Cholesterol-dependent cytolysins, a family of versatile pore-forming toxins

The cholesterol-dependent cytolysin family of gram-positive bacterial toxins

The cholesterol-dependent cytolysins (CDCs) are a family of beta-barrel pore-forming toxins secreted by Gram-positive bacteria. These toxins are produced as water-soluble monomeric proteins that after binding to the target cell oligomerize on the membrane surface forming a ring-like pre-pore complex, and finally insert a large beta-barrel into the membrane (about 250 A in diameter). Formation of such a large transmembrane structure requires multiple and coordinated conformational changes. The presence of cholesterol in the target membrane is absolutely required for pore-formation, and therefore it was long thought that cholesterol was the cellular receptor for these toxins. However, not all the CDCs require cholesterol for binding. Intermedilysin, secreted by Streptoccocus intermedius only binds to membranes containing a protein receptor, but forms pores only if the membrane contains sufficient cholesterol. In contrast, perfringolysin O, secreted by Clostridium perfringens, only binds to membranes containing substantial amounts of cholesterol. The mechanisms by which cholesterol regulates the cytolytic activity of the CDCs are not understood at the molecular level. The C-terminus of perfringolysin O is involved in cholesterol recognition, and changes in the conformation of the loops located at the distal tip of this domain affect the toxin-membrane interactions. At the same time, the distribution of cholesterol in the membrane can modulate toxin binding. Recent studies support the concept that there is a dynamic interplay between the cholesterol-binding domain of the CDCs and the excess of cholesterol molecules in the target membrane.

Toxins from bacteria

Bacterial toxins damage the host at the site of bacterial infection or distant from the site. Bacterial toxins can be single proteins or oligomeric protein complexes that are organized with distinct AB structure-function properties. The A domain encodes a catalytic activity. ADP ribosylation of host proteins is the earliest post-translational modification determined to be performed by bacterial toxins; other modifications include glucosylation and proteolysis. Bacterial toxins also catalyze the non-covalent modification of host protein function or can modify host cell properties through direct protein-protein interactions. The B domain includes two functional domains: a receptor-binding domain, which defines the tropism of a toxin for a cell and a translocation domain that delivers the A domain across a lipid bilayer, either on the plasma membrane or the endosome. Bacterial toxins are often characterized based upon the secretion mechanism that delivers the toxin out of the bacterium, termed types I-VII. This review summarizes the major families of bacterial toxins and also describes the specific structure-function properties of the botulinum neurotoxins.

Role of membrane lipids for the activity of pore forming peptides and proteins

Bilayer lipids, far from being passive elements, have multiple roles in polypeptide-dependent pore formation. Lipids participate at all stages of the formation of pores by providing the binding site for proteins and peptides, conditioning their active structure and modulating the molecular reorganization of the membrane complex. Such general functions of lipids superimpose to other particular roles, from electrostatic and curvature effects to more specific actions in cases like cholesterol, sphingolipids or cardiolipin. Pores are natural phenomena in lipid membranes. Driven by membrane fluctuations and packing defects, transient water pores are related to spontaneous lipid flip-flop and non-assisted ion permeation. In the absence ofproteins or peptides, these are rare short living events, with properties dependent on the lipid composition of the membrane. Their frequency increases under conditions of internal membrane disturbance of the lipid packing, like in the presence of membrane-bound proteins or peptides. These latter molecules, in fact, form dynamic supramolecular assemblies together with the lipids and transmembrane pores are one of the possible structures of the complex. Active peptides and proteins can thus be considered inducers or enhancers of pores which increase their probability and lifetime by modifying the thermodynamic membrane balance. This includes destabilizing the membrane lamellar structure, lowering the activation energy for pore formation and stabilizing the open pore structure.

A high-affinity inhibitor of human CD59 enhances complement-mediated virolysis of HIV-1: implications for treatment of HIV-1/AIDS

Many pathogenic enveloped viruses, including HIV-1, escape complement-mediated virolysis by incorporating host cell regulators of complement activation into their own viral envelope. The presence of complement regulators including CD59 on the external surface of the viral envelope confers resistance to complement-mediated virolysis, which may explain why human pathogenic viruses such as HIV-1 are not neutralized by complement in human fluids, even in the presence of high Ab titers against the viral surface proteins. In this study, we report the development of a recombinant form of the fourth domain of the bacterial toxin intermedilysin (the recombinant domain 4 of intermedilysin [rILYd4]), a 114 aa protein that inhibits human CD59 function with high affinity and specificity. In the presence of rILYd4, HIV-1 virions derived from either cell lines or peripheral blood mononuclear cells of HIV-1-infected patients became highly sensitive to complement-mediated lysis activated by either anti-HIV-1 gp120 Abs or by viral infection-induced Abs present in the plasma of HIV-1-infected individuals. We also demonstrated that rILYd4 together with serum or plasma from HIV-1-infected patients as a source of anti-HIV-1 Abs and complement did not mediate complement-mediated lysis of either erythrocytes or peripheral blood mononuclear cells. These results indicate that rILYd4 may represent a novel therapeutic agent against HIV-1/AIDS.

Association of Vibrio parahaemolyticus thermostable direct hemolysin with lipid rafts is essential for cytotoxicity but not hemolytic activity

Thermostable direct hemolysin (TDH), a major virulence factor of Vibrio parahaemolyticus, induces cytotoxicity in cultured cells. However, the mechanism of TDH's cytotoxic effect including its target molecules on the plasma membrane of eukaryotic cells remains unclear. In this study, we identified the role of lipid rafts, cholesterol- and sphingolipid-enriched microdomains, in TDH cytotoxicity. Treatment of cells with methyl-beta-cyclodextrin (MbetaCD), a raft-disrupting agent, inhibited TDH cytotoxicity. TDH was associated with detergent-resistant membranes (DRMs), and MbetaCD eliminated this association. In contrast, there was no such association between a nontoxic TDH mutant and DRMs. The disruption of lipid rafts neither affected hemolysis nor inhibited Ca(2+) influx into HeLa cells induced by TDH. These findings indicate that the cytotoxicity but not the hemolytic activity of TDH is dependent on lipid rafts. The exogenous and endogenous depletion of cellular sphingomyelin also prevented TDH cytotoxicity, but a direct interaction between TDH and sphingomyelin was not detected with either a lipid overlay assay or a liposome absorption test. Treatment with sphingomyelinase (SMase) at 100 mU/ml disrupted the association of TDH with DRMs but did not affect the localization of lipid raft marker proteins (caveolin-1 and flotillin-1) with DRMs. These results suggest that sphingomyelin is important for the association of TDH with lipid rafts but is not a molecular target of TDH. We hypothesize that TDH may target a certain group of rafts that are sensitive to SMase at a certain concentration, which does not affect other types of rafts.

Role of host cell polarity and leading edge properties in Pseudomonas type III secretion

Type III secretion (T3S) functions in establishing infections in a large number of Gram-negative bacteria, yet little is known about how host cell properties might function in this process. We used the opportunistic pathogen Pseudomonas aeruginosa and the ability to alter host cell sensitivity to Pseudomonas T3S to explore this problem. HT-29 epithelial cells were used to study cellular changes associated with loss of T3S sensitivity, which could be induced by treatment with methyl-beta-cyclodextrin or perfringolysin O. HL-60 promyelocytic cells are innately resistant to Pseudomonas T3S and were used to study cellular changes occurring in response to induction of T3S sensitivity, which occurred following treatment with phorbol esters. Using both cell models, a positive correlation was observed between eukaryotic cell adherence to tissue culture wells and T3S sensitivity. In examining the type of adhesion process linked to T3S sensitivity in HT-29 cells, a hierarchical order of protein involvement was identified that paralleled the architecture of leading edge (LE) focal complexes. Conversely, in HL-60 cells, induction of T3S sensitivity coincided with the onset of LE properties and the development of actin-rich projections associated with polarized cell migration. When LE architecture was examined by immunofluorescent staining for actin, Rac1, IQ-motif-containing GTPase-activating protein 1 (IQGAP1) and phosphatidylinositol 3 kinase (PI3 kinase), intact LE structure was found to closely correlate with host cell sensitivity to P. aeruginosa T3S. Our model for host cell involvement in Pseudomonas T3S proposes that cortical actin polymerization at the LE alters membrane properties to favour T3S translocon function and the establishment of infections, which is consistent with Pseudomonas infections targeting wounded epithelial barriers undergoing cell migration.

Recombinant Listeria monocytogenes expressing a cell wall-associated listeriolysin O is weakly virulent but immunogenic

Listeriolysin O (LLO) is an essential virulence factor for the gram-positive bacterium Listeria monocytogenes. Our goal was to determine if altering the topology of LLO would alter the virulence and toxicity of L. monocytogenes in vivo. A recombinant strain was generated that expressed a surface-associated LLO (sLLO) variant secreted at 40-fold-lower levels than the wild type. In culture, the sLLO strain grew in macrophages, translocated to the cytosol, and induced cell death. However, the sLLO strain showed decreased infectivity, reduced lymphocyte apoptosis, and decreased virulence despite a normal in vitro phenotype. Thus, the topology of LLO in L. monocytogenes was a factor in the pathogenesis of the infection and points to a role of LLO secretion during in vivo infection. The sLLO strain was cleared by severe combined immunodeficient (SCID) mice. Despite the attenuation of virulence, the sLLO strain was immunogenic and capable of eliciting protective T-cell responses.

Listeriolysin O as cytotoxic component of an immunotoxin

Monoclonal antibodies (mAbs) have been developed over the past years as promising anticancer therapeutics. The conjugation of tumor specific mAbs with cytotoxic molecules has been shown to improve their efficacy dramatically. These bifunctional immunotoxins, consisting of covalently linked antibodies and protein toxins, possess considerable potential in cancer therapy. Many of them are under investigation in clinical trials. As a result of general interest in new toxic components, we describe here the suitability of the bacterial protein Listeriolysin O (LLO) as cytotoxic component of an immunotoxin. Unique characteristics of LLO, such as its acidic pH optimum and the possibility to regulate the cytolytic activity by cysteine-oxidation, make LLO an interesting toxophore. Oxidized LLO shows a substantially decreased cytolytic activity when compared with the reduced protein as analyzed by hemolysis. Both oxidized and reduced LLO exhibit a cell-type-unspecific toxicity in cell culture with a significantly higher toxicity of reduced LLO. For cell-type-specific targeting of LLO to tumor cells, LLO was coupled to the dsFv fragment of the monoclonal antibody B3, which recognizes the tumor-antigen Lewis Y. The coupling of LLO to dsFv-B3 was performed via cysteine-containing polyionic fusion peptides that act as a specific heterodimerization motif. The novel immunotoxin B3-LLO could be shown to specifically eliminate antigen positive MCF7 cells with an EC(50) value of 2.3 nM, whereas antigen negative cell lines were 80- to 250-fold less sensitive towards B3-LLO.

New roles for perforins and proteases in apicomplexan egress

Egress is a pivotal step in the life cycle of intracellular pathogens initiating the transition from an expiring host cell to a fresh target cell. While much attention has been focused on understanding cell invasion by intracellular pathogens, recent work is providing a new appreciation of mechanisms and therapeutic potential of microbial egress. This review highlights recent insight into cell egress by apicomplexan parasites and emerging contributions of membranolytic and proteolytic secretory products, along with host proteases. New findings suggest that Toxoplasma gondii secretes a pore-forming protein, TgPLP1, during egress that facilitates parasite escape from the cell by perforating the parasitophorous membrane. Also, in a cascade of proteolytic events, Plasmodium falciparum late-stage schizonts activate and secrete a subtilisin, PfSUB1, which processes enigmatic putative proteases called serine-repeat antigens that contribute to merozoite egress. A new report also suggests that calcium-activated host proteases called calpains aid parasite exit, possibly by acting upon the host cytoskeleton. Together these discoveries reveal important new molecular players involved in the principal steps of egress by apicomplexans.

Human alpha-defensins inhibit hemolysis mediated by cholesterol-dependent cytolysins

Many pathogenic gram-positive bacteria release exotoxins that belong to the family of cholesterol-dependent cytolysins. Here, we report that human alpha-defensins HNP-1 to HNP-3 acted in a concentration-dependent manner to protect human red blood cells from the lytic effects of three of these exotoxins: anthrolysin O (ALO), listeriolysin O, and pneumolysin. HD-5 was very effective against listeriolysin O but less effective against the other toxins. Human alpha-defensins HNP-4 and HD-6 and human beta-defensin-1, -2, and -3 lacked protective ability. HNP-1 required intact disulfide bonds to prevent toxin-mediated hemolysis. A fully linearized analog, in which all six cysteines were replaced by aminobutyric acid (Abu) residues, showed greatly reduced binding and protection. A partially unfolded HNP-1 analog, in which only cysteines 9 and 29 were replaced by Abu residues, showed intact ALO binding but was 10-fold less potent in preventing hemolysis. Surface plasmon resonance assays revealed that HNP-1 to HNP-3 bound all three toxins at multiple sites and also that solution-phase HNP molecules could bind immobilized HNP molecules. Defensin concentrations that inhibited hemolysis by ALO and listeriolysin did not prevent these toxins from binding either to red blood cells or to cholesterol. Others have shown that HNP-1 to HNP-3 inhibit lethal toxin of Bacillus anthracis, toxin B of Clostridium difficile, diphtheria toxin, and exotoxin A of Pseudomonas aeruginosa; however, this is the first time these defensins have been shown to inhibit pore-forming toxins. An "ABCDE mechanism" that can account for the ability of HNP-1 to HNP-3 to inhibit so many different exotoxins is proposed.

Cholesterol exposure at the membrane surface is necessary and sufficient to trigger perfringolysin O binding

Perfringolysin O (PFO) is the prototype for the cholesterol-dependent cytolysins, a family of bacterial pore-forming toxins that act on eukaryotic membranes. The pore-forming mechanism of PFO exhibits an absolute requirement for membrane cholesterol, but the complex interplay between the structural arrangement of the PFO C-terminal domain and the distribution of cholesterol in the target membrane is poorly understood. Herein we show that PFO binding to the bilayer and the initiation of the sequence of events that culminate in the formation of a transmembrane pore depend on the availability of free cholesterol at the membrane surface, while changes in the acyl chain packing of the phospholipids and cholesterol in the membrane core, or the presence or absence of detergent-resistant domains do not correlate with PFO binding. Moreover, PFO association with the membrane was inhibited by the addition of sphingomyelin, a typical component of membrane rafts in cell membranes. Finally, addition of molecules that do not interact with PFO, but intercalate into the membrane and displace cholesterol from its association with phospholipids (e.g., epicholesterol), reduced the amount of cholesterol required to trigger PFO binding. Taken together, our studies reveal that PFO binding to membranes is triggered when the concentration of cholesterol exceeds the association capacity of the phospholipids, and this cholesterol excess is then free to associate with the toxin.

Membrane injury by pore-forming proteins

The plasma membrane defines the boundary of every living cell, and its integrity is essential for life. The plasma membrane may, however, be challenged by mechanical stress or pore-forming proteins produced by the organism itself or invading pathogens. We will here review recent findings about pore-forming proteins from different organisms, highlighting their structural and functional similarities, and describe the mechanisms that lead to membrane repair, since remarkably, cells can repair breaches in their plasma membrane of up to 10,000 microm(2).

Aegerolysins: structure, function, and putative biological role

Aegerolysins, discovered in fungi, bacteria and plants, are highly similar proteins with interesting biological properties. Certain aegerolysins possess antitumoral, antiproliferative, and antibacterial activities. Further possible medicinal applications include their use in the prevention of atherosclerosis, or as vaccines. Additional biotechnological value of fungal aegerolysins lies in their involvement in development, which could improve cultivation of commercially important edible mushrooms. Besides, new insights on microheterogeneity of raft-like membrane domains could be gained by using aegerolysins as specific markers in cell and molecular biology. Although the exact function of aegerolysins in their producing organisms remains to be explained, they are biochemically well characterized all-beta structured proteins sharing the following common features: low isoelectric points, similar molecular weights (15-17 kDa), and stability in a wide pH range.

Multiple effector mechanisms induced by recombinant Listeria monocytogenes anticancer immunotherapeutics

Listeria monocytogenes is a facultative intracellular gram-positive bacterium that naturally infects professional antigen presenting cells (APC) to target antigens to both class I and class II antigen processing pathways. This infection process results in the stimulation of strong innate and adaptive immune responses, which make it an ideal candidate for a vaccine vector to deliver heterologous antigens. This ability of L. monocytogenes has been exploited by several researchers over the past decade to specifically deliver tumor-associated antigens that are poorly immunogenic such as self-antigens. This review describes the preclinical studies that have elucidated the multiple immune responses elicited by this bacterium that direct its ability to influence tumor growth.

Cellular functions and X-ray structure of anthrolysin O, a cholesterol-dependent cytolysin secreted by Bacillus anthracis

Anthrolysin O (ALO) is a pore-forming, cholesterol-dependent cytolysin (CDC) secreted by Bacillus anthracis, the etiologic agent for anthrax. Growing evidence suggests the involvement of ALO in anthrax pathogenesis. Here, we show that the apical application of ALO decreases the barrier function of human polarized epithelial cells as well as increases intracellular calcium and the internalization of the tight junction protein occludin. Using pharmacological agents, we also found that barrier function disruption requires increased intracellular calcium and protein degradation. We also report a crystal structure of the soluble state of ALO. Based on our analytical ultracentrifugation and light scattering studies, ALO exists as a monomer. Our ALO structure provides the molecular basis as to how ALO is locked in a monomeric state, in contrast to other CDCs that undergo antiparallel dimerization or higher order oligomerization in solution. ALO has four domains and is globally similar to perfringolysin O (PFO) and intermedilysin (ILY), yet the highly conserved undecapeptide region in domain 4 (D4) adopts a completely different conformation in all three CDCs. Consistent with the differences within D4 and at the D2-D4 interface, we found that ALO D4 plays a key role in affecting the barrier function of C2BBE cells, whereas PFO domain 4 cannot substitute for this role. Novel structural elements and unique cellular functions of ALO revealed by our studies provide new insight into the molecular basis for the diverse nature of the CDC family.

Molecular mechanism of pore formation by actinoporins

Actinoporins are effective pore-forming toxins produced by sea anemones. These extremely potent, basic 20 kDa proteins readily form pores in membranes that contain sphingomyelin. Much has been learned about the molecular basis of their pore-forming mechanism in recent years. Pore formation is a multi-step process that involves recognition of membrane sphingomyelin, firm binding to the membrane accompanied by the transfer of the N-terminal region to the lipid-water interface and finally pore formation after oligomerisation of three to four monomers. The final conductive pathway is formed by amphipathic alpha-helices, hence actinoporins are an important example of so-called alpha-helical pore-forming toxins. Actinoporins have become useful model proteins to study protein-membrane interactions, specific recognition of lipids in the membrane, and protein oligomerisation in the lipid milieu. Recent sequence and structural data of proteins similar to actinoporins indicate that they are not a unique family restricted to sea anemones as was long believed. An AF domain superfamily (abbreviated from actinoporin-like proteins and fungal fruit-body lectins) was defined and shown to contain members from three animal and two plant phyla. On the basis of functional properties of some members we hypothesise that AF domain proteins are peripheral membrane proteins. Finally, ability of actinoporins to form transmembrane pores has been exploited in some novel biomedical applications.

Identification of the high affinity binding site in the Streptococcus intermedius toxin intermedilysin for its membrane receptor, the human complement regulator CD59

The unique species specificity of the bacterial cytolysin intermedilysin is explained by its requirement for the human complement regulator CD59 as the primary receptor. Binding studies using individual domains of intermedilysin mapped the CD59-binding site to domain 4 and swap mutants between human and rabbit (non-intermedilysin-binding) CD59 implicated a short sequence (residues 42–59) in human CD59 in binding intermedilysin. We set out to map more closely the CD59 binding site in intermedilysin. We first looked for regions of homology between domain 4 in intermedilysin and the terminal complement components that bind CD59, C8 and C9. A nine amino acid sequence immediately adjacent the undecapeptide segment in intermedilysin domain 4 matched (5 of 9 identical, 3 of 9 conserved) a sequence in C9. A peptide containing this sequence caused dose-dependent inhibition of intermedilysin-mediated lysis of human erythrocytes and rendered erythrocytes more susceptible to complement lysis. Surface plasmon resonance analysis of intermedilysin binding to immobilized CD59 revealed saturable fast-on, fast-off binding and a calculated affinity of 4.9 nM. Substitution of three residues from the putative binding site caused a 5-fold reduction in lytic potency of intermedilysin and reduced affinity for immobilized CD59 by 2.5-fold. The demonstration that a peptide modeled on the CD59-binding site inhibits intermedilysin-mediated haemolysis leads us to suggest that such peptides might be useful in treating infections caused by intermedilysin-producing bacteria.

Vibrio vulnificus cytolysin induces apoptosis in HUVEC, SGC-7901 and SMMC-7721 cells via caspase-9/3-dependent pathway

Vibrio vulnificus cytolysin (VVC) is known to be a pore-forming toxin which shows cytotoxicity for mammalian cells in culture and induces apoptosis in endothelial cells. In order to determine whether VVC induces apoptosis in vascular endothelial cells and tumor cells, the cytotoxicity induced by recombinant VVC (rVVC) and its potential mechanism in HUVEC, SGC-7901 and SMMC-7721 cells were investigated. Our study demonstrated that rVVC induced the release of intracellular K(+) from all the target cells, yet lactate dehydrogenase was not released by rVVC. It indicates that osmotic lysis might not contribute to the cytolysin-induced cytotoxicity. The study also demonstrated that rVVC induced apoptosis in HUVEC, SGC-7901 and SMMC-7721 cells in time- and dosage-dependent manners, which was associated with the activation of caspase-9 and -3, but not caspase-8. During the apoptotic process of the target cells, rVVC labeled with FITC was monitored to attach initially to the surface of the cells and entered the cytoplasma subsequently. These findings suggest that VVC may be not only a pore-forming toxin, but also a transmembrane toxin with powerful ability to induce apoptosis in human vascular endothelial cells and tumor cells.

Rapid membrane disruption by a perforin-like protein facilitates parasite exit from host cells

Perforin-like proteins are expressed by many bacterial and protozoan pathogens, yet little is known about their function or mode of action. Here, we describe Toxoplasma perforin-like protein 1 (TgPLP1), a secreted perforin-like protein of the intracellular protozoan pathogen Toxoplasma gondii that displays structural features necessary for pore formation. After intracellular growth, TgPLP1-deficient parasites failed to exit normally, resulting in entrapment within host cells. We show that this defect is due to an inability to rapidly permeabilize the parasitophorous vacuole membrane and host plasma membrane during exit. TgPLP1 ablation had little effect on growth in culture but resulted in a reduction greater than five orders of magnitude of acute virulence in mice. Perforin-like proteins from other intracellular pathogens may play a similar role in microbial egress and virulence.

Demonstration of a cholesterol-dependent cytolysin in a noninsecticidal Bacillus sphaericus strain and evidence for widespread distribution of the toxin within the species

During the course of screening Bacillus species from food and water in Norway, we isolated a strain of Bacillus sphaericus of DNA homology group V, not previously recognized to contain entomopathogenic strains, that was cytotoxic to Vero cell epithelia. Peptide mass fingerprinting of a protein purified from the culture supernatant of B. sphaericus B354 identified a cholesterol-dependent cytolysin (CDC) with high amino acid sequence identity with sphaericolysin, a CDC identified recently in B. sphaericus DNA homology group IIA. The toxin was haemolytic against erythrocytes from a range of species. Haemolysis was potentiated by dithiothreitol and inhibited by preincubation with cholesterol. The toxin induced lactate dehydrogenase release from Vero cells and formed pores in planar lipid bilayers. The distribution of CDC genes in B. sphaericus was examined, with CDC gene products obtained in 13 out of 17 strains representing four of the six DNA homology groups. Thus, we demonstrate the presence of a CDC in a nonentomopathogenic DNA homology group of B. sphaericus (group V) with typical CDC characteristics. CDCs appear to be present in a high proportion of B. sphaericus strains and are not restricted to group IIA insecticidal strains.

Disparate proteins use similar architectures to damage membranes

Membrane disruption can efficiently alter cellular function; indeed, pore-forming toxins (PFTs) are well known as important bacterial virulence factors. However, recent data have revealed that structures similar to those found in PFTs are found in membrane active proteins across disparate phyla. Many similarities can be identified only at the 3D-structural level. Of note, domains found in membrane-attack complex proteins of complement and perforin (MACPF) resemble cholesterol-dependent cytolysins from Gram-positive bacteria, and the Bcl family of apoptosis regulators share similar architectures with Escherichia coli pore-forming colicins. These and other correlations provide considerable help in understanding the structural requirements for membrane binding and pore formation.

Granzymes drive a rapid listeriolysin O-induced T cell apoptosis

The Listeria monocytogenes protein listeriolysin O (LLO) is a pore-forming protein essential for virulence. Although the major role for LLO is to allow L. monocytogenes entry into the cytosol, it also induces apoptosis of activated lymphocytes, an obligatory cellular response that modulates the infection. Induction of apoptosis by LLO proceeds through a fast, caspase-dependent pathway and a slow, caspase-independent pathway. Polyclonal T cell lines were generated from either normal mice or mice deficient in granzyme and perforin proteins, and then treated with apoptogenic doses of LLO. In this study we show that apoptosis of lymphocytes induced by LLO was characterized by activation of caspases as quickly as 30 min that was dependent on the expression of granzymes. In the absence of granzymes, all parameters of apoptosis such as caspase activation, phosphatidylserine exposure, mitochondrial depolarization, and DNA fragmentation were dramatically reduced in magnitude. Removal of perforin inhibited the apoptotic effect of LLO on cells by approximately 50%. Neutralization of intracellular acidification using chloroquine inhibited the rapid apoptotic death. In agreement with these findings granzyme-deficient mice harbored lower bacterial titers and decrease splenic pathology compared with normal mice following L. monocytogenes infection. Thus, LLO exploits apoptotic enzymes of the adaptive immune response to eliminate immune cells and increase its virulence.

Differential cholesterol binding by class II fusion proteins determines membrane fusion properties

The class II fusion proteins of the alphaviruses and flaviviruses mediate virus infection by driving the fusion of the virus membrane with that of the cell. These fusion proteins are triggered by low pH, and their structures are strikingly similar in both the prefusion dimer and the postfusion homotrimer conformations. Here we have compared cholesterol interactions during membrane fusion by these two groups of viruses. Using cholesterol-depleted insect cells, we showed that fusion and infection by the alphaviruses Semliki Forest virus (SFV) and Sindbis virus were strongly promoted by cholesterol, with similar sterol dependence in laboratory and field isolates and in viruses passaged in tissue culture. The E1 fusion protein from SFV bound cholesterol, as detected by labeling with photocholesterol and by cholesterol extraction studies. In contrast, fusion and infection by numerous strains of the flavivirus dengue virus (DV) and by yellow fever virus 17D were cholesterol independent, and the DV fusion protein did not show significant cholesterol binding. SFV E1 is the first virus fusion protein demonstrated to directly bind cholesterol. Taken together, our results reveal important functional differences conferred by the cholesterol-binding properties of class II fusion proteins.

The MACPF/CDC family of pore-forming toxins

Pore-forming toxins (PFTs) are commonly associated with bacterial pathogenesis. In eukaryotes, however, PFTs operate in the immune system or are deployed for attacking prey (e.g. venoms). This review focuses upon two families of globular protein PFTs: the cholesterol-dependent cytolysins (CDCs) and the membrane attack complex/perforin superfamily (MACPF). CDCs are produced by Gram-positive bacteria and lyse or permeabilize host cells or intracellular organelles during infection. In eukaryotes, MACPF proteins have both lytic and non-lytic roles and function in immunity, invasion and development. The structure and molecular mechanism of several CDCs are relatively well characterized. Pore formation involves oligomerization and assembly of soluble monomers into a ring-shaped pre-pore which undergoes conformational change to insert into membranes, forming a large amphipathic transmembrane β-barrel. In contrast, the structure and mechanism of MACPF proteins has remained obscure. Recent crystallographic studies now reveal that although MACPF and CDCs are extremely divergent at the sequence level, they share a common fold. Together with biochemical studies, these structural data suggest that lytic MACPF proteins use a CDC-like mechanism of membrane disruption, and will help understand the roles these proteins play in immunity and development.

Characterization of a streptococcal cholesterol-dependent cytolysin with a lewis y and b specific lectin domain

The cholesterol-dependent cytolysins (CDCs) are a large family of pore-forming toxins that often exhibit distinct structural changes that modify their pore-forming activity. A soluble platelet aggregation factor from Streptococcus mitis (Sm-hPAF) was characterized and shown to be a functional CDC with an amino-terminal fucose-binding lectin domain. Sm-hPAF, or lectinolysin (LLY) as renamed herein, is most closely related to CDCs from Streptococcus intermedius (ILY) and Streptococcus pneumoniae (pneumolysin or PLY). The LLY gene was identified in strains of S. mitis, S. pneumoniae, and Streptococcus pseudopneumoniae. LLY induces pore-dependent changes in the light scattering properties of the platelets that mimic those induced by platelet aggregation but does not induce platelet aggregation. LLY monomers form the typical large homooligomeric membrane pore complex observed for the CDCs. The pore-forming activity of LLY on platelets is modulated by the amino-terminal lectin domain, a structure that is not present in other CDCs. Glycan microarray analysis showed the lectin domain is specific for difucosylated glycans within Lewis b (Le (b)) and Lewis y (Le (y)) antigens. The glycan-binding site is occluded in the soluble monomer of LLY but is apparently exposed after cell binding, since it significantly increases LLY pore-forming activity in a glycan-dependent manner. Hence, LLY represents a new class of CDC whose pore-forming mechanism is modulated by a glycan-binding domain.

IFN-beta increases listeriolysin O-induced membrane permeabilization and death of macrophages

Type I IFN (IFN-I) signaling is detrimental to cells and mice infected with Listeria monocytogenes. In this study, we investigate the impact of IFN-I on the activity of listeriolysin O (LLO), a pore-forming toxin and virulence protein released by L. monocytogenes. Treatment of macrophages with IFN-beta increased the ability of sublytic LLO concentrations to cause transient permeability of the plasma membrane. At higher LLO concentrations, IFN-beta enhanced the complete breakdown of membrane integrity and cell death. This activity of IFN-beta required Stat1. Perturbation of the plasma membrane by LLO resulted in activation of the p38MAPK pathway. IFN-beta pretreatment enhanced LLO-mediated signaling through this pathway, consistent with its ability to increase membrane damage. p38MAPK activation in response to LLO was independent of TLR4, a putative LLO receptor, and inhibition of p38MAPK neither enhanced nor prevented LLO-induced death. IFN-beta caused cells to express increased amounts of caspase 1 and to produce a detectable caspase 1 cleavage product after LLO treatment. Contrasting recent reports with another pore-forming toxin, this pathway did not aid cell survival as caspase 1-deficient cells were equally sensitive to lysis by LLO. Key lipogenesis enzymes were suppressed in IFN-beta-treated cells, which may exacerbate the membrane damage caused by LLO.

Functional and phylogenetic characterization of Vaginolysin, the human-specific cytolysin from Gardnerella vaginalis

Pore-forming toxins are essential to the virulence of a wide variety of pathogenic bacteria. Gardnerella vaginalis is a bacterial species associated with bacterial vaginosis (BV) and its significant adverse sequelae, including preterm birth and acquisition of human immunodeficiency virus. G. vaginalis makes a protein toxin that generates host immune responses and has been hypothesized to be involved in the pathogenesis of BV. We demonstrate that G. vaginalis produces a toxin (vaginolysin [VLY]) that is a member of the cholesterol-dependent cytolysin (CDC) family, most closely related to intermedilysin from Streptococcus intermedius. Consistent with this predicted relationship, VLY lyses target cells in a species-specific manner, dependent upon the complement regulatory molecule CD59. In addition to causing erythrocyte lysis, VLY activates the conserved epithelial p38 mitogen-activated protein kinase pathway and induces interleukin-8 production by human epithelial cells. Transfection of human CD59 into nonsusceptible cells renders them sensitive to VLY-mediated lysis. In addition, a single amino acid substitution in the VLY undecapeptide [VLY(P480W)] generates a toxoid that does not form pores, and introduction of the analogous proline residue into another CDC, pneumolysin, significantly decreases its cytolytic activity. Further investigation of the mechanism of action of VLY may improve understanding of the functions of the CDC family as well as diagnosis and therapy for BV.

Crystal structure of the MACPF domain of human complement protein C8 alpha in complex with the C8 gamma subunit

Human C8 is one of five complement components (C5b, C6, C7, C8, and C9) that assemble on bacterial membranes to form a porelike structure referred to as the "membrane attack complex" (MAC). C8 contains three genetically distinct subunits (C8 alpha, C8 beta, C8 gamma) arranged as a disulfide-linked C8 alpha-gamma dimer that is noncovalently associated with C8 beta. C6, C7 C8 alpha, C8 beta, and C9 are homologous. All contain N- and C-terminal modules and an intervening 40-kDa segment referred to as the membrane attack complex/perforin (MACPF) domain. The C8 gamma subunit is unrelated and belongs to the lipocalin family of proteins that display a beta-barrel fold and generally bind small, hydrophobic ligands. Several hundred proteins with MACPF domains have been identified based on sequence similarity; however, the structure and function of most are unknown. Crystal structures of the secreted bacterial protein Plu-MACPF and the human C8 alpha MACPF domain were recently reported and both display a fold similar to those of the bacterial pore-forming cholesterol-dependent cytolysins (CDCs). In the present study, we determined the crystal structure of the human C8 alpha MACPF domain disulfide-linked to C8 gamma (alphaMACPF-gamma) at 2.15 A resolution. The alphaMACPF portion has the predicted CDC-like fold and shows two regions of interaction with C8 gamma. One is in a previously characterized 19-residue insertion (indel) in C8 alpha and fills the entrance to the putative C8 gamma ligand-binding site. The second is a hydrophobic pocket that makes contact with residues on the side of the C8 gamma beta-barrel. The latter interaction induces conformational changes in alphaMACPF that are likely important for C8 function. Also observed is structural conservation of the MACPF signature motif Y/W-G-T/S-H-F/Y-X(6)-G-G in alphaMACPF and Plu-MACPF, and conservation of several key glycine residues known to be important for refolding and pore formation by CDCs.

Repair of injured plasma membrane by rapid Ca2+-dependent endocytosis

Ca²⁺ influx through plasma membrane lesions triggers a rapid repair process that was previously shown to require the exocytosis of lysosomal organelles (Reddy, A., E. Caler, and N. Andrews. 2001. Cell. 106:157–169). However, how exocytosis leads to membrane resealing has remained obscure, particularly for stable lesions caused by pore-forming proteins. In this study, we show that Ca²⁺-dependent resealing after permeabilization with the bacterial toxin streptolysin O (SLO) requires endocytosis via a novel pathway that removes SLO-containing pores from the plasma membrane. We also find that endocytosis is similarly required to repair lesions formed in mechanically wounded cells. Inhibition of lesion endocytosis (by sterol depletion) inhibits repair, whereas enhancement of endocytosis through disruption of the actin cytoskeleton facilitates resealing. Thus, endocytosis promotes wound resealing by removing lesions from the plasma membrane. These findings provide an important new insight into how cells protect themselves not only from mechanical injury but also from microbial toxins and pore-forming proteins produced by the immune system.

Pore formation: an ancient yet complex form of attack

Bacteria, as well as higher organisms such as sea anemones or earthworms, have developed sophisticated virulence factors such as the pore-forming toxins (PFTs) to mount their attack against the host. One of the most fascinating aspects of PFTs is that they can adopt a water-soluble form at the beginning of their lifetime and become an integral transmembrane protein in the membrane of the target cells. There is a growing understanding of the sequence of events and the various conformational changes undergone by these toxins in order to bind to the host cell surface, to penetrate the cell membranes and to achieve pore formation. These points will be addressed in this review.

Structural elements of the cholesterol-dependent cytolysins that are responsible for their cholesterol-sensitive membrane interactions

The pore-forming mechanism of the cholesterol-dependent cytolysins (CDCs) exhibits an absolute requirement for membrane cholesterol. The structural elements of the CDCs that mediate this interaction are not well understood. Three short hydrophobic loops (L1-L3) and a highly conserved undecapeptide sequence at the tip of domain 4 of the CDC structure are known to anchor the CDC to the membrane. It has been thought that the undecapeptide directly mediates the interaction of the CDCs with a cholesterol-rich cell surface. Herein we show that the L1-L3 loops, not the undecapeptide, are responsible for mediating the specific interaction of the CDCs with cholesterol-rich membranes. The membrane insertion of the undecapeptide was uncoupled from membrane binding by the covalent modification of the undecapeptide cysteine thiol. Modification of the cysteine prevented prepore to pore conversion, but did not affect membrane binding, thus demonstrating that undecapeptide membrane insertion follows that of the L1-L3 loops. These studies provide an example of a structural motif that specifically mediates the interaction of a bacterial toxin with a cholesterol-rich membrane.

Bacillus anthracis: interactions with the host and establishment of inhalational anthrax

Due to its potential as a bioweapon, Bacillus anthracis has received a great deal of attention in recent years, and a significant effort has been devoted to understanding how this organism causes anthrax. There has been a particular focus on the inhalational form of the disease, and studies over the past several years have painted an increasingly complex picture of how B. anthracis enters the mammalian host, survives the host's defense mechanisms, disseminates throughout the body and causes death. This article reviews recent advances in these areas, with a focus on how the bacterium interacts with its host in establishing infection and causing anthrax.

Elucidating the in vivo targets of bacterial toxins

Many bacterial pathogens release soluble proteins, referred to as toxins, which damage host cells during disease. In the past, bacterial toxins have been studied extensively using cultured cells, and in vitro biochemical systems. However, little is known about the types of cells targeted by toxins during the disease process while within the host. This has limited our understanding of these important virulence factors. To address this problem, we have recently used transparent zebrafish embryos to follow toxin activity in a multiorgan system in real-time. Zebrafish provide many advantages over more traditional animal models, since major organs can be directly visualized by light microscopy. This allows one to follow toxin activity and organ damage as it occurs following intoxication. As proof-of-principle, we have recently exploited the zebrafish embryo to identify the activities of Clostridium difficile toxin B, an intracellular bacterial toxin. By using the zebrafish system we have been able to identify a major organ, the heart, targeted by this toxin.

Listeriolysin O secreted by Listeria monocytogenes into the host cell cytosol is degraded by the N-end rule pathway

The intracellular pathogen Listeria monocytogenes escapes from a phagosomal compartment into the cytosol by secreting the pore-forming cytolysin listeriolysin O (LLO). During the proliferation of L. monocytogenes bacteria in the mammalian cell cytosol, the secreted LLO is targeted for degradation by the ubiquitin system. We report here that LLO is a substrate of the ubiquitin-dependent N-end rule pathway, which recognizes LLO through its N-terminal Lys residue. Specifically, we demonstrated by reverse-genetic and pharmacological methods that LLO was targeted for degradation by the N-end rule pathway in reticulocyte extracts and mouse NIH 3T3 cells and after its secretion by intracellular bacteria into the mouse cell cytosol. Replacing the N-terminal Lys of LLO with a stabilizing residue such as Val increased the in vivo half-life of LLO but did not strongly affect the intracellular growth or virulence of L. monocytogenes. Nevertheless, this replacement decreased the virulence of L. monocytogenes by nearly twofold, suggesting that a destabilizing N-terminal residue of LLO may stem from positive selection during the evolution of this and related bacteria. A double mutant strain of L. monocytogenes in which upregulated secretion of LLO was combined with a stabilizing N-terminal residue was severely toxic to infected mammalian cells, resulting in reduced intracellular growth of bacteria and an approximately 100-fold-lower level of virulence. In summary, we showed that LLO is degraded by the N-end rule pathway and that the degradation of LLO can reduce the toxicity of L. monocytogenes during infection, a property of LLO that may have been selected for its positive effects on fitness during the evolution of L. monocytogenes.

Kinetics of cholesterol extraction from lipid membranes by methyl-beta-cyclodextrin--a surface plasmon resonance approach

The kinetics of cholesterol extraction from cellular membranes is complex and not yet completely understood. In this paper we have developed an experimental approach to directly monitor the extraction of cholesterol from lipid membranes by using surface plasmon resonance and model lipid systems. Methyl-beta-cyclodextrin was used to selectively remove cholesterol from large unilamellar vesicles of various compositions. The amount of extracted cholesterol is highly dependent on the composition of lipid membrane, i.e. the presence of sphingomyelin drastically reduced and slowed down cholesterol extraction by methyl-beta-cyclodextrin. This was confirmed also in the erythrocyte ghosts system, where more cholesterol was extracted after erythrocytes were treated with sphingomyelinase. We further show that the kinetics of the extraction is mono-exponential for mixtures of 1,2-dioleoyl-sn-glycero-3-phosphocholine and cholesterol. The kinetics is complex for ternary lipid mixtures composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine, bovine brain sphingomyelin and cholesterol. Our results indicate that the complex kinetics observed in experiments with cells may be the consequence of lateral segregation of lipids in cell plasma membrane.

Structure of C8alpha-MACPF reveals mechanism of membrane attack in complement immune defense

Membrane attack is important for mammalian immune defense against invading microorganisms and infected host cells. Proteins of the complement membrane attack complex (MAC) and the protein perforin share a common MACPF domain that is responsible for membrane insertion and pore formation. We determined the crystal structure of the MACPF domain of complement component C8alpha at 2.5 angstrom resolution and show that it is structurally homologous to the bacterial, pore-forming, cholesterol-dependent cytolysins. The structure displays two regions that (in the bacterial cytolysins) refold into transmembrane beta hairpins, forming the lining of a barrel pore. Local hydrophobicity explains why C8alpha is the first complement protein to insert into the membrane. The size of the MACPF domain is consistent with known C9 pore sizes. These data imply that these mammalian and bacterial cytolytic proteins share a common mechanism of membrane insertion.

Ultrastructural analysis of the membrane insertion of domain 3 of streptolysin O

Streptolysin O (SLO) is a membrane-damaging toxic protein produced by group A streptococci. We performed an ultrastructural analysis of pore formation and the mechanism of hemolysis by SLO, using a mutant form of SLO [SLO(C/A)-SS] and native SLO. SLO(C/A)-SS was unable to penetrate the erythrocyte membrane as a consequence of immobilization that was due to a disulfide bond between domains. The SLO(C/A)-SS molecules that bound to membranes formed numerous single-layered ring-shaped structures that did not result in pores on the membranes. These structures were similar to the structures formed by native SLO at 0 degrees C. After treatment with dithiothreitol, SLO(C/A)-SS that had bound to membranes formed double-layered rings with pores on the membranes, as does native SLO at room temperature. Our morphological evidence demonstrates that an increase in temperature is necessary for the occurrence of conformational changes and for the formation of double-layered rings after the insertion of domain 3 into the host cell membrane. On the basis of a model of the oligomeric structure of SLO, we propose some new details of the mechanism of hemolysis by SLO.

Nod1 mediates cytoplasmic sensing of combinations of extracellular bacteria

During mucosal colonization, epithelial cells are concurrently exposed to numerous microbial species. Epithelial cytokine production is an early component of innate immunity and contributes to mucosal defence. We have previously demonstrated a synergistic response of respiratory epithelial cells to costimulation by two human pathogens, Streptococcus pneumoniae and Haemophilus influenzae. Here we define a molecular mechanism for the synergistic activation of epithelial signalling during polymicrobial colonization. H. influenzae peptidoglycan synergizes with the pore-forming toxin pneumolysin from S. pneumoniae. Radiolabelled peptidoglycan enters epithelial cells more efficiently in the presence of pneumolysin, consistent with peptidoglycan gaining access to the cytoplasm via toxin pores. Other pore-forming toxins (including anthrolysin O from Bacillus anthracis and Staphylococcus aureus alpha-toxin) can substitute for pneumolysin in the generation of synergistic responses. Consistent with a requirement for pore formation, S. pneumoniae expressing pneumolysin but not an isogenic mutant expressing a non-pore-forming toxoid prime epithelial responses. Nod1, a host cytoplasmic peptidoglycan-recognition molecule, is crucial to the epithelial response. Taken together, these findings demonstrate a role for cytosolic recognition of peptidoglycan in the setting of polymicrobial epithelial stimulation. We conclude that combinations of extracellular organisms can activate innate immune pathways previously considered to be reserved for the detection of intracellular microorganisms.

Histone modifications induced by a family of bacterial toxins

Upon infection, pathogens reprogram host gene expression. In eukaryotic cells, genetic reprogramming is induced by the concerted activation/repression of transcription factors and various histone modifications that control DNA accessibility in chromatin. We report here that the bacterial pathogen Listeria monocytogenes induces a dramatic dephosphorylation of histone H3 as well as a deacetylation of histone H4 during early phases of infection. This effect is mediated by the major listerial toxin listeriolysin O in a pore-forming-independent manner. Strikingly, a similar effect also is observed with other toxins of the same family, such as Clostridium perfringens perfringolysin and Streptococcus pneumoniae pneumolysin. The decreased levels of histone modifications correlate with a reduced transcriptional activity of a subset of host genes, including key immunity genes. Thus, control of epigenetic regulation emerges here as an unsuspected function shared by several bacterial toxins, highlighting a common strategy used by intracellular and extracellular pathogens to modulate the host response early during infection.

Listeriolysin O: a phagosome-specific lysin

Listeriolysin O (LLO) is a pore-forming toxin of the cholesterol-dependent cytolysin family and a primary virulence factor of the gram-positive, facultative intracellular pathogen Listeria monocytogenes. During the intracellular life cycle of L. monocytogenes, LLO is largely responsible for mediating rupture of the phagosomal membrane, thereby allowing the bacterium access to the host cytosol, its replicative niche. In the host cytosol, LLO activity is controlled at numerous levels to prevent perforation of the plasma membrane and loss of the intracellular environment. In this review, we focus primarily on the role of LLO in phagosomal escape and the multiple regulatory mechanisms that control LLO activity in the host cytosol.

Cholesterol as treatment for pneumococcal keratitis: cholesterol-specific inhibition of pneumolysin in the cornea

PURPOSE

The purpose of this study was to determine whether cholesterol, the host cell receptor for pneumolysin of Streptococcus pneumoniae, could effectively treat pneumococcal keratitis.

METHODS

New Zealand White rabbits were intrastromally injected with 10(5) colony-forming units (CFUs) of S. pneumoniae D39. Corneas were treated with topical drops of 1% cholesterol every 2 hours beginning 25 hours after infection and were examined by slit lamp microscopy 24, 36, and 48 hours after infection. Rabbits were killed, and CFUs were recovered from the corneas after the final slit lamp examination (SLE). Minimal inhibitory concentration (MIC) assays of cholesterol against bacteria were performed. Specific inhibition of pneumolysin by cholesterol in the rabbit cornea was tested by intrastromal injection of pneumolysin with or without cholesterol and was compared with cholesterol inhibition of pneumolysin in vitro using hemolysis assays with rabbit erythrocytes.

RESULTS

Corneas treated with cholesterol had significantly lower SLE scores 48 hours after infection than corneas treated with vehicle (P = 0.0015). Treated corneas also had significantly less log(10) CFUs than vehicle-treated corneas (P = 0.0006). Cholesterol at a 1% concentration was bactericidal to bacteria in vitro, and lower concentrations of cholesterol were partially inhibitory in a concentration-dependent manner. Cholesterol also specifically inhibited 1 mug pneumolysin in vivo and up to 50 ng pneumolysin in vitro.

CONCLUSIONS

Topical cholesterol is an effective treatment for S. pneumoniae keratitis. Cholesterol not only inhibits pneumolysin, it is also bactericidal.

Characterization of a small PlcR-regulated gene co-expressed with cereolysin O

Background

In the human pathogen Bacillus cereus, the expression of most extracellular virulence factors is controlled by the transcriptional activator PlcR. Among these virulence factors, cereolysin O (Clo) is an haemolysin belonging to the cholesterol-dependant cytolysins, a protein family extensively studied in Gram-positive bacteria.

Results

In the genomes of bacteria belonging to the B. cereus group, including Bacillus anthracis and Bacillus thuringiensis, a small gene encoding a 26-amino acid peptide was present in multicopy. One copy was always found upstream from the gene encoding Clo. In B. cereus ATCC 14579, the small gene and the clo gene are co-transcribed. Transcriptional fusions showed that the three paralogues identified in this strain were expressed in a PlcR-dependent manner. We propose to name these peptides Spp for small PlcR-regulated peptides. We show that a synthetic peptide corresponding to the deduced product of the spp genes displayed antibacterial activity.

Conclusion

The co-expression of spp, a small PlcR-regulated multicopy gene with clo suggests a yet unidentified relationship between Spp and the cholesterol-dependent cytolysin in bacteria belonging to the B.cereus group.

Bacillus anthracis anthrolysin O and three phospholipases C are functionally redundant in a murine model of inhalation anthrax

Although traditionally considered to be an extracellular pathogen, Bacillus anthracis has a brief intracellular step to initiate anthrax. At the onset of infection, B. anthracis must withstand the bactericidal activities of the macrophage. Recently, three phospholipases C (PLCs) were shown to contribute to macrophage-associated growth of B. anthracis by presumably aiding in the escape of the bacterium from phagocytic vacuoles following phagocytosis. However, in the absence of all three PLCs, vegetative bacilli were still observed growing in association with the macrophage, albeit to a lesser extent, implicating that additional factors are involved in this process. In this study, the contributions of the previously identified cholesterol-dependent cytolysin anthrolysin O (ALO) to B. anthracis pathogenesis were investigated following challenges of bone marrow-derived macrophages and intratracheal inoculations of mice. Disruption of ALO alone yielded no differences in virulence in mice. However, combinatorial deletions of ALO with the three PLCs resulted in attenuation in both tissue culture and murine challenges, suggesting that these toxins may have overlapping roles in anthrax pathogenesis.

Molecular mechanisms exploited by Listeria monocytogenes during host cell invasion

The facultative intracellular bacterial pathogen Listeria monocytogenes has evolved multiple strategies to invade a large panel of mammalian cells. Host cell invasion is critical for several stages of listeriosis pathology such as the initial crossing of the host intestinal barrier and the successive colonization of diverse target organs including the placenta. In this review, we address the main molecular mechanisms known to be used by L. monocytogenes during invasion of nonphagocytic cells and host tissues.