO molecules in the inner and outer layers were calculated to be 77 and 88 nm3. On the basis of a model of the structure of SLO, we propose some new details of the mechanisms of hemolysis by SLO toxin.

Genetic conservation of hlyA determinants and serological conservation of HlyA: basis for developing a broadly cross-reactive subunit Escherichia coli alpha-hemolysin vaccine

The HlyA determinant among Escherichia coli isolates from patients with symptomatic urinary tract infection was compared in this report with a prototype HlyA encoded by pSF4000 by DNA-DNA hybridization tests with 20-base synthetic oligonucleotides and monoclonal antibody binding and neutralization assays. Hybridization results demonstrated that 349 (98%) of 357 definitive reactions among 54 hemolytic strains shared homology with seven DNA probes spanning many HlyA regions corresponding to residues (R) 41 to 47, 55 to 61, 248 to 254, 306 to 312, 336 to 343, 402 to 408, and 929 to 935. Genetic divergence was identified by lack of hybridization signals among 17 to 76% of the hemolytic strains within the distal portion of a predicted hydrophobic region corresponding to R491 to 319 and within a predicted hydrophilic region corresponding to R491 to 497 and R532 to 538. Serological studies demonstrated that 26 (81%) culture supernatants of 32 hemolytic strains were bound by all 12 monoclonal anti-HlyA antibodies. Among five of six remaining strains, the culture supernatants were bound by 3 to 11 monoclonal antibody preparations. There was only one hemolytic culture supernatant that failed to be bound by any monoclonal antibody, although the strain hybridized with nine hemolysin DNA probes. In addition, hemolytic activity of all 24 different culture supernatants tested was reduced by at least twofold by one monoclonal antibody specific for R2-161. These data extend and support previous views that the HlyA determinant is conserved among E. coli strains and suggest that a broadly cross-reactive HlyA subunit vaccine can be developed.

Dual specificity of sterol-mediated glycoalkaloid induced membrane disruption

In this study the effects of the glycoalkaloids alpha-solanine, alpha-chaconine, alpha-tomatine and the aglycone solanidine on model membranes composed of PC in the absence and presence of sterols have been analysed via permeability measurements and different biophysical methods. The main result is that glycoalkaloids are able to interact strongly with sterol containing membranes thereby causing membrane disruption in a way which is specific for the type of glycoalkaloid and sterol. For this dual specificity both the sugar moiety of the glycoalkaloid and the side-chain of the sterol on position 24 turned out to be of major importance for the membrane disrupting activity. The order of potency of the glycoalkaloids was alpha-tomatine > alpha-chaconine > alpha-solanine. The plant sterols beta-sitosterol and fucosterol showed higher affinity for glycoalkaloids as compared to cholesterol and ergosterol. The mode of action of the glycoalkaloids is proposed to consist of three main steps: (1) Insertion of the aglycone part in the bilayer. (2) Complex formation of the glycoalkaloid with the sterols present. (3) Rearrangement of the membrane caused by the formation of a network of sterol-glycoalkaloid complexes resulting in a transient disruption of the bilayer during which leakage occurs.

Capacity of listeriolysin O, streptolysin O, and perfringolysin O to mediate growth of Bacillus subtilis within mammalian cells

The Listeria monocytogenes hemolysin listeriolysin O (LLO) plays a major role in mediating the escape of L. monocytogenes from a vacuolar compartment. In a previous report, it was shown that Bacillus subtilis expressing LLO could escape from a host vacuolar compartment and grow in the cytoplasm (J. Bielecki, P. Youngman, P. Connelly, and D. A. Portnoy, Nature [London] 345:175-176, 1990). In the present study, two related thiol-activated hemolysins, streptolysin O (SLO) and perfringolysin O (PFO), were expressed in B. subtilis and their ability to mediate intracellular growth was monitored by visual inspection and by assaying for CFU. Like LLO, PFO was active within the vacuolar environment, whereas SLO showed negligible activity. However, expression of PFO seemed to damage the host cells. The pH of the vacuole probably had little to do with these results, since all three hemolysins showed full or enhanced activity at pH 5.5, although LLO showed greatly reduced activity at pH 7. In addition, neutralization of the pH within host vacuoles by using weak bases had little effect on the lysis of the vacuole. The lack of SLO activity is probably caused by its lower specific activity; the purified protein had 10-fold less activity on a molar basis. These results suggest that LLO is not unique in its capacity to mediate intracellular growth of B. subtilis.

Structural and functional relationships among the RTX toxin determinants of gram-negative bacteria

The RTX (repeats in toxin) cytolytic toxins represent a family of important virulence factors that have disseminated widely among Gram-negative bacteria. They are characterised by a series of glycine-rich repeat units at the C-terminal end of each protein. They also have other features in common. Secretion from the cell occurs without a periplasmic intermediate by a novel mechanism which involves recognition of a signal sequence at the C-terminus of the toxin by membrane-associated proteins that export the toxin directly to the outside of the cell. The structural gene for each protein encodes an inactive toxin which is modified post-translationally to an active cytotoxic form by another gene product before secretion. The genes for toxin synthesis, activation and secretion are for the most part grouped together on the chromosome and form an operon. The toxins all create pores in the cell membrane of target cells leading to eventual cell lysis and they appear to require Ca2+ for cytotoxic activity. Although the toxins have a similar mode of action, they vary in target cell specificity. Some are cytotoxic for a wide variety of eukaryotic cell types while others exhibit precise target cell specificity and are only active against leukocytes from certain host species. The characteristic glycine-rich repeat units have been identified in other exoproteins besides those with cytotoxic activity and it is likely that the novel secretory mechanism has been harnessed by a variety of pathogens to release important virulence-associated factors from the cell or to locate them on the cell surface.

Calcium is required for binding of Escherichia coli hemolysin (HlyA) to erythrocyte membranes

The calcium requirement for hemolytic activity of Escherichia coli hemolysin was investigated by using hemolytic assays and immunoblotting of toxin-treated erythrocytes. The hemolytic activity of cell culture supernatants obtained during growth of E. coli in Luria-Bertani (LB) broth or calcium-free LB broth was calcium dependent. The hemolytic activity of culture supernatants obtained during growth in LB broth supplemented with calcium was calcium independent. Osmotic protection experiments using Dextran 4 to prevent cell lysis indicated that calcium was required for the binding of hemolysin to erythrocytes at both 4 and 37 degrees C. The binding efficiency at 4 degrees C was 50% of that occurring at 37 degrees C. The calcium-dependent binding was confirmed by immunoblotting saline-washed, toxin-treated erythrocytes with a monoclonal antibody after sodium dodecyl sulfate-polyacrylamide gel electrophoresis separation of membrane proteins. Bound hemolysin increased the calcium permeability of the cell membranes as evidenced by calcium-induced membrane protein alterations. The alterations in membrane proteins did not directly cause lysis of the cells. The results were consistent with a mechanism of lysis involving the formation of cation-selective pores in the membranes of target cells.

Cholesterol as a target for toxins

A mechanism is proposed for the way in which cholesterol facilitates channel formation by polyene antibiotics and bacterial protein toxins. Central elements of the model are: (i) interactions between the ring system of the sterol and rigid elements of the polyene or toxin molecule, and (ii) the specific orientation of cholesterol within the membrane.

Surface hydrophobicity and electrophoretic mobilities of staphylococcal exotoxins with special reference to toxic shock syndrome toxin-1

The surface hydrophobicities of eleven staphylococcal toxins were estimated and compared with those of standard proteins on an octyl agarose column by high-performance hydrophobic-interaction chromatography (HP-HIC). Staphylococcal enterotoxins (SE) D, C3, C2, C1 and B showed a low surface hydrophobicity whereas alpha-toxin and gamma-toxin had a moderate surface hydrophobicity. SEA, toxic shock syndrome toxin-1 (TSST-1) and staphylococcal epidermolytic toxin (SET) showed high surface hydrophobicity and delta-toxin was the most hydrophobic protein. The electrophoretic mobility of the toxins was determined by free zone electrophoresis (FZE). All toxins except SEC1 and one of the two SEA species showed negative charge at pH 8.6. Charge heterogeneity was observed in SEA, SEC1, SEC3 and TSST-1: SEA and SEC1 had two overlapping components, whereas SEC3 and TSST-1 were resolved into two distinct components. The mobilities of the two TSST-1 components were estimated at -2.12 x 10(-5) and -3.60 x 10(-5) cm2v-1s-1, respectively, at 10 degrees C, and both fractions were immunologically indistinguishable as tested by specific TSST-1 antibodies with ELISA. An asymmetric peak was obtained in hydrophobic-interaction chromatography of TSST-1 indicating heterogeneity.

The thiol-activated toxin streptolysin O does not require a thiol group for cytolytic activity

Site-directed mutagenesis of the TGC codon in a cloned streptolysin O (SLO) gene exchanged the single Cys residue in SLO for either Ala or Ser. The parent wild-type SLO (SLO.Cys-530) and the SLO.Ala-530 and SLO.Ser-530 mutant toxins, expressed in Escherichia coli, were purified and analyzed. Wild-type SLO.Cys-530 and the SLO.Ala-530 mutant showed no significant differences in their specific hemolytic activities, while the SLO.Ser-530 mutant had a reduced (ca. 25%), but still considerable, specific hemolytic activity as compared with that of wild-type SLO. The parent and mutant toxins extracted from lysed erythrocyte membranes had similar sedimentation profiles on sucrose density gradients, suggesting that the mutations did not affect the ability of SLO to form oligomers in membranes. These results show that the widely held assumption that the in vitro cytolytic activity of SLO requires an essential Cys residue is not true.

Transmembrane pore size and role of cell swelling in cytotoxicity caused by Pasteurella haemolytica leukotoxin

Pasteurella haemolytica A1 leukotoxin causes rapid (5 to 15 min) leakage of intracellular K+ and cell swelling and slower (15 to 60 min), Ca2+-dependent formation of large plasma membrane defects (congruent to 100 nm) and leakage of lactate dehydrogenase from bovine lymphoma cells (BL3 cells) (K. D. Clinkenbeard, D. A. Mosier, A. L. Timko, and A. W. Confer, Am. J. Vet. Res., in press). Incubation of BL3 cells in medium made hypertonic by inclusion of 75 mM sucrose blocked leukotoxin-induced cell swelling, formation of large plasma membrane defects, and leakage of lactate dehydrogenase but did not block leukotoxin-induced leakage of intracellular K+. Carbohydrates with molecular weights less than that of sucrose, e.g., mannitol, did not block leukotoxin-induced cell swelling of BL3 cells. Increasing the concentration of mannitol to twice that of sucrose still resulted in no protective effect. Assuming that leukotoxin acts as a transmembrane molecular sieve, then the functional transmembrane pore size formed by leukotoxin in BL3 cells is slightly less than the size of sucrose, i.e., 0.9 nm. Exposure of BL3 cells to leukotoxin for 15 or 45 min followed by the addition of hypertonic sucrose to the incubation medium reversed leukotoxin-induced cell swelling and prevented further leakage of lactate dehydrogenase. Leukotoxin-induced leakage of lactate dehydrogenase required both cell swelling and Ca2+-dependent processes. The Ca2+-dependent steps can occur before or concurrent with cell swelling.

Expression of the pneumolysin gene in Escherichia coli: rapid purification and biological properties

The gene for pneumolysin, the thiol-activated toxin from Streptococcus pneumoniae, has been expressed in Escherichia coli. The recombinant protein has been purified using a rapid, high yield, purification procedure and has been shown to be identical with respect to N-terminal amino-acid sequence, specific activity, effect on human polymorphonuclear phagocytes and effect on human complement to the native toxin purified from the pneumococcus. This provides a large enough source of material to begin investigation of pneumolysin as a candidate for inclusion in a pneumococcal vaccine.

Effects of divalent cations and saccharides on Vibrio metschnikovii cytolysin-induced hemolysis of rabbit erythrocytes

Divalent cations and polysaccharides such as inulin and dextran reversibly inhibited hemolysis of rabbit erythrocytes caused by Vibrio metschnikovii cytolysin. On the basis of the 50% inhibitory doses, the cations were divided into two groups, group I (Cd2+, Cu2+, Ni2+, Sn2+, and Zn2+) and group II (Ba2+, Ca2+, Co2+, Mg2+, Mn2+, and Sr2+). Neither divalent cations nor polysaccharides interfered with the binding of toxins to the erythrocyte membrane. Group I cations disturbed tetramer formation of cytolysin on the cytolysin-lysed erythrocyte membrane, although group II cations and dextran did not affect the process. Erythrocytes treated with cytolysin in the presence of group II cations or dextran lysed after transfer to toxin- and inhibitor (group II cations or dextran)-free buffer at both 37 and 4 degrees C. However, erythrocytes treated in the presence of group I cations lysed at 37 degrees C but not at 4 degrees C, indicating that group I cations block the temperature-dependent lesion (tetramer)-forming step subsequent to the binding of cytolysin to the erythrocytes. The cytolysin-treated erythrocytes swelled in a colloid osmotic manner, and the swelling was preceded by the binding and the lesion-forming steps. It is also suggested that the lysis of the erythrocytes proceeds in a temperature-independent manner and that the cytolysin does not bind to the erythrocytes at 4 degrees C. These findings suggest that the sequence of V. metschnikovii cytolysin-induced hemolysis is defined by three steps: (i) a temperature-dependent binding step, (ii) a temperature-dependent lesion-forming step, and (iii) a temperature-independent lysis step.

Molecular biology and the diagnosis, epidemiology and pathogenesis of infectious diseases

In this short review, the impact of molecular biology on microbiology in general is described. Specifically, molecular biology is increasingly enlarging the available choice of methods for the diagnosis of microbial disease. In situ hybridization seems to be a particularly promising procedure. In epidemiology, an interesting facet is the high mutation rate of RNA viruses. In pathogenesis, molecular biology will help to elucidate pathways of infection and the targeting of pathogenic macromolecules within the cell and within an organism.

Comparative effects of sugars on the hemolytic activity of Schistosoma mansoni and other hemolytic agents

1. The rate of red blood cell lysis by the hemolytic agent in adult worm homogenates of Schistosoma mansoni is slowed in the presence of added sugars (50 mM). 2. Trisaccharides were the most effective in slowing and reducing lysis. Disaccharides were more effective than monosaccharides. 3. The addition of sodium, potassium or lithium chloride salts (25 mM) stimulated hemolysis by the S. mansoni agent. 4. Hemolysins with known mechanisms were tested to determine the effects of added sugars (50 mM) or salts (25 mM). 5. The S. mansoni hemolytic agent responds to the addition of sugars and salts in a manner similar to small membrane pore formers.

Kinetics of hemolysis induced by a toxin from Bacillus thuringiensis israelensis

The kinetics of hemolysis resulting from the action on rabbit erythrocytes of a highly purified cytolytic toxin (26,000 mol. wt) isolated from a spore-crystal mixture of Bacillus thuringiensis israelensis was studied. Course of hemolysis, as determined by release of hemoglobin, yielded sigmoid curves whose maximum slopes were taken as a measure of the rate of lysis. Hemolysis occurred without an induction period, and the rate of lysis was a linear function of toxin concentration. Rate of hemolysis as a function of temperature yielded an Arrhenius constant of 9300 calories per mole. The toxin was active between pH 4.5 and 8.0. Lysis was strongly inhibited by Cu2+, Fe2+ and Zn2+ in concentrations as low as 0.025 M. Phosphatidylserine, phosphatidylinositol, phosphatidylcholine and sphingomyelin inhibited lysis, whereas phosphatidylethanolamine, cerebroside, cholesterol and major integral erythrocyte membrane proteins caused little or no inhibition. Inhibition of lysis by sucrose indicates that hemolysis is of the colloid-osmotic type.

Staphylococcal alpha toxin--recent advances

The elucidation of the amino acid sequence of alpha toxin in 1984 has greatly promoted our understanding of the basic biochemistry and interaction of this toxin with membranes. These aspects are discussed and the concept of alpha toxin as a channel forming protein is critically evaluated. The lethal action of alpha toxin has not yet been clarified, but the previously postulated action as a neurotoxin is not supported by recent observations.

Quantitative analysis of the binding and oligomerization of staphylococcal alpha-toxin in target erythrocyte membranes

The binding of staphylococcal alpha-toxin to rabbit and human erythrocytes was quantitated over a wide range of toxin concentrations (3 x 10(-11) to 3 x 10(-6) M) with the use of an enzyme-linked immunosorbent assay that permitted simultaneous quantitation of monomeric and oligomeric toxin forms. Three basic observations were made. First, in no range of concentrations did the binding of alpha-toxin to rabbit erythrocytes display characteristics of a receptor-ligand interaction. Net binding to rabbit cells was nil at sublytic concentrations (10(-10) M or 3 ng/ml). The onset of binding occurred at around 10 ng/ml and remained fairly constant and ineffective (5 to 8% of toxin offered) over a wide concentration range (up to 10 micrograms/ml). Second, hemolysis of rabbit and human erythrocytes at 37 degrees C was always accompanied by the formation of toxin oligomers in the membrane. Third, overall toxin binding at 0 degree C followed a pattern similar to that at 37 degrees C. However, oligomer formation and cell lysis were retarded (but not totally inhibited) at 0 degree C. When rabbit erythrocytes were incubated with low levels of toxin at 0 degree C (0.5 microgram/ml) for 30 min, the toxin became bound exclusively in monomer form, and no lysis occurred. When cells thus treated were washed and suspended at 37 degrees C, lysis rapidly ensued, and native monomeric toxin was replaced by oligomeric toxin. The collective results directly support the oligomer pore concept of toxin action and also indicate that toxin oligomers form by lateral aggregation of bound monomers in the bilayer. They speak against the existence of specific binding sites for alpha-toxin on rabbit erythrocytes.

Nucleotide sequence of the streptolysin O (SLO) gene: structural homologies between SLO and other membrane-damaging, thiol-activated toxins

The complete nucleotide sequence of a cloned streptolysin O (SLO) gene and the amino acid sequence of SLO, predicted from the DNA sequence, are reported. SLO contains a single cysteine residue located close to the C terminus of the molecule and shares extensive structural homologies with other thiol-activated toxins, which allow us to predict functionally important features.

Quantitation of monomeric and oligomeric forms of membrane-bound staphylococcal alpha-toxin by enzyme-linked immunosorbent assay with a neutralizing monoclonal antibody

A murine monoclonal antibody generated against staphylococcal alpha-toxin was shown to react only with the monomeric (native), 3S form of the toxin. A sensitive sandwich enzyme-linked immunosorbent assay (ELISA) constructed with this antibody permitted detection of 0.25 to 0.5 ng of native toxin per ml. Toxin oligomers formed either by heat aggregation in solution, on target erythrocyte membranes, or on phosphatidylcholine-cholesterol liposomes were unreactive in the ELISA when membranes were solubilized with the nondenaturing detergent Triton X-100. After dissociation of the oligomers by boiling in sodium dodecyl sulfate, however, the ELISA reactivity of the liberated 3S toxin was fully restored. Parallel determinations of membrane-bound toxin with sodium dodecyl sulfate and Triton X-100 solubilization thus permitted direct quantitation of total and monomeric toxin, respectively; the difference between these two values was represented by toxin oligomers. The detection limits for membrane-bound oligomeric and monomeric toxin on erythrocyte membranes are in the order of 100 molecules and 1 molecule per cell, respectively. Using this ELISA, we show that over 90% of alpha-toxin molecules bound to target membranes at 37 degrees C are in oligomeric form. Evidence is given that the monoclonal antibody neutralizes alpha-toxin by inhibiting its binding to both rabbit and human erythrocytes. This ELISA is the first assay that quantitatively discriminates between mono- and oligomeric forms of a pore-forming protein on target cell membranes.

Some properties of flammutoxin from the edible mushroom Flammulina velutipes

A cytolytic toxin from the basidiocarps of the edible mushroom Flammulina velutipes was purified to homogeneity. The lysin, flammutoxin, is a single polypeptide chain of Mr 32,000 and pK about 5.4. It contains unusually large amounts of tryptophan, serine and glycine, and few or none of the sulfur-containing amino acids. Erythrocytes of rat, rabbit, guinea pig, man, mouse, cat and dog were sensitive to lysis, in that order, whereas erythrocytes of sheep, ox, goat, swine and horse were largely or completely resistant to lysis. The toxin appears not to be a phospholipase and it was not inhibitable by any of a variety of lipids. Hemolysis probably involves alteration of the erythrocyte membrane, with formation of submicroscopic ion channels, and it appears to be of the osmotic type. In some respects flammutoxin resembles phallolysin, a cytolytic toxin obtained from the mushroom Amanita phalloides.

Identification with monoclonal antibodies of hemolysin produced by clinical isolates of Escherichia coli

Murine monoclonal antibodies were generated against the 107,000-dalton hemolysin encoded by the hemolytic determinant from Escherichia coli LE 2001, and colony blotting was used to assay for production of the hemolysin by 35 hemolytic strains of E. coli and other hemolytic members of the family Enterobacteriaceae of clinical origin. All hemolytic E. coli strains gave positive reactions with two monoclonal antibodies. In contrast, none of the hemolytic, non-E. coli isolates yielded positive colony blots. In addition, Western blotting showed that the hemolysins produced by all clinical E. coli isolates had a similar molecular weight of about 107,000. Discrete antigenic variation may occur in the molecule, since a third monoclonal antibody did not react with the hemolysin from a number of wild-type E. coli strains. Western blot analysis was used to assess the presence of immunoglobulin G (IgG), IgA, and IgM antibodies to E. coli hemolysin in human sera. All 20 of the tested sera from healthy adults contained antibodies to the toxin, with various constellations among the antibody classes. In contrast, sera from five of eight infants aged 8 to 36 months contained no antihemolysin antibodies. We conclude that the 107,000-dalton hemolysin of E. coli is a widespread immunogen that is produced by most or all hemolytic E. coli strains in the human host.