In vivo covalent cross-linking of cellular actin by the Vibrio cholerae RTX toxin

Vibrio parahaemolyticus disruption of epithelial cell tight junctions occurs independently of toxin production

Vibrio parahaemolyticus is a leading cause of seafood-borne gastroenteritis worldwide. Virulence is commonly associated with the production of two toxins, thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH). Although the majority of clinical isolates produce TDH and/or TRH, clinical samples lacking toxin genes have been identified. In the present study, we investigated the effects of V. parahaemolyticus on transepithelial resistance (TER) and paracellular permeability in Caco-2 cultured epithelial cells. We found that V. parahaemolyticus profoundly disrupts epithelial barrier function in Caco-2 cells and that this disruption occurs independently of toxin production. Clinical isolates with different toxin genotypes all led to a significant decrease in TER, which was accompanied by an increased flux of fluorescent dextran across the Caco-2 monolayer, and profound disruption of actin and the tight junction-associated proteins zonula occludin protein 1 and occludin. Purified TDH, even at concentrations eightfold higher than those produced by the bacteria, had no effect on either TER or paracellular permeability. We used lactate dehydrogenase release as a measure of cytotoxicity and found that this parameter did not correlate with the ability to disrupt tight junctions. As the effect on barrier function occurs independently of toxin production, we used PCR to determine the toxin genotypes of V. parahaemolyticus isolates obtained from both clinical and environmental sources, and we found that 5.6% of the clinical isolates were toxin negative. These data strongly indicate that the effect on tight junctions is not due to TDH and suggest that there are other virulence factors.

Genetics of stress adaptation and virulence in toxigenic Vibrio cholerae

Vibrio cholerae, a Gram-negative bacterium belonging to the gamma-subdivision of the family Proteobacteriaceae is the etiologic agent of cholera, a devastating diarrheal disease which occurs frequently as epidemics. Any bacterial species encountering a broad spectrum of environments during the course of its life cycle is likely to develop complex regulatory systems and stress adaptation mechanisms to best survive in each environment encountered. Toxigenic V. cholerae, which has evolved from environmental nonpathogenic V. cholerae by acquisition of virulence genes, represents a paradigm for this process in that this organism naturally exists in an aquatic environment but infects human beings and cause cholera. The V. cholerae genome, which is comprised of two independent circular mega-replicons, carries the genetic determinants for the bacterium to survive both in an aquatic environment as well as in the human intestinal environment. Pathogenesis of V. cholerae involves coordinated expression of different sets of virulence associated genes, and the synergistic action of their gene products. Although the acquisition of major virulence genes and association between V. cholerae and its human host appears to be recent, and reflects a simple pathogenic strategy, the establishment of a productive infection involves the expression of many more genes that are crucial for survival and adaptation of the bacterium in the host, as well as for its onward transmission and epidemic spread. While a few of the virulence gene clusters involved directly with cholera pathogenesis have been characterized, the potential exists for identification of yet new genes which may influence the stress adaptation, pathogenesis, and epidemiological characteristics of V. cholerae. Coevolution of bacteria and mobile genetic elements (plasmids, transposons, pathogenicity islands, and phages) can determine environmental survival and pathogenic interactions between bacteria and their hosts. Besides horizontal gene transfer mediated by genetic elements and phages, the evolution of pathogenic V. cholerae involves a combination of selection mechanisms both in the host and in the environment. The occurrence of periodic epidemics of cholera in endemic areas appear to enhance this process.

Vibrio cholerae strains with mutations in an atypical type I secretion system accumulate RTX toxin intracellularly

This study shows that the Vibrio cholerae RTX toxin is secreted by a four-component type I secretion system (TISS) encoded by rtxB, rtxD, rtxE, and tolC. ATP-binding site mutations in both RtxB and RtxE blocked secretion, demonstrating that this atypical TISS requires two transport ATPases that may function as a heterodimer.

The Neisseria meningitidis outer membrane lipoprotein FrpD binds the RTX protein FrpC

At conditions of low iron availability, Neisseria meningitidis produces a family of FrpC-like, type I-secreted RTX proteins of unknown role in meningococcal lifestyle. It is shown here that iron starvation also induces production of FrpD, the other protein expressed from a gene located immediately upstream of the frpC gene in a predicted iron-regulated frpDC operon. We found that FrpD is highly conserved in a set of meningococcal strains representative of all serogroups and does not exhibit any similarity to known sequences of other organisms. Subcellular localization and [3H]palmitic acid labeling in Escherichia coli revealed that FrpD is synthesized with a type II signal peptide for export across the cytoplasmic membrane and is, upon processing to a lipoprotein, sorted to the outer bacterial membrane. Furthermore, the biological function of FrpD appears to be linked to that of the RTX protein FrpC, because FrpD was found to bind the amino-proximal portion of FrpC (first 300 residues) with very high affinity (apparent Kd approximately 0.2 nM). These results suggest that FrpD represents an rtx loci-encoded accessory lipoprotein that could be involved in anchoring of the secreted RTX protein to the outer bacterial membrane.

Identification of a domain within the multifunctional Vibrio cholerae RTX toxin that covalently cross-links actin

The Gram-negative pathogen Vibrio cholerae causes diarrheal disease through the export of enterotoxins. The V. cholerae RTX toxin was previously identified and characterized by its ability to round human laryngeal epithelial (HEp-2) cells. Further investigation determined that cell rounding is caused by the depolymerization of actin stress fibers, through the unique mechanism of covalent actin cross-linking. In this study, we identify a domain within the full-length RTX toxin that is capable of mediating the cross-linking reaction when transiently expressed within eukaryotic cells. A structure/function analysis of the actin cross-linking domain (ACD) reveals that a 412-aa, or a 47.8-kDa, region is essential for cross-linking activity. When this domain is deleted from the full-length toxin gene, actin cross-linking, but not cell rounding, is eliminated, indicating that this toxin carries multiple dissociable activities. The ACD shares 59% amino acid identity with a hypothetical protein from V. cholerae, VC1416, and transient expression of the C-terminal domain of VC1416 also results in actin cross-linking in eukaryotic cells. The presence of this second ACD linked to an Rhs-like element suggests that V. cholerae acquired the domain by horizontal gene transfer and the ACD was inserted into the RTX toxin by gene duplication through the evolution of V. cholerae.

The genome sequence of the entomopathogenic bacterium Photorhabdus luminescens

Photorhabdus luminescens is a symbiont of nematodes and a broad-spectrum insect pathogen. The complete genome sequence of strain TT01 is 5,688,987 base pairs (bp) long and contains 4,839 predicted protein-coding genes. Strikingly, it encodes a large number of adhesins, toxins, hemolysins, proteases and lipases, and contains a wide array of antibiotic synthesizing genes. These proteins are likely to play a role in the elimination of competitors, host colonization, invasion and bioconversion of the insect cadaver, making P. luminescens a promising model for the study of symbiosis and host-pathogen interactions. Comparison with the genomes of related bacteria reveals the acquisition of virulence factors by extensive horizontal transfer and provides clues about the evolution of an insect pathogen. Moreover, newly identified insecticidal proteins may be effective alternatives for the control of insect pests.

Bacterial toxins that modify the actin cytoskeleton

Bacterial pathogens utilize several strategies to modulate the organization of the actin cytoskeleton. Some bacterial toxins catalyze the covalent modification of actin or the Rho GTPases, which are involved in the control of the actin cytoskeleton. Other bacteria produce toxins that act as guanine nucleotide exchange factors or GTPase-activating proteins to modulate the nucleotide state of the Rho GTPases. This latter group of toxins provides a temporal modulation of the actin cytoskeleton. A third group of bacterial toxins act as adenylate cyclases, which directly elevate intracellular cAMP to supra-physiological levels. Each class of toxins gives the bacterial pathogen a selective advantage in modulating host cell resistance to infection.

A constitutively active variant of the quorum-sensing regulator LuxO affects protease production and biofilm formation in Vibrio cholerae

Vibrio cholerae normally inhabits aquatic habitats but can cause a severe diarrheal illness in humans. Its arsenal of virulence factors includes a secreted hemagglutinin (HA) protease. An HA protease-deficient mutant of V. cholerae was isolated and designated E7946 mpc. E7946 mpc was found to contain a point mutation in the luxO quorum-sensing regulator. In accordance with this finding, E7946 mpc exhibits a defect in quorum sensing. The mutant luxO allele [luxO(Con)] produces a protein with a leucine-to-glutamine substitution at amino acid 104. Transfer of the luxO(Con) allele to an otherwise wild-type background was sufficient to eliminate HA protease expression; conversely, deletion of luxO(Con) from E7946 mpc restored protease activity. We demonstrate that LuxO(Con) constitutively represses the transcription of hapR, an essential positive regulator of HA protease. Interestingly, strains harboring luxO(Con) form enhanced biofilms, and enhanced biofilm formation does not appear to be dependent on reduced HA protease expression. Taken together, the results confirm the role of LuxO as a central "switch" that coordinately regulates virulence-related phenotypes such as protease production and biofilm formation.

ToxR regulon of Vibrio cholerae and its expression in vibrios shed by cholera patients

Toxigenic Vibrio cholerae cause cholera, a severe diarrheal disease responsible for significant morbidity and mortality worldwide. Two determinants, cholera enterotoxin (CT) and toxin coregulated pilus (TCP) are critical factors responsible for this organism's virulence. The genes for these virulence determinants belong to a network of genes (the ToxR regulon) whose expression is modulated by transcriptional regulators encoded by the toxRS, tcpPH, and toxT genes. To define the ToxR regulon more fully, mutants defective in these regulatory genes were transcriptionally profiled by using V. cholerae genomic microarrays. This study identified 13 genes that were transcriptionally repressed by the toxT mutation (all involved in CT and TCP biogenesis), and 27 and 60 genes that were transcriptionally repressed by the tcpPH and toxRS mutations, respectively. During the course of this analysis, we validated the use of a genomic DNA-based reference sample as a means to standardize and normalize data obtained in different microarray experiments. This method allowed the accurate transcriptional profiling of V. cholerae cells present in stools from cholera patients and the comparison of these profiles to those of wild-type and mutant strains of V. cholerae grown under optimal conditions for CT and TCP expression. Our results suggest that vibrios present in cholera stools carry transcripts for these two virulence determinants, albeit at relatively low levels compared with optimal in vitro conditions. The transcriptional profile of vibrios present in cholera stools also suggests that the bacteria experienced conditions of anaerobiosis, iron limitation, and nutrient deprivation within the human gastrointestinal tract.

Transglutaminases: crosslinking enzymes with pleiotropic functions

Blood coagulation, skin-barrier formation, hardening of the fertilization envelope, extracellular-matrix assembly and other important biological processes are dependent on the rapid generation of covalent crosslinks between proteins. These reactions--which are catalysed by transglutaminases--endow the resulting supramolecular structure with extra rigidity and resistance against proteolytic degradation. Some transglutaminases function as molecular switches in cytoskeletal scaffolding and modulate protein-protein interactions. Having knowledge of these enzymes is essential for understanding the aetiologies of diverse hereditary diseases of the blood and skin, and various autoimmune, inflammatory and degenerative conditions.

The contribution of accessory toxins of Vibrio cholerae O1 El Tor to the proinflammatory response in a murine pulmonary cholera model

The contribution of accessory toxins to the acute inflammatory response to Vibrio cholerae was assessed in a murine pulmonary model. Intranasal administration of an El Tor O1 V. cholerae strain deleted of cholera toxin genes (ctxAB) caused diffuse pneumonia characterized by infiltration of PMNs, tissue damage, and hemorrhage. By contrast, the ctxAB mutant with an additional deletion in the actin-cross-linking repeats-in-toxin (RTX) toxin gene (rtxA) caused a less severe pathology and decreased serum levels of proinflammatory molecules interleukin (IL)-6 and murine macrophage inflammatory protein (MIP)-2. These data suggest that the RTX toxin contributes to the severity of acute inflammatory responses. Deletions within the genes for either hemagglutinin/protease (hapA) or hemolysin (hlyA) did not significantly affect virulence in this model. Compound deletion of ctxAB, hlyA, hapA, and rtxA created strain KFV101, which colonized the lung but induced pulmonary disease with limited inflammation and significantly reduced serum titers of IL-6 and MIP-2. 100% of mice inoculated with KFV101 survive, compared with 20% of mice inoculated with the ctxAB mutant. Thus, the reduced virulence of KFV101 makes it a prototype for multi-toxin deleted vaccine strains that could be used for protection against V. cholerae without the adverse effects of the accessory cholera toxins.

Vibrio cholerae and cholera: out of the water and into the host

The facultative human pathogen Vibrio cholerae can be isolated from estuarine and aquatic environments. V. cholerae is well recognized and extensively studied as the causative agent of the human intestinal disease cholera. In former centuries cholera was a permanent threat even to the highly developed populations of Europe, North America, and the northern part of Asia. Today, cholera still remains a burden mainly for underdeveloped countries, which cannot afford to establish or to maintain necessary hygienic and medical facilities. Especially in these environments, cholera is responsible for significant mortality and economic damage. During the last three decades, intensive research has been undertaken to unravel the virulence properties and to study the epidemiology of this significant human pathogen. More recently, researchers have been elucidating the environmental lifestyle of V. cholerae. This review provides an overview of the current knowledge of both the host- and environment-specific physiological attributes of V. cholerae.

Vibrio cholerae-induced cellular responses of polarized T84 intestinal epithelial cells are dependent on production of cholera toxin and the RTX toxin

To study the utility of in vitro-polarized intestinal cell monolayers for modeling Vibrio cholerae-host cell interactions, we added live V. cholerae bacteria to the apical surfaces of polarized T84 cell monolayers and monitored changes in electrical properties. We found that both classical and El Tor strains produce cholera toxin after addition to the monolayer, but induction is most likely due to medium components rather than bacterium-cell interactions. We also found that the RTX toxin is produced by El Tor strains. This toxin caused a loss of the barrier function of the paracellular tight junction that was measured as a decrease in transepithelial resistance. This decrease occurred when bacteria were added to either the apical or basolateral surfaces, indicating that the RTX toxin receptor is expressed on both surfaces. These results are discussed with regard to the applicability of the polarized T84 cell monolayers as an in vitro model of host-pathogen interactions.

Detection of RTX toxin gene in Vibrio cholerae by PCR

A PCR that amplifies a recently discovered Vibrio cholerae RTX (repeat in toxin) toxin gene was developed. Among 166 clinical and environmental isolates of V. cholerae causing epidemics and sporadic cases of cholera in various parts of the world, all were found to be toxigenic by both PCR and HEp-2 cell cytotoxicity assay. Standard strains of the classical biotype containing a deletion within the gene cluster exhibited negative results by both assays. This is the first rapid genotyping method for differentiation of V. cholerae O1 classical biotype strains from El Tor biotype strains as well as strains of other non-O1 serogroups including serogroup O139. The PCR assay that was developed also specifically detects RTX toxin genes in V. cholerae, as clinical isolates of Vibrio parahaemolyticus, diarrheagenic Escherichia coli, Aeromonas species, and Plesiomonas species were all negative by the RTX toxin-specific PCR as well as the HEp-2 cytotoxicity assay. These findings highlight the characteristics of the RTX toxins in V. cholerae. Their role in the pathogenicity of the bacterium requires further investigation.

Vibrio cholerae tolC is required for bile resistance and colonization

TolC and its homologues are outer membrane proteins that are essential for the transport of many molecules across the cell envelope. In this study we characterized the gene encoding Vibrio cholerae TolC. V. cholerae tolC mutants failed to secrete the RTX cytotoxin, were hypersensitive to antimicrobial agents, and were deficient in intestinal colonization.

Regulation of virulence in Vibrio cholerae

Vibrio cholerae causes the diarrheal disease cholera primarily because it expresses a colonization factor (toxin-coregulated pilus; TCP) and a potent toxin (cholera toxin; CT) within the human intestine. While the true environmental signals that induce CT and TCP expression within the intestine remain unknown, much progress has been made identifying the regulatory factors that modulate their expression. Transcriptional regulation of the genes encoding TCP and CT involves a cascade consisting of a number of regulatory factors located on recently acquired mobile genetic elements as well as others residing within the ancestral Vibrio genome. In vivo studies have revealed interesting differences between the regulation of TCP and CT expression in the laboratory and within the intestine.