Molecular analysis of the cytolytic protein ClyA (SheA) from Escherichia coli

Cholesterol catalyzes unfolding in membrane-inserted motifs of the pore forming protein cytolysin A

Plasma membrane-induced protein folding and conformational transitions play a central role in cellular homeostasis. Several transmembrane proteins are folded in the complex lipid milieu to acquire a specific structure and function. Bacterial pore forming toxins (PFTs) are proteins expressed by a large class of pathogenic bacteria that exploit the plasma membrane environment to efficiently undergo secondary structure changes, oligomerize, and form transmembrane pores. Unregulated pore formation causes ion imbalance, leading to cell death and infection. Determining the free energy landscape of these membrane-driven-driven transitions remains a challenging problem. Although cholesterol recognition is required for lytic activity of several proteins in the PFT family of toxins, the regulatory role of cholesterol for the α-PFT, cytolysin A expressed by Escherichia coli remains unexplained. In a recent free energy computation, we showed that the β tongue, a critical membrane-inserted motif of the ClyA toxin, has an on-pathway partially unfolded intermediate that refolds into the helix-turn-helix motif of the pore state. To understand the molecular role played by cholesterol, we carry out string-method-based computations in membranes devoid of cholesterol, which reveals an increase of ∼30 times in the free energy barrier for the loss of β sheet secondary structure when compared with membranes containing cholesterol. Specifically, the tyrosine-cholesterol interaction was found to be critical to creating the unfolded intermediate. Cholesterol also increases the packing and hydrophobicity of the bilayer, resulting in enhanced interactions of the bound protein before complete membrane insertion. Our study illustrates that cholesterol is critical to catalyzing and stabilizing the membrane-inserted unfolded state of the β tongue motif of ClyA, opening up fresh insights into cholesterol-assisted unfolding of membrane proteins.

The role of bacteria and its derived biomaterials in cancer radiotherapy

Bacteria-mediated anti-tumor therapy has received widespread attention due to its natural tumor-targeting ability and specific immune-activation characteristics. It has made significant progress in breaking the limitations of monotherapy and effectively eradicating tumors, especially when combined with traditional therapy, such as radiotherapy. According to their different biological characteristics, bacteria and their derivatives can not only improve the sensitivity of tumor radiotherapy but also protect normal tissues. Moreover, genetically engineered bacteria and bacteria-based biomaterials have further expanded the scope of their applications in radiotherapy. In this review, we have summarized relevant researches on the application of bacteria and its derivatives in radiotherapy in recent years, expounding that the bacteria, bacterial derivatives and bacteria-based biomaterials can not only directly enhance radiotherapy but also improve the anti-tumor effect by improving the tumor microenvironment (TME) and immune effects. Furthermore, some probiotics can also protect normal tissues and organs such as intestines from radiation via anti-inflammatory, anti-oxidation and apoptosis inhibition. In conclusion, the prospect of bacteria in radiotherapy will be very extensive, but its biological safety and mechanism need to be further evaluated and studied.

Conformational Flexibility Is a Key Determinant for the Lytic Activity of the Pore-Forming Protein, Cytolysin A

Several bacterial infections are mediated by pore-forming toxins (PFTs), a subclass of proteins that oligomerize on mammalian cell membranes forming lytic nanopores. Cytolysin A (ClyA), an α-PFT, undergoes a dramatic conformational change restructuring its two membrane-binding motifs (the β-tongue and the N-terminus helix), during pore formation. A complete molecular picture for this key transition and the driving force behind the secondary structure change upon membrane binding remain elusive. Using all-atom molecular dynamics (MD) simulations of the ClyA monomer and string method based free energy computations with path collective variables, we illustrate that an unfolded β-tongue motif is an on-pathway intermediate during the transition to the helix-turn-helix motif of the protomer. An aggregate of 28 μs of all-atom thermal unfolding MD simulations of wild-type ClyA and its single point mutants reveal that the membrane-binding motifs of the ClyA protein display high structural flexibility in water. However, point mutations in these motifs lead to a distinct reduction in the flexibility, especially in the β-tongue, thereby stabilizing the pretransition secondary structure. Resistance to unfolding was further corroborated by MD simulations of the β-tongue mutant motif in the membrane. Combined with the thermal unfolding simulations, we posit that the β-tongue as well as N-terminal mutants that lower the tendency to unfold and disorder the β-tongue are detrimental to pore formation by ClyA and its lytic activity. Erythrocyte turbidity and vesicle leakage assays indeed reveal a loss of activity for the β-tongue mutant, and delayed kinetics for the N-terminus mutants. On the other hand, a point mutation in the extracellular domain that did not abrogate lytic activity displayed similar unfolding characteristics as the wild type. Thus, attenuation of conformational flexibility in membrane-binding motifs correlates with reduced lytic and leakage activity. Combined with secondary structure changes observed in the membrane bound states, our study shows that the tendency to unfold in the β-tongue region is a critical step in the conformational transition and bistability of the ClyA protein and mutants that disrupt this tendency reduced pore formation. Overall, our finding suggests that inherent flexibility in the protein could play a wider and hitherto unrecognized role in membrane-mediated conformational transitions of PFTs and other membrane protein transformations.

Bacterial protein MakA causes suppression of tumour cell proliferation via inhibition of PIP5K1α/Akt signalling

Recently, we demonstrated that a novel bacterial cytotoxin, the protein MakA which is released by Vibrio cholerae, is a virulence factor, causing killing of Caenorhabditis elegans when the worms are grazing on the bacteria. Studies with mammalian cell cultures in vitro indicated that MakA could affect eukaryotic cell signalling pathways involved in lipid biosynthesis. MakA treatment of colon cancer cells in vitro caused inhibition of growth and loss of cell viability. These findings prompted us to investigate possible signalling pathways that could be targets of the MakA-mediated inhibition of tumour cell proliferation. Initial in vivo studies with MakA producing V. cholerae and C. elegans suggested that the MakA protein might target the PIP5K1α phospholipid-signalling pathway in the worms. Intriguingly, MakA was then found to inhibit the PIP5K1α lipid-signalling pathway in cancer cells, resulting in a decrease in PIP5K1α and pAkt expression. Further analyses revealed that MakA inhibited cyclin-dependent kinase 1 (CDK1) and induced p27 expression, resulting in G2/M cell cycle arrest. Moreover, MakA induced downregulation of Ki67 and cyclin D1, which led to inhibition of cell proliferation. This is the first report about a bacterial protein that may target signalling involving the cancer cell lipid modulator PIP5K1α in colon cancer cells, implying an anti-cancer effect.

Protein-lipid interaction at low pH induces oligomerization of the MakA cytotoxin from Vibrio cholerae

The α-pore-forming toxins (α-PFTs) from pathogenic bacteria damage host cell membranes by pore formation. We demonstrate a remarkable, hitherto unknown mechanism by an α-PFT protein from Vibrio cholerae. As part of the MakA/B/E tripartite toxin, MakA is involved in membrane pore formation similar to other α-PFTs. In contrast, MakA in isolation induces tube-like structures in acidic endosomal compartments of epithelial cells in vitro. The present study unravels the dynamics of tubular growth, which occurs in a pH-, lipid-, and concentration-dependent manner. Within acidified organelle lumens or when incubated with cells in acidic media, MakA forms oligomers and remodels membranes into high-curvature tubes leading to loss of membrane integrity. A 3.7 Å cryo-electron microscopy structure of MakA filaments reveals a unique protein-lipid superstructure. MakA forms a pinecone-like spiral with a central cavity and a thin annular lipid bilayer embedded between the MakA transmembrane helices in its active α-PFT conformation. Our study provides insights into a novel tubulation mechanism of an α-PFT protein and a new mode of action by a secreted bacterial toxin.

Cytolysin A (ClyA): A Bacterial Virulence Factor with Potential Applications in Nanopore Technology, Vaccine Development, and Tumor Therapy

Cytolysin A (ClyA) is a pore-forming toxin that is produced by some bacteria from the Enterobacteriaceae family. This review provides an overview of the current state of knowledge regarding ClyA, including the prevalence of the encoding gene and its transcriptional regulation, the secretion pathway used by the protein, and the mechanism of protein assembly, and highlights potential applications of ClyA in biotechnology. ClyA expression is regulated at the transcriptional level, primarily in response to environmental stressors, and ClyA can exist stably both as a soluble monomer and as an oligomeric membrane complex. At high concentrations, ClyA induces cytolysis, whereas at low concentrations ClyA can affect intracellular signaling. ClyA is secreted in outer membrane vesicles (OMVs), which has important implications for biotechnology applications. For example, the native pore-forming ability of ClyA suggests that it could be used as a component of nanopore-based technologies, such as sequencing platforms. ClyA has also been exploited in vaccine development owing to its ability to present antigens on the OMV surface and provoke a robust immune response. In addition, ClyA alone or OMVs carrying ClyA fusion proteins have been investigated for their potential use as anti-tumor agents.

Surface Engineering of Escherichia coli-Derived OMVs as Promising Nano-Carriers to Target EGFR-Overexpressing Breast Cancer Cells

Bacterial outer membrane vesicles (OMVs) have recently drawn a great deal of attention due to their therapeutic efficiency and ability to target specific cells. In the present study, we sought to probe engineered OMVs as novel and promising carriers to target breast cancer cells. Following the fusion of the affi_EGFR-GALA structure to the C-terminal of ClyA as an anchor protein, the ClyA-affi_EGFR-GALA construct was successfully expressed on the surface of ∆msbB/∆pagP E. coli W3110-derived OMVs. Morphological features of the engineered and wild-type OMVs were identical. The engineered OMVs induced no endotoxicity, cytotoxicity, or immunogenicity, indicating the safety of their application. These OMVs could specifically bind to EGF receptors of MDA-MB-468 cells expressing high levels of EGFR and not to those with low levels of EGFR (HEK293T cells). Interestingly, despite a lower binding affinity of the engineered OMVs relative to the positive control Cetuximab, it was strong enough to identify these cells. Moreover, confocal microscopy revealed no uptake of the modified OMVs by the EGFR-overexpressing cells in the presence of EGFR competitors. These results suggest that OMVs might internalize into the cells with EGF receptors, as no OMVs entered the cells with any EGFR expression or those pretreated with EGF or Cetuximab. Regarding the EGFR-binding affinity of the engineered OMVs and their cellular uptake, they are presented here as a potential carrier for cell-specific drug delivery to treat a wide variety of cancer cells. Interestingly, the engineered OMVs are capable of reaching the cytoplasm while escaping the endosome due to the incorporation of a fusogenic GALA peptide in the construct.

A tripartite cytolytic toxin formed by Vibrio cholerae proteins with flagellum-facilitated secretion

Vibrio cholerae, responsible for outbreaks of cholera disease, is a highly motile organism by virtue of a single flagellum. We describe how the flagellum facilitates the secretion of three V. cholerae proteins encoded by a hitherto-unrecognized genomic island. The proteins MakA/B/E can form a tripartite toxin that lyses erythrocytes and is cytotoxic to cultured human cells. A structural basis for the cytolytic activity of the Mak proteins was obtained by X-ray crystallography. Flagellum-facilitated secretion ensuring spatially coordinated delivery of Mak proteins revealed a role for the V. cholerae flagellum considered of particular significance for the bacterial environmental persistence. Our findings will pave the way for the development of diagnostics and therapeutic strategies against pathogenic Vibrionaceae.

The protein MakA was discovered as a motility-associated secreted toxin from Vibrio cholerae. Here, we show that MakA is part of a gene cluster encoding four additional proteins: MakB, MakC, MakD, and MakE. MakA, MakB, and MakE were readily detected in culture supernatants of wild-type V. cholerae, whereas secretion was very much reduced from a flagellum-deficient mutant. Crystal structures of MakA, MakB, and MakE revealed a structural relationship to a superfamily of bacterial pore-forming toxins. Expression of MakA/B/E in Escherichia coli resulted in toxicity toward Caenorhabditis elegans used as a predatory model organism. None of these Mak proteins alone or in pairwise combinations were cytolytic, but an equimolar mixture of MakA, MakB, and MakE acted as a tripartite cytolytic toxin in vitro, causing lysis of erythrocytes and cytotoxicity on cultured human colon carcinoma cells. Formation of oligomeric complexes on liposomes was observed by electron microscopy. Oligomer interaction with membranes was initiated by MakA membrane binding followed by MakB and MakE joining the assembly of a pore structure. A predicted membrane insertion domain of MakA was shown by site-directed mutagenesis to be essential for toxicity toward C. elegans. Bioinformatic analyses revealed that the makCDBAE gene cluster is present as a genomic island in the vast majority of sequenced genomes of V. cholerae and the fish pathogen Vibrio anguillarum. We suggest that the hitherto-unrecognized cytolytic MakA/B/E toxin can contribute to Vibrionaceae fitness and virulence potential in different host environments and organisms.

Categorizing sequences of concern by function to better assess mechanisms of microbial pathogenesis

To identify sequences with a role in microbial pathogenesis, we assessed the adequacy of their annotation by existing controlled vocabularies and sequence databases. Our goal was to regularize descriptions of microbial pathogenesis for improved integration with bioinformatic applications. Here, we review the challenges of annotating sequences for pathogenic activity. We relate the categorization of more than 2,750 sequences of pathogenic microbes through a controlled vocabulary called Functions of Sequences of Concern (FunSoCs). These allow for an ease of description by both humans and machines. We provide a subset of 220 fully annotated sequences in the supplemental material as examples. The use of this compact (∼30 terms), controlled vocabulary has potential benefits for research in microbial genomics, public health, biosecurity, biosurveillance, and the characterization of new and emerging pathogens.

Artificial Sweeteners Negatively Regulate Pathogenic Characteristics of Two Model Gut Bacteria, E. coli and E. faecalis

Artificial sweeteners (AS) are synthetic sugar substitutes that are commonly consumed in the diet. Recent studies have indicated considerable health risks which links the consumption of AS with metabolic derangements and gut microbiota perturbations. Despite these studies, there is still limited data on how AS impacts the commensal microbiota to cause pathogenicity. The present study sought to investigate the role of commonly consumed AS on gut bacterial pathogenicity and gut epithelium-microbiota interactions, using models of microbiota (Escherichia coli NCTC10418 and Enterococcus faecalis ATCC19433) and the intestinal epithelium (Caco-2 cells). Model gut bacteria were exposed to different concentrations of the AS saccharin, sucralose, and aspartame, and their pathogenicity and changes in interactions with Caco-2 cells were measured using in vitro studies. Findings show that sweeteners differentially increase the ability of bacteria to form a biofilm. Co-culture with human intestinal epithelial cells shows an increase in the ability of model gut bacteria to adhere to, invade and kill the host epithelium. The pan-sweet taste inhibitor, zinc sulphate, effectively blocked these negative impacts. Since AS consumption in the diet continues to increase, understanding how this food additive affects gut microbiota and how these damaging effects can be ameliorated is vital.

Using multiscale molecular dynamics simulations to obtain insights into pore forming toxin mechanisms

Pore forming toxins (PFTs) are virulent proteins released by several species, including many strains of bacteria, to attack and kill host cells. In this article, we focus on the utility of molecular dynamics (MD) simulations and the molecular insights gleaned from these techniques on the pore forming pathways of PFTs. In addition to all-atom simulations which are widely used, coarse-grained MARTINI models and structure-based models have also been used to study PFTs. Here, the emphasis is on methods and techniques involved while setting up, monitoring, and evaluating properties from MD simulations of PFTs in a membrane environment. We draw from several case studies to illustrate how MD simulations have provided molecular insights into protein-protein and protein-lipid interactions, lipid dynamics, conformational transitions and structures of both the oligomeric intermediates and assembled pore structures.

X-ray crystallography shines a light on pore-forming toxins

A common form of cellular attack by pathogenic bacteria is to secrete pore-forming toxins (PFTs). Capable of forming transmembrane pores in various biological membranes, PFTs have also been identified in a diverse range of other organisms such as sea anemones, earthworms and even mushrooms and trees. The mechanism of pore formation by PFTs is associated with substantial conformational changes in going from the water-soluble to transmembrane states of the protein. The determination of the crystal structures for numerous PFTs has shed much light on our understanding of these proteins. Other than elucidating the atomic structural details of PFTs and the conformational changes that must occur for pore formation, crystal structures have revealed structural homology that has led to the discovery of new PFTs and new PFT families. Here we review some key crystallographic results together with complimentary approaches for studying PFTs. We discuss how these studies have impacted our understanding of PFT function and guided research into biotechnical applications.

The Food Poisoning Toxins of Bacillus cereus

Bacillus cereus is a ubiquitous soil bacterium responsible for two types of food-associated gastrointestinal diseases. While the emetic type, a food intoxication, manifests in nausea and vomiting, food infections with enteropathogenic strains cause diarrhea and abdominal pain. Causative toxins are the cyclic dodecadepsipeptide cereulide, and the proteinaceous enterotoxins hemolysin BL (Hbl), nonhemolytic enterotoxin (Nhe) and cytotoxin K (CytK), respectively. This review covers the current knowledge on distribution and genetic organization of the toxin genes, as well as mechanisms of enterotoxin gene regulation and toxin secretion. In this context, the exceptionally high variability of toxin production between single strains is highlighted. In addition, the mode of action of the pore-forming enterotoxins and their effect on target cells is described in detail. The main focus of this review are the two tripartite enterotoxin complexes Hbl and Nhe, but the latest findings on cereulide and CytK are also presented, as well as methods for toxin detection, and the contribution of further putative virulence factors to the diarrheal disease.

Mechanistic Insights into Pore Formation by an α-Pore Forming Toxin: Protein and Lipid Bilayer Interactions of Cytolysin A

Pore forming toxins (PFTs) are the largest class of bacterial toxins playing a central role in bacterial pathogenesis. They are proteins specifically designed to form nanochannels in the membranes of target cells, ultimately resulting in cell death and establishing infection. PFTs are broadly classified as α- and β-PFTs, depending on secondary structures that form the transmembrane channel. A unique feature about this class of proteins is the drastic conformational changes and complex oligomerization pathways that occur upon exposure to the plasma membrane. A molecular understanding of pore formation has implications in designing novel intervention strategies to combat rising antimicrobial resistance, targeted-cancer therapy, as well as designing nanopores for specialized technologies. Central to unraveling the pore formation pathway is the availability of high resolution crystal structures. In this regard, β-toxins are better understood, when compared with α-toxins whose pore forming mechanisms are complicated by an incomplete knowledge of the driving forces for amphiphatic membrane-inserted helices to organize into functional pores. With the publication of the first crystal structure for an α-toxin, cytolysin A (ClyA), in 2009 we embarked on an extensive multiscale study to unravel its pore forming mechanism. This Account represents the collective mechanistic knowledge gained in our laboratories using a variety of experimental and theoretical techniques which include large scale molecular dynamics (MD) simulations, kinetic modeling studies, single-molecule fluorescence imaging, and super-resolution spectroscopy. We reported MD simulations of the ClyA protomer, oligomeric intermediates, and full pore complex in a lipid bilayer and mapped the conformational transitions that accompany membrane binding. Using single-molecule fluorescence imaging, the conformational transition was experimentally verified by analysis of various diffusion states of membrane bound ClyA. Importantly, we have uncovered a hitherto unknown putative cholesterol binding motif in the membrane-inserted helix of ClyA. Distinct binding pockets for cholesterol formed by adjacent membrane-inserted helices are revealed in MD simulations. Cholesterol appears to play a dual role by stabilizing both the membrane-inserted protomer as well as oligomeric intermediates. Molecular dynamics simulations and kinetic modeling studies suggest that the membrane-inserted arcs oligomerize reversibly to form the predominant transmembrane oligomeric intermediates during pore formation. We posit that this mechanistic understanding of the complex action of α-PFTs has implications in unraveling pore assembly across the wider family of bacterial toxins. With emerging antimicrobial resistance, alternate therapies may rely on disrupting pore functionality or oligomerization of these pathogenic determinants utilized by bacteria, and our study includes assessing the potential for dendrimers as pore blockers.

Characteristics of the Protein Complexes and Pores Formed by Bacillus cereus Hemolysin BL

Bacillus cereus Hemolysin BL is a tripartite toxin responsible for a diarrheal type of food poisoning. Open questions remain regarding its mode of action, including the extent to which complex formation prior to cell binding contributes to pore-forming activity, how these complexes are composed, and the properties of the pores formed in the target cell membrane. Distinct complexes of up to 600 kDa were found on native gels, whose structure and size were primarily defined by Hbl B. Hbl L1 and L2 were also identified in these complexes using Western blotting and an LC-MS approach. LC-MS also revealed that many other proteins secreted by B. cereus exist in complexes. Further, a decrease of toxic activity at temperatures ≥60 °C was shown, which was unexpectedly restored at higher temperatures. This could be attributed to a release of Hbl B monomers from tight complexation, resulting in enhanced cell binding. In contrast, Hbl L1 was rather susceptible to heat, while heat treatment of Hbl L2 seemed not to be crucial. Furthermore, Hbl-induced pores had a rather small single-channel conductance of around 200 pS and a probable channel diameter of at least 1 nm on planar lipid bilayers. These were highly instable and had a limited lifetime, and were also slightly cation-selective. Altogether, this study provides astonishing new insights into the complex mechanism of Hbl pore formation, as well as the properties of the pores.

Genomic characterization of multidrug-resistant ESBL-producing Escherichia coli ST58 causing fatal colibacillosis in critically endangered Brazilian merganser (Mergus octosetaceus)

Even though antimicrobial‐resistant bacteria have begun to be detected in wildlife, raising important issues related to their transmission and persistence of clinically important pathogens in the environment, little is known about the role of these bacteria on wildlife health, especially on endangered species. The Brazilian merganser (Mergus octosetaceus) is one of the most threatened waterfowl in the world, classified as Critically Endangered by the International Union for Conservation of Nature. In 2019, a fatal case of sepsis was diagnosed in an 8‐day‐old Brazilian merganser inhabiting a zoological park. At necropsy, major gross lesions were pulmonary and hepatic congestion. Using microbiologic and genomic methods, we identified a multidrug‐resistant (MDR) extended‐spectrum β‐lactamase (ESBL) CTX‐M‐8‐producing Escherichia coli (designed as PMPU strain) belonging to the international clone ST58, in coelomic cavity, oesophagus, lungs, small intestine and cloaca samples. PMPU strain harboured a broad resistome against antibiotics (cephalosporins, tetracyclines, aminoglycosides, sulphonamides, trimethoprim and quinolones), domestic/hospital disinfectants and heavy metals (arsenic, mercury, lead, copper and silver). Additionally, the virulence of E. coli PMPU strain was confirmed using a wax moth (Galleria mellonella) infection model, and it was supported by the presence of virulence genes encoding toxins, adherence factors, invasins and iron acquisition systems. Broad resistome and virulome of PMPU contributed to therapeutic failure and death of the animal. In brief, we report for the first time a fatal colibacillosis by MDR ESBL‐producing E. coli in critically endangered Brazilian merganser, highlighting that besides colonization, critical priority pathogens are threatening wildlife. E. coli ST58 clone has been previously reported in humans, food‐producing animals, wildlife and environment, supporting broad adaptation and persistence at human–animal–environment interface.

High-resolution cryo-EM structures of the E. coli hemolysin ClyA oligomers

Pore-forming proteins (PFPs) represent a functionally important protein family, that are found in organisms from viruses to humans. As a major branch of PFPs, bacteria pore-forming toxins (PFTs) permeabilize membranes and usually cause the death of target cells. E. coli hemolysin ClyA is the first member with the pore complex structure solved among α-PFTs, employing α-helices as transmembrane elements. ClyA is proposed to form pores composed of various numbers of protomers. With high-resolution cryo-EM structures, we observe that ClyA pore complexes can exist as newly confirmed oligomers of a tridecamer and a tetradecamer, at estimated resolutions of 3.2 Å and 4.3 Å, respectively. The 2.8 Å cryo-EM structure of a dodecamer dramatically improves the existing structural model. Structural analysis indicates that protomers from distinct oligomers resemble each other and neighboring protomers adopt a conserved interaction mode. We also show a stabilized intermediate state of ClyA during the transition process from soluble monomers to pore complexes. Unexpectedly, even without the formation of mature pore complexes, ClyA can permeabilize membranes and allow leakage of particles less than ~400 Daltons. In addition, we are the first to show that ClyA forms pore complexes in the presence of cholesterol within artificial liposomes. These findings provide new mechanistic insights into the dynamic process of pore assembly for the prototypical α-PFT ClyA.

Cholesterol promotes Cytolysin A activity by stabilizing the intermediates during pore formation

Pore-forming toxins (PFTs) form nanoscale pores across target membranes causing cell death. Cytolysin A (ClyA) from Escherichia coli is a prototypical α-helical toxin that contributes to cytolytic phenotype of several pathogenic strains. It is produced as a monomer and, upon membrane exposure, undergoes conformational changes and finally oligomerizes to form a dodecameric pore, thereby causing ion imbalance and finally cell death. However, our current understanding of this assembly process is limited to studies in detergents, which do not capture the physicochemical properties of biological membranes. Here, using single-molecule imaging and molecular dynamics simulations, we study the ClyA assembly pathway on phospholipid bilayers. We report that cholesterol stimulates pore formation, not by enhancing initial ClyA binding to the membrane but by selectively stabilizing a protomer-like conformation. This was mediated by specific interactions by cholesterol-interacting residues in the N-terminal helix. Additionally, cholesterol stabilized the oligomeric structure using bridging interactions in the protomer-protomer interfaces, thereby resulting in enhanced ClyA oligomerization. This dual stabilization of distinct intermediates by cholesterol suggests a possible molecular mechanism by which ClyA achieves selective membrane rupture of eukaryotic cell membranes. Topological similarity to eukaryotic membrane proteins suggests evolution of a bacterial α-toxin to adopt eukaryotic motifs for its activation. Broad mechanistic correspondence between pore-forming toxins hints at a wider prevalence of similar protein membrane insertion mechanisms.

Assembly mechanism of the α-pore-forming toxin cytolysin A from Escherichia coli

The cytolytic toxin cytolysin A (ClyA) from Escherichia coli is probably one of the best-characterized examples of bacterial, α-pore-forming toxins (α-PFTs). Like other PFTs, ClyA exists in a soluble, monomeric form that assembles to an annular, homo-oligomeric pore complex upon contact with detergent or target membranes. Comparison of the three-dimensional structures of the 34 kDa monomer and the protomer in the context of the dodecameric pore complex revealed that ClyA undergoes one of the largest conformational transitions described for proteins so far, in which 55% of the residues change their position and 16% of the residues adopt a different secondary structure in the protomer. Studies on the assembly of ClyA revealed a unique mechanism that differs from the assembly mechanism of other PFTs. The rate-liming step of pore formation proved to be the unimolecular conversion of the monomer to an assembly-competent protomer, during which a molten globule-like off-pathway intermediate accumulates. The oligomerization of protomers to pore complexes is fast and follows a kinetic scheme in which mixtures of linear oligomers of different size are formed first, followed by very rapid and specific association of pairs of oligomers that can directly perform ring closure to the dodecameric pore complex.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

Disruption of the open conductance in the β-tongue mutants of Cytolysin A

Cytolysin A (ClyA) is a water-soluble alpha pore-forming toxin that assembles to form an oligomeric pore on host cell membranes. The ClyA monomer possesses an α-helical bundle with a β-sheet subdomain (the β-tongue) previously believed to be critical for pore assembly and/or insertion. Oligomerization of ClyA pores transforms the β-tongue into a helix-turn-helix that embeds into the lipid bilayer. Here, we show that mutations of the β-tongue did not prevent oligomerization or transmembrane insertion. Instead, β-tongue substitution mutants yielded pores with decreased conductance while a deletion mutation resulted in pores that rapidly closed following membrane association. Our results suggest that the β-tongue may play an essential structural role in stabilizing the open conformation of the transmembrane domain.

Cry6Aa1, a Bacillus thuringiensis nematocidal and insecticidal toxin, forms pores in planar lipid bilayers at extremely low concentrations and without the need of proteolytic processing

Cry6Aa1 is a Bacillus thuringiensis (Bt) toxin active against nematodes and corn rootworm insects. Its 3D molecular structure, which has been recently elucidated, is unique among those known for other Bt toxins. Typical three-domain Bt toxins permeabilize receptor-free planar lipid bilayers (PLBs) by forming pores at doses in the 1-50 μg/ml range. Solubilization and proteolytic activation are necessary steps for PLB permeabilization. In contrast to other Bt toxins, Cry6Aa1 formed pores in receptor-free bilayers at doses as low as 200 pg/ml in a wide range of pH (5.5-9.5) and without the need of protease treatment. When Cry6Aa1 was preincubated with Western corn rootworm (WCRW) midgut juice or trypsin, 100 fg/ml of the toxin was sufficient to form pores in PLBs. The overall biophysical properties of the pores were similar for all three forms of the toxin (native, midgut juice- and trypsin-treated), with conductances ranging from 28 to 689 pS, except for their ionic selectivity, which was slightly cationic for the native and midgut juice-treated Cry6Aa1, whereas dual selectivity (to cations or anions) was observed for the pores formed by the trypsin-treated toxin. Enrichment of PLBs with WCRW midgut brush-border membrane material resulted in a 2000-fold reduction of the amount of native Cry6Aa1 required to form pores and affected the biophysical properties of both the native and trypsin-treated forms of the toxin. These results indicate that, although Cry6Aa1 forms pores, the molecular determinants of its mode of action are significantly different from those reported for other Bt toxins.

Multi-isotype antibody responses against the multimeric Salmonella Typhi recombinant hemolysin E antigen

The detection and measurement of different antibody isotypes in the serum provide valuable indicators of the different stages of typhoid infection. Here, the ability of S. Typhi recombinant hemolysin E (HlyE) to detect multi-isotype antibody responses in sera of patients with typhoid and paratyphoid A was investigated using an indirect antibody immunoassay. Nanogram amounts of HlyE were found to be sufficient for detection of IgG and IgA isotypes and, in a study of individuals' sera (n = 100), the immunoassay was able to distinguish between typhoid and non-typhoid sera. The overall sensitivity, specificity and efficiency of the ELISA were 70% (39/56), 100% (44/44) and 83% respectively.

A pore-forming toxin requires a specific residue for its activity in membranes with particular physicochemical properties

The physicochemical landscape of the bilayer modulates membrane protein function. Actinoporins are a family of potent hemolytic proteins from sea anemones acting at the membrane level. This family of cytolysins preferentially binds to target membranes containing sphingomyelin, where they form lytic pores giving rise to cell death. Although the cytolytic activity of the actinoporin fragaceatoxin C (FraC) is sensitive to vesicles made of various lipid compositions, it is far from clear how this toxin adjusts its mechanism of action to a broad range of physiochemical landscapes. Herein, we show that the conserved residue Phe-16 of FraC is critical for pore formation in cholesterol-rich membranes such as those of red blood cells. The interaction of a panel of muteins of Phe-16 with model membranes composed of raft-like lipid domains is inactivated in cholesterol-rich membranes but not in cholesterol-depleted membranes. These results indicate that actinoporins recognize different membrane environments, resulting in a wider repertoire of susceptible target membranes (and preys) for sea anemones. In addition, this study has unveiled promising candidates for the development of protein-based biosensors highly sensitive to the concentration of cholesterol within the membrane.

Colonization factors of enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is a major cause of life-threatening diarrheal disease around the world. The major aspects of ETEC virulence are colonization of the small intestine and the secretion of enterotoxins which elicit diarrhea. Intestinal colonization is mediated, in part, by adhesins displayed on the bacterial cell surface. As colonization of the intestine is the critical first step in the establishment of an infection, it represents a potential point of intervention for the prevention of infections. Therefore, colonization factors (CFs) have been important subjects of research in the field of ETEC virulence. Research in this field has revealed that ETEC possesses a large array of serologically distinct CFs that differ in composition, structure, and function. Most ETEC CFs are pili (fimbriae) or related fibrous structures, while other adhesins are simple outer membrane proteins lacking any macromolecular structure. This chapter reviews the genetics, structure, function, and regulation of ETEC CFs and how such studies have contributed to our understanding of ETEC virulence and opened up potential opportunities for the development of preventive and therapeutic interventions.

Outer membrane vesicles mediate transport of biologically active Vibrio cholerae cytolysin (VCC) from V. cholerae strains

Background

Outer membrane vesicles (OMVs) released from Gram-negative bacteria can serve as vehicles for the translocation of virulence factors. Vibrio cholerae produce OMVs but their putative role in translocation of effectors involved in pathogenesis has not been well elucidated. The V. cholerae cytolysin (VCC), is a pore-forming toxin that lyses target eukaryotic cells by forming transmembrane oligomeric β-barrel channels. It is considered a potent toxin that contributes to V. cholerae pathogenesis. The mechanisms involved in the secretion and delivery of the VCC have not been extensively studied.

Methodology/Principal Findings

OMVs from V. cholerae strains were isolated and purified using a differential centrifugation procedure and Optiprep centrifugation. The ultrastructure and the contents of OMVs were examined under the electron microscope and by immunoblot analyses respectively. We demonstrated that VCC from V. cholerae strain V:5/04 was secreted in association with OMVs and the release of VCC via OMVs is a common feature among V. cholerae strains. The biological activity of OMV-associated VCC was investigated using contact hemolytic assay and epithelial cell cytotoxicity test. It showed toxic activity on both red blood cells and epithelial cells. Our results indicate that the OMVs architecture might play a role in stability of VCC and thereby can enhance its biological activities in comparison with the free secreted VCC. Furthermore, we tested the role of OMV-associated VCC in host cell autophagy signalling using confocal microscopy and immunoblot analysis. We observed that OMV-associated VCC triggered an autophagy response in the target cell and our findings demonstrated for the first time that autophagy may operate as a cellular defence mechanism against an OMV-associated bacterial virulence factor.

Conclusion/Significance

Biological assays of OMVs from the V. cholerae strain V:5/04 demonstrated that OMV-associated VCC is indeed biologically active and induces toxicity on mammalian cells and furthermore can induce autophagy.

Elevated recombinant clyA gene expression in the uropathogenic Escherichia coli strain 536, a clue to explain pathoadaptive mutations in a subset of extraintestinal E. coli strains

Background

Analysis of the Escherichia coli collection of reference strains (ECOR) for the presence of the gene locus clyA, which encodes the pore-forming protein ClyA (cytolysin A), revealed that a non-functional clyA locus is common among certain extraintestinal pathogenic E. coli (ExPEC). In fact, all 15 ECOR group B2 strains and several additionally examined extraintestinal pathogenic (uropathogenic (UPEC) and neonatal meningitis (NBM)) E. coli strains contained various ΔclyA alleles.

Results

There are at least four different variants of ΔclyA, suggesting that such deletions in clyA have arisen at more than one occasion. On the basis of this occurrence of the truncated clyA genes, we considered that there may be a patho-adaptive selection for deletions in clyA in extraintestinal pathogenic E. coli. In E. coli K-12 the clyA gene has been viewed as “cryptic” since it is tightly silenced by the nucleoid structuring protein H-NS. We constructed a restored clyA⁺ locus in derivatives of the UPEC strain 536 for further investigation of this hypothesis and, in particular, how the gene would be expressed. Our results show that the level of clyA⁺ expression is highly increased in the UPEC derivatives in comparison with the non-pathogenic E. coli K-12. Transcription of the clyA⁺ gene was induced to even higher levels when the SfaX regulatory protein was overproduced. The derivative with a restored clyA⁺ locus displayed a somewhat slower growth than the parental UPEC strain 536 when a sub-inhibitory concentration of the antimicrobial peptide Polymyxin B was added to the growth medium.

Conclusions

Taken together, our findings show that the clyA⁺ locus is expressed at an elevated level in the UPEC strain and we conclude that this is at least in part due to the effect of the SfaX/PapX transcriptional regulators.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0216-4) contains supplementary material, which is available to authorized users.

Prevalence of hemolysin genes and comparison of ehxA subtype patterns in Shiga toxin-producing Escherichia coli (STEC) and non-STEC strains from clinical, food, and animal sources

Shiga toxin-producing Escherichia coli (STEC) belonging to certain serogroups (e.g., O157 and O26) can cause serious conditions like hemolytic-uremic syndrome (HUS), but other strains might be equally pathogenic. While virulence factors, like stx and eae, have been well studied, little is known about the prevalence of the E. coli hemolysin genes (hlyA, ehxA, e-hlyA, and sheA) in association with these factors. Hemolysins are potential virulence factors, and ehxA and hlyA have been associated with human illness, but the significance of sheA is unknown. Hence, 435 E. coli strains belonging to 62 different O serogroups were characterized to investigate gene presence and phenotypic expression of hemolysis. We further investigated ehxA subtype patterns in E. coli isolates from clinical, animal, and food sources. While sheA and ehxA were widely distributed, e-hlyA and hlyA were rarely found. Most strains (86.7%) were hemolytic, and significantly more hemolytic (95%) than nonhemolytic strains (49%) carried stx and/or eae (P < 0.0001). ehxA subtyping, as performed by using PCR in combination with restriction fragment length polymorphism analysis, resulted in six closely related subtypes (>94.2%), with subtypes A/D being eae-negative STECs and subtypes B, C, E, and F eae positive. Unexpectedly, ehxA subtype patterns differed significantly between isolates collected from different sources (P < 0.0001), suggesting that simple linear models of exposure and transmission need modification; animal isolates carried mostly subtypes A/C (39.3%/42.9%), food isolates carried mainly subtype A (81.9%), and clinical isolates carried mainly subtype C (66.4%). Certain O serogroups correlated with particular ehxA subtypes: subtype A with O104, O113, and O8; B exclusively with O157; C with O26, O111, and O121.

A non-classical assembly pathway of Escherichia coli pore-forming toxin cytolysin A

Cytolysin A (ClyA) is an α-pore forming toxin from pathogenic Escherichia coli (E. coli) and Salmonella enterica. Here, we report that E. coli ClyA assembles into an oligomeric structure in solution in the absence of either bilayer membranes or detergents at physiological temperature. These oligomers can rearrange to create transmembrane pores when in contact with detergents or biological membranes. Intrinsic fluorescence measurements revealed that oligomers adopted an intermediate state found during the transition between monomer and transmembrane pore. These results indicate that the water-soluble oligomer represents a prepore intermediate state. Furthermore, we show that ClyA does not form transmembrane pores on E. coli lipid membranes. Because ClyA is delivered to the target host cell in an oligomeric conformation within outer membrane vesicles (OMVs), our findings suggest ClyA forms a prepore oligomeric structure independently of the lipid membrane within the OMV. The proposed model for ClyA represents a non-classical pathway to attack eukaryotic host cells.

YaxAB, a Yersinia enterocolitica pore-forming toxin regulated by RovA

The transcriptional regulator RovA positively regulates transcription of the Yersinia enterocolitica virulence gene inv. Invasin, encoded by inv, is important for establishment of Y. enterocolitica infection. However, a rovA mutant is more attenuated for virulence than an inv mutant, implying that RovA regulates additional virulence genes. When the Y. enterocolitica RovA regulon was defined by microarray analysis, YE1984 and YE1985 were among the genes identified as being upregulated by RovA. Since these genes are homologous to Xenorhabdus nematophila cytotoxin genes xaxA and xaxB, we named them yaxA and yaxB, respectively. In this work, we demonstrate the effects of YaxAB on the course of infection in the murine model. While a yaxAB mutant (ΔyaxAB) is capable of colonizing mice at the same level as the wild type, it slightly delays the course of infection and results in differing pathology in the spleen. Further, we found that yaxAB encode a probable cytotoxin capable of lysing mammalian cells, that both YaxA and YaxB are required for cytotoxic activity, and that the two proteins associate. YaxAB-mediated cell death occurs via osmotic lysis through the formation of distinct membrane pores. In silico tertiary structural analysis identified predicted structural homology between YaxA and proteins in pore-forming toxin complexes from Bacillus cereus (HBL-B) and Escherichia coli (HlyE). Thus, it appears that YaxAB function as virulence factors by inducing cell lysis through the formation of pores in the host cell membrane. This characterization of YaxAB supports the hypothesis that RovA regulates expression of multiple virulence factors in Y. enterocolitica.

Cloning, expression, and purification of the hemolysin/cytolysin (HlyE antigen) from Salmonella enterica serovar Typhi: potential application for immunoassay development

The hemolysin (HlyE) protein of Salmonella enterica serovar Typhi was reported to be antigenic. This work describes the cloning, expression, and purification of a hexahistidine-tagged HlyE protein under native conditions. Immunoblot analysis and a competitive enzyme-linked immunosorbent assay using sera from typhoid patients showed the presence of HlyE-specific antibodies in circulation.

Staphylococcus aureus α-toxin-dependent induction of host cell death by membrane-derived vesicles

Staphylococcus aureus causes a wide spectrum of infections in humans, ranging from superficial cutaneous infections, infections in the circum-oral region, to life-threatening bacteremia. It was recently demonstrated that Gram-positive organisms such as S. aureus liberate membrane-derived vesicles (MVs), which analogously to outer membrane vesicles (OMVs) of Gram-negative bacteria can play a role in delivering virulence factors to host cells. In the present study we have shown that cholesterol-dependent fusion of S. aureus MVs with the plasma membrane represents a route for delivery of a key virulence factor, α-toxin (α-hemolysin; Hla) to human cells. Most S. aureus strains produce this 33-kDa pore-forming protein, which can lyse a wide range of human cells, and induce apoptosis in T-lymphocytes. Our results revealed a tight association of biologically active α-toxin with membrane-derived vesicles isolated from S. aureus strain 8325-4. Concomitantly, α-toxin contributed to HeLa cell cytotoxicity of MVs, and was the main vesicle-associated protein responsible for erythrocyte lysis. In contrast, MVs obtained from an isogenic hla mutant were significantly attenuated with regards to both causing lysis of erythrocytes and death of HeLa cells. This is to our knowledge the first recognition of an S. aureus MV-associated factor contributing to host cell cytotoxicity.

Overexpression of Ipe protein from the coliphage mEp021 induces pleiotropic effects involving haemolysis by HlyE-containing vesicles and cell death

Lysogenic Escherichia coli K-12 harbouring the prophage mEp021 displays haemolytic activity. From a genomic library of mEp021, we identified an open reading frame (ORF 4) that was responsible for the haemolytic activity. However, the ORF 4 sequence contains four initiation codons in the same frame: ORF 4.1-ORF 4.4, coding for 83-a.a., 82-a.a., 77-a.a. and 72-a.a. products, respectively. The expression of the cloned ORF 4.3, or inducer of pleiotropic effects (ipe), reproduced the haemolytic phenotype in a native strain carrying the gene hlyE(+), but not in the mutant hlyE(-) strain. The overexpression of Ipe induced several pleiotropic effects, such as the inhibition of cell growth and the deregulation of cell division, which resulted in a mixture of normal and desiccated-like cells: normal-filamentous, desiccated-like-filamentous bacilli, minicells etc. Other effects included abnormalities of the cell membrane, the production of vesicles containing HlyE, and finally, cell death. These events were analysed at the molecular level by microarray assays. The global transcription profile of E. coli K-12 strain MC4100, which expressed Ipe after 4 h, revealed differential expression of various genes, most of which were related either to cell membrane and murein biosynthesis or to cell division. The up-regulation of some of these transcripts was confirmed by qRT-PCR. Additional research is needed to determine whether these effects are directly related to Ipe activity or are consequences of the cellular responses to putative structural damage induced by Ipe.

Haemolysin E- and enterohaemolysin-derived haemolytic activity of O55/O157 strains and other Escherichia coli lineages

Among three haemolysins identified thus far in Escherichia coli, alpha-haemolysin (HlyA) is encoded on the pathogenicity islands of extraintestinal pathogenic strains, while enterohaemolysin (EhxA) is encoded on the virulence plasmids of enterohaemorrhagic E. coli (EHEC) strains. In contrast, the gene for haemolysin E (HlyE) is located on the E. coli chromosome backbone and is therefore widely distributed among E. coli strains. However, because hlyE gene expression is repressed by the H-NS protein and because the gene has been disrupted in many strains, its haemolytic activity cannot be detected in wild-type strains by routine screening on blood agar plates. In this study, we found that the HlyE-derived haemolytic activity of enteropathogenic E. coli (EPEC) O55 : H7 can be detected after anaerobic cultivation on a washed blood agar plate (EHX plate) that is used to detect the production of EhxA. We also found that the haemolytic activity of EHEC O157 : H7 observed on EHX plates under aerobic and anaerobic growth conditions is derived from EhxA and HlyE, respectively; this differential expression of the two haemolysins occurs at the transcriptional level. Our analysis of 60 E. coli strains of various pathotypes and phylogenies for their repertoires of haemolysin genes, haemolytic phenotypes and hlyE gene sequences revealed that HlyE activity can generally be detected on EHX plates under anaerobic growth conditions if the gene is intact. Furthermore, our results indicate that hlyE gene inactivation occurred in three of the five E. coli lineages (phylogroups A, B1 and B2), which demonstrates phylogroup-specific gene disruption patterns.

Analysis of Shigella flexneri-mediated infections in model organism Caenorhabditis elegans

BACKGROUND

Shigella flexneri is the causative agent of bacillary dysentery and generates a significant global disease burden. The aim of this study was to analyze the pathogenesis and host immune response, at both the physiological and molecular level, using the model organism Caenorhabditis elegans, in response to S. flexneri. C. elegans is a nematode that responds to infection with a simple innate immune system, key aspects of which have been shown to be conserved.

METHODS

S. flexneri-mediated infection of C. elegans was performed in both solid and liquid assays. The expression and subsequent regulation of host candidate antimicrobial genes such as lysozymes, C-type lectins and pathogen virulence genes were kinetically analyzed in the S. flexneri-exposed nematode.

RESULTS

In solid assays, worms fed with S. flexneri showed complete killing at 153 ± 9 h. The kinetic studies showed that S. flexneri killed the worms upon continuous exposure at 41 ± 1.7 h. However, short-time exposure of the host to S. flexneri indicated that 14 h of exposure resulted in a loss of progeny, and death occurred after 46 h. Semi-quantitative reverse transcriptase polymerase chain reaction (RT-PCR) analysis revealed that mRNA levels of host candidate antimicrobial genes and pathogen virulence genes varied significantly at the time of early infection.

CONCLUSIONS

The killing of C. elegans requires live bacteria, and a minimal exposure time is sufficient for S. flexneri to have a lethal effect. The candidate antimicrobial genes and virulence genes are kinetically regulated within C. elegans during S. flexneri-mediated infections, thereby exhibiting their role and contribution in the host innate immune system.

Molecular cloning and characterization of clyA genes in various serotypes of Salmonella enterica

Cytolysin A (ClyA) is a pore-forming hemolytic protein encoded by the clyA gene. It has been identified in Salmonella enterica serovars Typhi and Paratyphi A. To identify and characterize the clyA genes in various Salmonella enterica strains, 21 different serotypes of strains isolated from clinical specimens were presently examined. Full-length clyA genes were found in S. enterica serovar Brandenburg, Indiana, Panama, and Schwarzengrund strains by polymerase chain reaction amplification. The ClyA proteins from these four strains showed >97% amino acid identity to that of S. enterica serovar Typhi. Although all four serovars expressed detectable levels of ClyA as determined by Western blot analysis, they did not show a strong hemolytic effect on blood agar, indicating that ClyA may not be efficiently expressed or secreted. Escherichia coli transformed with clyA genes from the four serovars enhanced production of ClyA proteins and hemolytic activities to a level similar to S. enterica serovar Typhi ClyA. The present results suggest that ClyA may play a role in the pathogenesis of S. enterica serovar Brandenburg, Indiana, Panama and Schwarzengrund.

Cytotoxicity of the Bacillus cereus Nhe enterotoxin requires specific binding order of its three exoprotein components

This study focuses on the interaction of the three components of the Bacillus cereus Nhe enterotoxin with particular emphasis on the functional roles of NheB and NheC. The results demonstrated that both NheB and NheC were able to bind to Vero cells directly while NheA lacked this ability. It was also shown that Nhe-induced cytotoxicity required a specific binding order of the individual components whereby the presence of NheC in the priming step as well as the presence of NheA in the final incubation step was mandatory. Priming of cells with NheB alone and addition of NheA plus NheC in the second step failed to induce toxic effects. Furthermore, in solution, excess NheC inhibited binding of NheB to Vero cells, whereas priming of cells with excess NheC resulted in full toxicity if unbound NheC was removed before addition of NheB. By using mutated NheC proteins where the two cysteine residues in the predicted beta-tongue were replaced with glycine (NheCcys-) or where the entire hydrophobic stretch was deleted (NheChr-), the predicted hydrophobic beta-tongue of NheC was found essential for binding to cell membranes but not for interaction with NheB in solution. All data presented here are compatible with the following model. The first step in the mode of action of Nhe is associated with binding of NheC and NheB to the cell surface and probably accompanied by conformational changes. These events allow subsequent binding of NheA, leading to cell lysis.

Mutations affecting export and activity of cytolysin A from Escherichia coli

Cytolysin A (known as ClyA, HlyE, and SheA) is a cytolytic pore-forming protein toxin found in several Escherichia coli and Salmonella enterica strains. The structure of its water-soluble monomeric form and that of dodecameric ClyA pores is known, but the mechanisms of ClyA export from bacterial cells and of pore assembly are only partially understood. Here we used site-directed mutagenesis to study the importance of different regions of the E. coli ClyA protein for export and activity. The data indicate that ClyA translocation to the periplasm requires several protein segments located closely adjacent to each other in the "tail" domain of the ClyA monomer, namely, the N- and C-terminal regions and the hydrophobic sequence ranging from residues 89 to 101. Deletion of most of the "head" domain of the monomer (residues 181 to 203), on the other hand, did not strongly affect ClyA secretion, suggesting that the tail domain plays a particular role in export. Furthermore, we found that the N-terminal amphipathic helix alphaA1 of ClyA is crucial for the formation and the properties of the transmembrane channel, and hence for hemolytic activity. Several mutations affecting the C-terminal helix alphaG, the "beta-tongue" region in the head domain, or the hydrophobic region in the tail domain of the ClyA monomer strongly impaired the hemolytic activity and reduced the activity toward planar lipid bilayer membranes but did not totally prevent formation of wild-type-like channels in these artificial membranes. The latter regions thus apparently promote membrane interaction without being directly required for pore formation in a lipid bilayer.

So similar, yet so different: uncovering distinctive features in the genomes of Salmonella enterica serovars Typhimurium and Typhi

Salmonella enterica represents a major human and animal pathogen. Many S. enterica genomes have been completed and many more genome sequencing projects are underway, constituting an excellent resource for comparative genome analysis studies leading to a better understanding of bacterial evolution and pathogenesis. Salmonella enterica serovar Typhimurium and Typhi are the best-characterized serovars, with the first being involved in localized gastroenteritis in many hosts and the latter causing a systemic human-specific disease. Here, we summarize the major genetic differences between the two different serovars. We detail the divergent repertoires of the virulence factors responsible for the pathogenesis of the organisms and that ultimately result in the distinct clinical outcomes of infection. This comparative genomic overview highlights hypotheses for future investigations on S. enterica pathogenesis and the basis of host specificity.

Inhibition of tumor growth and metastasis by a combination of Escherichia coli-mediated cytolytic therapy and radiotherapy

We have reported that Escherichia coli K-12 colonizes hypoxic and necrotic tumor regions after intravenous injection into tumor-bearing mice. In this study, we established a novel strategy for cancer therapy using engineered bacteria to enhance the therapeutic effects of radiation. E. coli strain K-12 was engineered to produce cytolysin A (ClyA), and its effects on tumor growth in primary and metastatic tumor models were evaluated. A single treatment with E. coli-expressing ClyA significantly decreased tumor growth rates initially (9 days after treatment); however, the tumors tended to grow thereafter. With only radiotherapy (RT; 21 Gy), the tumor growth rates were retarded, but not the tumor sizes. A combination of therapy with E. coli-expressing ClyA and radiation [a total of 5 x 10(7) colony-forming units (CFU) and 21 Gy] resulted in significant tumor shrinkage and even complete disappearance of tumors in mice with tumors derived from murine CT26 colon cancer. Furthermore, treatment with E. coli-expressing ClyA markedly suppressed metastatic tumor growth and prolonged the survival time in mice. The results described here indicate that therapy with engineered E. coli could significantly improve the results of RT, and could exert a striking inhibitory effect on the development of lung metastasis.

Effect of iron on cytolysin A expression in Salmonella enterica serovar Typhi

Previously, a novel protein ClyA (Cytolysin A) has been identified in Escherichia coli K-12, Salmonella enterica serovars Typhi and Paratyphi A and Shigella. Salmonella spp. synthesize substantial amounts of ClyA upon infection of the human host, although the mechanism by which ClyA is induced in vivo is unclear. Since environmental signals could control the expression of virulence determinants, ClyA expression in S. Typhi Ty2 was tested by Western blotting in the presence of normal pooled human serum (NPS). The level of ClyA expression increased in the presence of NPS in a concentration-dependent manner. RPMI 1640 medium similarly induced ClyA expression. ClyA expression was inversely proportional to the concentration of iron in RPMI medium. Therefore, we speculated that iron inhibited the expression of ClyA in S. Typhi Ty2, and free iron depletion may be one of the causes of S. Typhi-mediated induction of ClyA in vivo. Transcription from a clyA-lacZ fusion gene decreased as iron concentration increased, but not as significantly as the ClyA protein expression. It is concluded that the regulatory effect of iron on clyA expression is mainly at translational level.