Purification and characterization of a hemolysin produced by Vibrio cholerae biotype El Tor: another toxic substance produced by cholera vibrios

Abundance and biofilm formation capability of Vibrio cholerae in aquatic environment with an emphasis on Hilsha fish (Tenualosa ilisha)

The potentially deadly and sporadic diarrhea-causing agent, Vibrio cholerae, is present in a great number in the freshwater aquatic environment and can be transmitted to humans by different aquatic organisms. In the perspective of Bangladesh, an anadromous fish species Hilsha (Tenualosa ilisha) can act as a transmission vehicle of V. cholerae from the aquatic to the household kitchen environment. The present study was carried out to investigate the presence of V. cholerae in the aquatic habitat of Bangladesh with a major emphasis on freshly caught Hilsha fish, along with river water and plankton samples from the fish capture site. The study also detected the biofilm formation capability of V. cholerae within Hilsha fish that might help the transmission and persistence of the pathogen in aquatic habitat. Twenty out of 65 freshly caught fish (30.8%) and 1 out of 15 water samples (6.67%) showed the presence of V. cholerae and none of the plankton samples were positive for V. cholerae. The isolated strains were identified as non-O1 and non-O139 serogroups of V. cholerae and contain some major toxin and virulence genes. A few strains showed cellular cytotoxicity on the HeLa cell line. All strains were able to form biofilm on the microtiter plate and the detection of three genes related to biofilm formation (vpsA, vpsL, and vpsR) were also assayed using qPCR. In this study, the in vitro biofilm formation ability of the isolated strains may indicate the long-term persistence of V. cholerae in different parts of Hilsha fish. The abundance of V. cholerae only in freshly caught Hilsha fish and the absence of the pathogen in the surrounding aquatic environment could stipulate the role of Hilsha fish as one of the major transmission routes of V. cholerae from the freshwater aquatic environment of Bangladesh to the household kitchen environment.

Pore formation-independent cell death induced by a β-barrel pore-forming toxin

Vibrio cholerae cytolysin (VCC) is a β-barrel pore-forming toxin (β-PFT). It exhibits potent hemolytic activity against erythrocytes that appears to be a direct outcome of its pore-forming functionality. However, VCC-mediated cell-killing mechanism is more complicated in the case of nucleated mammalian cells. It induces apoptosis in the target nucleated cells, mechanistic details of which are still unclear. Furthermore, it has never been explored whether the ability of VCC to trigger programmed cell death is stringently dependent on its pore-forming activity. Here, we show that VCC can evoke hallmark features of the caspase-dependent apoptotic cell death even in the absence of the pore-forming ability. Our study demonstrates that VCC mutants with abortive pore-forming hemolytic activity can trigger apoptotic cell death responses and cytotoxicity, similar to those elicited by the wild-type toxin. VCC as well as its pore formation-deficient mutants display prominent propensity to translocate to the target cell mitochondria and cause mitochondrial membrane damage. Therefore, our results for the first time reveal that VCC, despite being an archetypical β-PFT, can kill target nucleated cells independent of its pore-forming functionality. These findings are intriguing for a β-PFT, whose destination is generally expected to remain limited on the target cell membranes, and whose mode of action is commonly attributed to the membrane-damaging pore-forming ability. Taken together, our study provides critical new insights regarding distinct implications of the two important virulence functionalities of VCC for the V. cholerae pathogenesis process: hemolytic activity for iron acquisition and cytotoxicity for tissue damage by the bacteria.

Glu289 residue in the pore-forming motif of Vibrio cholerae cytolysin is important for efficient β-barrel pore formation

Vibrio cholerae cytolysin (VCC) is a potent membrane-damaging β-barrel pore-forming toxin (β-PFT). Upon binding to the target membranes, VCC monomers first assemble into oligomeric pre-pore intermediates, and subsequently transform into transmembrane β-barrel pores. VCC harbors a designated pore-forming motif, which, during oligomeric pore formation, inserts into the membrane and generates a transmembrane β-barrel scaffold. It remains an enigma how the molecular architecture of the pore-forming motif regulates the VCC pore-formation mechanism. Here, we show that a specific pore-forming motif residue, E289, plays crucial regulatory roles in the pore-formation mechanism of VCC. We find that the mutation of E289A drastically compromises pore-forming activity, without affecting the structural integrity and membrane-binding potential of the toxin monomers. Although our single-particle cryo-EM analysis reveals wild type-like oligomeric β-barrel pore formation by E289A-VCC in the membrane, we demonstrate that the mutant shows severely delayed kinetics in terms of pore-forming ability that can be rescued with elevated temperature conditions. We find that the pore-formation efficacy of E289A-VCC appears to be more profoundly dependent on temperature as compared to that of the wild type toxin. Our results suggest that the E289A mutation traps membrane-bound toxin molecules in the pre-pore-like intermediate state that is hindered from converting into the functional β-barrel pores by a large energy barrier, thus highlighting the importance of this residue for the pore-formation mechanism of VCC.

Curcumin Inhibits Membrane-Damaging Pore-Forming Function of the β-Barrel Pore-Forming Toxin Vibrio cholerae Cytolysin

Vibrio cholerae cytolysin (VCC) is a β-barrel pore-forming toxin (β-PFT). Upon encountering the target cells, VCC forms heptameric β-barrel pores and permeabilizes the cell membranes. Structure-function mechanisms of VCC have been extensively studied in the past. However, the existence of any natural inhibitor for VCC has not been reported yet. In the present study, we show that curcumin can compromise the membrane-damaging activity of VCC. Curcumin is known to modulate a wide variety of biological processes and functions. However, the application of curcumin in the physiological scenario often gets limited due to its extremely poor solubility in the aqueous environment. Interestingly, we find that VCC can associate with the insoluble fraction of curcumin in the aqueous medium and thus gets separated from the solution phase. This, in turn, reduces the availability of VCC to attack the target membranes and thus blocks the membrane-damaging action of the toxin. We also observe that the soluble aqueous extract of curcumin, generated by the heat treatment, compromises the pore-forming activity of VCC. Interestingly, in the presence of such soluble extract of curcumin, VCC binds to the target membranes and forms the oligomeric assembly. However, such oligomers appear to be non-functional, devoid of the pore-forming activity. The ability of curcumin to bind to VCC and neutralize its membrane-damaging activity suggests that curcumin has the potential to act as an inhibitor of this potent bacterial β-PFT.

Nonhemolysis of epidemic El Tor biotype strains of Vibrio cholerae is related to multiple functional deficiencies of hemolysin A

Background

Hemolysis of bacteria is an important phenotype used for typing and characterizing strains with specific biomarkers and even a virulence factor in bacterial pathogenesis. In Vibrio cholerae, hemolysin HlyA is responsible for hemolysis of sheep red blood cells, and this hemolytic phenotype is used as a biotyping indicator and considered one of the virulence factors. At the beginning of the seventh cholera pandemic, the El Tor biotype strains of serogroup O1 were distinguished by hemolysis from the sixth pandemic O1 classical biotype strains, whereas during the following epidemics, nonhemolytic El Tor strains appeared, suggesting phenotypic and genetic variations in these strains. This study aimed to investigate the possible mechanisms involved in nonhemolysis of El Tor strains.

Results

Five sequence types of hlyA genes were found in the studied O1 El Tor strains isolated during the seventh pandemic. A 4-base deletion in hlyA caused the HlyA protein mutation and non-hemolytic phenotype. Some strains carry wildtype hlyA genes but are still non-hemolytic, and greatly reduced hlyA transcription and blocked secretion of hemolysin were observed in hemolysis tests of the subcellular components and transcription/expression analysis of hlyA.

Conclusions

Mechanisms responsible for nonhemolysis of the epidemic O1 El Tor strains are complex and not only confined to gene mutation but also deficiencies of transcription and extracellular transport of HlyA. Mutations in gene regulation and protein secretion systems of HlyA in the nonhemolytic V. cholerae strains should be areas of concern in future studies.

Exploring the Genomic Traits of Non-toxigenic Vibrio parahaemolyticus Strains Isolated in Southern Chile

Vibrio parahaemolyticus is the leading cause of seafood-borne gastroenteritis worldwide. As reported in other countries, after the rise and fall of the pandemic strain in Chile, other post-pandemic strains have been associated with clinical cases, including strains lacking the major toxins TDH and TRH. Since the presence or absence of tdh and trh genes has been used for diagnostic purposes and as a proxy of the virulence of V. parahaemolyticus isolates, the understanding of virulence in V. parahaemolyticus strains lacking toxins is essential to detect these strains present in water and marine products to avoid possible food-borne infection. In this study, we characterized the genome of four environmental and two clinical non-toxigenic strains (tdh-, trh-, and T3SS2-). Using whole-genome sequencing, phylogenetic, and comparative genome analysis, we identified the core and pan-genome of V. parahaemolyticus of strains of southern Chile. The phylogenetic tree based on the core genome showed low genetic diversity but the analysis of the pan-genome revealed that all strains harbored genomic islands carrying diverse virulence and fitness factors or prophage-like elements that encode toxins like Zot and RTX. Interestingly, the three strains carrying Zot-like toxin have a different sequence, although the alignment showed some conserved areas with the zot sequence found in V. cholerae. In addition, we identified an unexpected diversity in the genetic architecture of the T3SS1 gene cluster and the presence of the T3SS2 gene cluster in a non-pandemic environmental strain. Our study sheds light on the diversity of V. parahaemolyticus strains from the southern Pacific which increases our current knowledge regarding the global diversity of this organism.

The Relationship between Glycan Binding and Direct Membrane Interactions in Vibrio cholerae Cytolysin, a Channel-forming Toxin

Bacterial pore-forming toxins (PFTs) are structurally diverse pathogen-secreted proteins that form cell-damaging channels in the membranes of host cells. Most PFTs are released as water-soluble monomers that first oligomerize on the membrane before inserting a transmembrane channel. To modulate specificity and increase potency, many PFTs recognize specific cell surface receptors that increase the local toxin concentration on cell membranes, thereby facilitating channel formation. Vibrio cholerae cytolysin (VCC) is a toxin secreted by the human pathogen responsible for pandemic cholera disease and acts as a defensive agent against the host immune system. Although it has been shown that VCC utilizes specific glycan receptors on the cell surface, additional direct contacts with the membrane must also play a role in toxin binding. To better understand the nature of these interactions, we conducted a systematic investigation of the membrane-binding surface of VCC to identify additional membrane interactions important in cell targeting. Through cell-based assays on several human-derived cell lines, we show that VCC is unlikely to utilize high affinity protein receptors as do structurally similar toxins from Staphylococcus aureus. Next, we identified a number of specific amino acid residues that greatly diminish the VCC potency against cells and investigated the interplay between glycan binding and these direct lipid contacts. Finally, we used model membranes to parse the importance of these key residues in lipid and cholesterol binding. Our study provides a complete functional map of the VCC membrane-binding surface and insights into the integration of sugar, lipid, and cholesterol binding interactions.

Transmembrane oligomeric form of Vibrio cholerae cytolysin triggers TLR2/TLR6–dependent proinflammatory responses in monocytes and macrophages

We show that the transmembrane oligomeric form of VCC evokes potent proinflammatory responses in the monocytes and macrophages of the innate immune system. VCC oligomer-induced proinflammatory responses depend critically on the TLR2/TLR6-dependent signalling pathways.

Identification of the target DNA sequence and characterization of DNA binding features of HlyU, and suggestion of a redox switch for hlyA expression in the human pathogen Vibrio cholerae from in silico studies

HlyU, a transcriptional regulator common in many Vibrio species, activates the hemolysin gene hlyA in Vibrio cholerae, the rtxA1 operon in Vibrio vulnificus and the genes of plp-vah1 and rtxACHBDE gene clusters in Vibrio anguillarum. The protein is also proposed to be a potential global virulence regulator for V. cholerae and V. vulnificus. Mechanisms of gene control by HlyU in V. vulnificus and V. anguillarum are reported. However, detailed elucidation of the interaction of HlyU in V. cholerae with its target DNA at the molecular level is not available. Here we report a 17-bp imperfect palindrome sequence, 5′-TAATTCAGACTAAATTA-3′, 173 bp upstream of hlyA promoter, as the binding site of HlyU. This winged helix-turn-helix protein binds necessarily as a dimer with the recognition helices contacting the major grooves and the β-sheet wings, the minor grooves. Such interactions enhance hlyA promoter activity in vivo. Mutations affecting dimerization as well as those in the DNA–protein interface hamper DNA binding and transcription regulation. Molecular dynamic simulations show hydrogen bonding patterns involving residues at the mutation sites and confirmed their importance in DNA binding. On binding to HlyU, DNA deviates by ∼68º from linearity. Dynamics also suggest a possible redox control in HlyU.

Trapping of Vibrio cholerae cytolysin in the membrane-bound monomeric state blocks membrane insertion and functional pore formation by the toxin

Vibrio cholerae cytolysin (VCC) is a potent membrane-damaging cytolytic toxin that belongs to the family of β barrel pore-forming protein toxins. VCC induces lysis of its target eukaryotic cells by forming transmembrane oligomeric β barrel pores. The mechanism of membrane pore formation by VCC follows the overall scheme of the archetypical β barrel pore-forming protein toxin mode of action, in which the water-soluble monomeric form of the toxin first binds to the target cell membrane, then assembles into a prepore oligomeric intermediate, and finally converts into the functional transmembrane oligomeric β barrel pore. However, there exists a vast knowledge gap in our understanding regarding the intricate details of the membrane pore formation process employed by VCC. In particular, the membrane oligomerization and membrane insertion steps of the process have only been described to a limited extent. In this study, we determined the key residues in VCC that are critical to trigger membrane oligomerization of the toxin. Alteration of such key residues traps the toxin in its membrane-bound monomeric state and abrogates subsequent oligomerization, membrane insertion, and functional transmembrane pore-formation events. The results obtained from our study also suggest that the membrane insertion of VCC depends critically on the oligomerization process and that it cannot be initiated in the membrane-bound monomeric form of the toxin. In sum, our study, for the first time, dissects membrane binding from the subsequent oligomerization and membrane insertion steps and, thus, defines the exact sequence of events in the membrane pore formation process by VCC.

Cardiolipin synthetase is involved in antagonistic interaction (reverse CAMP phenomenon) of Mycoplasma species with Staphylococcus aureus beta-hemolysis

Mycoplasma hyorhinis has been implicated in a variety of swine diseases. However, little is known about the hemolytic capabilities of Mycoplasma species in general or M. hyorhinis in particular. In this study, we show that M. hyorhinis possesses beta-hemolytic activity which may be involved in the invasion process. M. hyorhinis also possesses antagonistic cooperativity (reverse CAMP phenomenon) with Staphylococcus aureus beta-hemolysis, resulting in the protection of erythrocytes from the beta-hemolytic activity of S. aureus (reverse CAMP). The reversed CAMP phenomenon has been attributed to phospholipase D (PLD) activity. In silico analysis of the M. hyorhinis genome revealed the absence of the pld gene but the presence of the cls gene encoding cardiolipin synthetase, which contains two PLD active domains. The transformation of Mycoplasma gallisepticum that has neither the cls gene nor the reverse CAMP phenomenon with the cls gene from M. hyorhinis resulted in the reverse CAMP phenomenon, suggesting for the first time that reverse CAMP can be induced by cardiolipin synthetase.

Structural Requirements of Cholesterol for Binding toVibrio choleraeHemolysin

Cholesterol is necessary for the conversion of Vibrio cholerae hemolysin (VCH) monomers into oligomers in liposome membranes. Using different sterols, we determined the stereochemical structures of the VCH-binding active groups present in cholesterol. The VCH monomers are bound to cholesterol, diosgenin, campesterol, and ergosterol, which have a hydroxyl group at position C-3 (3betaOH) in the A ring and a C-C double bond between positions C-5 and C-6 (C-C Delta(5)) in the B ring. They are not bound to epicholesterol and dihydrocholesterol, which form a covalent link with a 3alphaOH group and a C-C single bond between positions C-5 and C-6, respectively. This result suggests that the 3betaOH group and the C-CDelta(5) bond in cholesterol are required for VCH monomer binding. We further examined VCH oligomer binding to cholesterol. However, this oligomer did not bind to cholesterol, suggesting that the disappearance of the cholesterol-binding potential of the VCH oligomer might be a result of the conformational change caused by the conversion of the monomer into the oligomer. VCH oligomer formation was observed in liposomes containing sterols with the 3betaOH group and the C-C Delta(5) bond, and it correlated with the binding affinity of the monomer to each sterol. Therefore, it seems likely that monomer binding to membrane sterol leads to the assembly of the monomer. However, since oligomer formation was induced by liposomes containing either epicholesterol or dihydrocholesterol, the 3betaOH group and the C-C Delta(5) bond were not essential for conversion into the oligomer.

Functional mapping of the lectin activity site on the β-prism domain of vibrio cholerae cytolysin: implications for the membrane pore-formation mechanism of the toxin

Vibrio cholerae cytolysin (VCC) is a prominent member in the family of β-barrel pore-forming toxins. It induces lysis of target eukaryotic cells by forming transmembrane oligomeric β-barrel channels. VCC also exhibits prominent lectin-like activity in interacting with β1-galactosyl-terminated glycoconjugates. Apart from the cytolysin domain, VCC harbors two lectin-like domains: the β-Trefoil and the β-Prism domains; however, precise contribution of these domains in the lectin property of VCC is not known. Also, role(s) of these lectin-like domains in the mode of action of VCC remain obscure. In the present study, we show that the β-Prism domain of VCC acts as the structural scaffold to determine the lectin activity of the protein toward β1-galactosyl-terminated glycoconjugates. Toward exploring the physiological implication of the β-Prism domain, we demonstrate that the presence of the β-Prism domain-mediated lectin activity is crucial for an efficient interaction of the toxin toward the target cells. Our results also suggest that such lectin activity may act to regulate the oligomerization ability of the membrane-bound VCC toxin. Based on the data presented here, and also consistent with the existing structural information, we propose a novel mechanism of regulation imposed by the β-Prism domain's lectin activity, implicated in the process of membrane pore formation by VCC.

Sigma E regulators control hemolytic activity and virulence in a shrimp pathogenic Vibrio harveyi

Members of the genus Vibrio are important marine and aquaculture pathogens. Hemolytic activity has been identified as a virulence factor in many pathogenic vibrios including V. cholerae, V. parahaemolyticus, V. alginolyticus, V. harveyi and V. vulnificus. We have used transposon mutagenesis to identify genes involved in the hemolytic activity of shrimp-pathogenic V. harveyi strain PSU3316. Out of 1,764 mutants screened, five mutants showed reduced hemolytic activity on sheep blood agar and exhibited virulence attenuation in shrimp (Litopenaeus vannamei). Mutants were identified by comparing transposon junction sequences to a draft of assembly of the PSU3316 genome. Surprisingly none of the disrupted open reading frames or gene neighborhoods contained genes annotated as hemolysins. The gene encoding RseB, a negative regulator of the sigma factor (σ(E)), was interrupted in 2 out of 5 transposon mutants, in addition, the transcription factor CytR, a threonine synthetase, and an efflux-associated cytoplasmic protein were also identified. Knockout mutations introduced into the rpoE operon at the rseB gene exhibited low hemolytic activity in sheep blood agar, and were 3-to 7-fold attenuated for colonization in shrimp. Comparison of whole cell extracted proteins in the rseB mutant (PSU4030) to the wild-type by 2-D gel electrophoresis revealed 6 differentially expressed proteins, including two down-regulated porins (OmpC-like and OmpN) and an upregulated protease (DegQ) which have been associated with σ(E) in other organisms. Our study is the first report linking hemolytic activity to the σ(E) regulators in pathogenic Vibrio species and suggests expression of this virulence-linked phenotype is governed by multiple regulatory pathways within the V. harveyi.

Cholesterol specificity of some heptameric beta-barrel pore-forming bacterial toxins: structural and functional aspects

Apart from the thiol-specific/cholesterol-dependent cytolysin family of toxins (see Chapter 20) there are a number of other unrelated bacterial toxins that also have an affinity for plasma membrane cholesterol. Emphasis is given here on the Vibrio cholerae cytolysin (VCC) and the cytolysins from related Vibrio species. The inhibition of the cytolytic activity of these toxins by prior incubation with extracellular cholesterol or low density lipoprotein emerges as a unifying feature, as does plasma membrane cholesterol depletion. Incubation of VCC with cholesterol produces a heptameric oligomer, which is not equivalent to the pre-pore since it is unable to penetrate the plasma membrane. In structural terms, the precise sequence of VCC monomer binding to membrane, oligomer formation and pore insertion through the bilayer has yet to be fully defined. Several other bacterial toxins have a dependency for cholesterol, although the available data is limited in most cases.

Vibrio cholerae cytolysin causes an inflammatory response in human intestinal epithelial cells that is modulated by the PrtV protease

BACKGROUND

Vibrio cholerae is the causal intestinal pathogen of the diarrheal disease cholera. It secretes the protease PrtV, which protects the bacterium from invertebrate predators but reduces the ability of Vibrio-secreted factor(s) to induce interleukin-8 (IL-8) production by human intestinal epithelial cells. The aim was to identify the secreted component(s) of V. cholerae that induces an epithelial inflammatory response and to define whether it is a substrate for PrtV.

METHODOLOGY/PRINCIPAL FINDINGS

Culture supernatants of wild type V. cholerae O1 strain C6706, its derivatives and pure V. cholerae cytolysin (VCC) were analyzed for the capacity to induce changes in cytokine mRNA expression levels, IL-8 and tumor necrosis factor-alpha (TNF-alpha) secretion, permeability and cell viability when added to the apical side of polarized tight monolayer T84 cells used as an in vitro model for human intestinal epithelium. Culture supernatants were also analyzed for hemolytic activity and for the presence of PrtV and VCC by immunoblot analysis.

CONCLUSIONS/SIGNIFICANCE

We suggest that VCC is capable of causing an inflammatory response characterized by increased permeability and production of IL-8 and TNF-alpha in tight monolayers. Pure VCC at a concentration of 160 ng/ml caused an inflammatory response that reached the magnitude of that caused by Vibrio-secreted factors, while higher concentrations caused epithelial cell death. The inflammatory response was totally abolished by treatment with PrtV. The findings suggest that low doses of VCC initiate a local immune defense reaction while high doses lead to intestinal epithelial lesions. Furthermore, VCC is indeed a substrate for PrtV and PrtV seems to execute an environment-dependent modulation of the activity of VCC that may be the cause of V. cholerae reactogenicity.

Necrotizing soft-tissue infections and primary sepsis caused by Vibrio vulnificus and Vibrio cholerae non-O1

BACKGROUND

Vibrio species are a rare cause of necrotizing soft-tissue infections and primary septicemia, which are likely to occur in patients with hepatic disease, diabetes, adrenal insufficiency, and immunocompromised conditions. These organisms thrive in warm seawater and are often present in raw oysters, shellfish, and other seafood. This study examined fulminating clinical characteristics of Vibrio vulnificus and Vibrio cholerae non-O1 soft-tissue infections and identified outcome predictors.

MATERIALS

Thirty patients with necrotizing fasciitis and sepsis caused by Vibrio species were retrospectively reviewed. Twenty-eight patients had a history of contact with seawater or raw seafood. Eight patients had hepatic disease such as hepatitis or liver cirrhosis, and seven patients had diabetes mellitus. Nine patients had hepatic dysfunction combined with diabetes mellitus. Microbiology laboratory culture studies confirmed V. vulnificus in 23 patients and V. cholerae non-O1 in seven patients.

RESULTS

Surgical debridement or immediate limb amputation was initially performed in all patients with necrotizing soft-tissue infections. Eleven patients (37%) died within several days of admission and 19 survived. The mortality of V. cholerae non-O1 group (57%) is higher than that of the V. vulnificus group (30%). A significantly higher mortality rate was noted in patients with initial presentations of a systolic blood pressure of < or =90 mm Hg, leukopenia, decreased platelet counts, and a combination of hepatic dysfunction and diabetes mellitus.

CONCLUSIONS

Vibrio necrotizing soft-tissue infections should be suspected in patients with appropriate clinical findings and history of contact with seawater or seafood. V. cholerae non-O1 may cause bacteremia more often than V. vulnificus in patients with liver cirrhosis. Early fasciotomy and culture-directed antimicrobial therapy are aggressively recommended in patients with hypotensive shock, leukopenia, high band forms of white blood cells, decreased platelet counts, severe hypoalbuminemia, and underlying chronic illness, such as hepatic dysfunction and diabetes mellitus.

Incidence, virulence factors, and clonality among clinical strains of non-O1, non-O139 Vibrio cholerae isolates from hospitalized diarrheal patients in Kolkata, India

The incidence of Vibrio cholerae non-O1, non-O139 strains from hospitalized patients with acute diarrhea constituted 27.4% (n = 54) of the total 197 V. cholerae strains isolated from patients in Kolkata, India, in 2003. Of 197 strains, 135 were identified as O1 serotype Ogawa and 2 were identified as O139. In the same time period, six O1 background rough strains that possessed all known virulence factors were identified. Serotype analysis of the non-O1, non-O139 strains placed 42 strains into 19 serogroups, while 12 remained O nontypeable (ONT); the existing serotyping scheme involved antisera to 206 serogroups. Detection of a good number of ONT strains suggested that additional serogroups have arisen that need to be added to the current serotyping scheme. The non-O1, non-O139 strains were nontoxigenic except for an O36 strain (SC124), which regulated expression of cholera toxin as O1 classical strains did. Additionally, strain SC124 carried alleles of tcpA and toxT that were different from those of the O1 counterpart, and these were also found in five clonally related strains belonging to different serogroups. Strains carrying tcpA exhibited higher colonization in an animal model compared to those lacking tcpA. PCR-based analyses revealed remarkable variations in the distribution of other virulence factors, including hlyA, rtxA, Vibrio seventh pandemic island I (VSP-I), VSP-II, and type III secretion system (TTSS). Most strains contained hlyA (87%) and rtxA (81.5%) and secreted cytotoxic factors when grown in vitro. Approximately one-third of the strains (31.5%) contained the TTSS gene cluster, and most of these strains were more motile and hemolytic against rabbit erythrocytes. Partial nucleotide sequence analysis of the TTSS-containing strains revealed silent nucleotide mutations within vcsN2 (type III secretion cytoplasmic ATPase), indicating functional conservation of the TTSS apparatus.

Vaccine development for the control of cholera and related toxin-induced diarrhoeal diseases

The toxin-induced diarrhoeal diseases in greatest need of effective vaccines for use in control programmes are cholera and diarrhoea due to enterotoxigenic Escherichia coli. Such vaccines, whether consisting of inactivated immunogens or live attenuated organisms, should be administered by the oral route to stimulate the gut mucosal immune system to a maximal extent. For optimal efficacy they should probably contain or produce immunogens evoking both antibacterial and antitoxic immunity that can interfere in a synergistically cooperative manner with colonization as well as toxin action (binding) events in the pathogenesis. The actual or predicted advantages and limitations of oral vaccines based on protective antigen cocktails and different approaches to live, attenuated organisms are discussed. A conclusion is that effective vaccines could play an important role in the control of diarrhoeal disease by reducing mortality and morbidity, and ideally also the transmission of disease.

LuxO controls extracellular protease, haemolytic activities and siderophore production in fish pathogen Vibrio alginolyticus

AIMS

To characterize the luxO gene in fish pathogen Vibrio alginolyticus MVP01 and investigate its roles in regulation of extracellular products (ECP) and siderophore production.

METHODS AND RESULTS

The luxO gene was cloned from V. alginolyticus MVP01. Genetic analysis revealed that it encoded a protein with high similarity to other LuxO homologues. The luxO in-frame deletion mutant and rpoN null mutant were constructed with suicide plasmids. We demonstrated that sole deletion in LuxO increased the secretion of extracellular protease and haemolytic products, but decreased siderophore production for V. alginolyticus MVP01. Mutants with null rpoN displayed significantly enhanced protease level and siderophore production while notable reduction in haemolytic activities of ECP.

CONCLUSIONS

Vibrio alginolyticus harbours functional luxO gene that regulates the secretion of extracellular protease and haemolytic materials as well as siderophore production in either sigma(54) dependent or independent manners.

SIGNIFICANCE AND IMPACT OF THE STUDY

The current study demonstrated that V. alginolyticus MVP01 produces extracellular protease and haemolytic activity material as well as siderophore, which may be characteristics of the virulence of the strain. Revelations that secretion of these products is under the regulation of LuxO and sigma(54) as well as the potential quorum sensing systems in V. alginolyticus MVP01 will expedite the understanding of vibriosis pathogenesis.

Pro-inflammatory feedback activation cycle evoked by attack of Vibrio cholerae cytolysin on human neutrophil granulocytes

Vibrio cholerae cytolysin (VCC) is a pore-forming toxin that is secreted in precursor form (pro-VCC) and requires proteolytic cleavage in order to attain membrane-permeabilizing properties. Pro-VCC can be activated both in solution and membrane-bound state. Processing of membrane-bound pro-VCC can in turn be achieved through the action of both cell-associated and soluble proteases. The current investigation describes the interaction of VCC with human neutrophil granulocytes. It is shown that pro-VCC binds to these cells and is cleaved by cell-bound serine proteases. Membrane permeabilization leads to granulocyte activation, as witnessed by the generation of reactive oxygen metabolites and liberation of granule constituents. A mutant toxin with unaltered binding properties but devoid of pore-forming activity did not elicit these effects. The secreted proteases cleave and activate further bound- and non-bound pro-VCC. A positive feedback loop is thus created that results in enhanced cytotoxicity towards both the targeted granulocytes and towards bystander cells that are not primarily killed by the protoxin. Thus, activation of neutrophil granulocytes by VCC fuels a positive feedback cycle that will cripple immune defence, augment inflammation, and enhance the cytotoxic action of the toxin on neighbouring tissue cells.

Hemolysin and the multifunctional autoprocessing RTX toxin are virulence factors during intestinal infection of mice with Vibrio cholerae El Tor O1 strains

The seventh cholera pandemic that started in 1961 was caused by Vibrio cholerae O1 strains of the El Tor biotype. These strains produce the pore-forming toxin hemolysin, a characteristic used clinically to distinguish classical and El Tor biotypes. Even though extensive in vitro data on the cytolytic activities of hemolysin exist, the connection of hemolysin to virulence in vivo is not well characterized. To study the contribution of hemolysin and other accessory toxins to pathogenesis, we utilized the model of intestinal infection in adult mice sensitive to the actions of accessory toxins. In this study, we showed that 4- to 6-week-old streptomycin-fed C57BL/6 mice were susceptible to intestinal infection with El Tor strains, which caused rapid death at high doses. Hemolysin had the predominant role in lethality, with a secondary contribution by the multifunctional autoprocessing RTX (MARTX) toxin. Cholera toxin and hemagglutinin/protease did not contribute to lethality in this model. Rapid death was not caused by increased dissemination due to a damaged epithelium since the numbers of CFU recovered from spleens and livers 6 h after infection did not differ between mice inoculated with hemolysin-expressing strains and those infected with non-hemolysin-expressing strains. Although accessory toxins were linked to virulence, a strain defective in the production of accessory toxins was still immunogenic since mice immunized with a multitoxin-deficient strain were protected from a subsequent lethal challenge with the wild type. These data suggest that hemolysin and MARTX toxin contribute to vaccine reactogenicity but that the genes for these toxins can be deleted from vaccine strains without affecting vaccine efficacy.

Antibacterial, antisecretory and antihemorrhagic activity of Azadirachta indica used to treat cholera and diarrhea in India

Indigenous uses of Azadirachta indica A. juss (Maliaceae) (locally known as neem) leaves in different parts of India for curing gastrointestinal disorder such as diarrhea and cholera is wide spread. The objective of the present study was to evaluate the antibacterial and antisecretory activity of neem extract against Vibrio cholerae, a causative agent of watery diarrhea such as cholera. The methanol extract of neem leaf was tested for its antibacterial, antisecretory and antihemorrhagic activity against Vibrio cholerae. Azadirachta indica extract had significant antibacterial activity against the multi-drug-resistant Vibrio cholerae of serotypes O1, O139 and non-O1, non-O139. The minimum inhibitory concentration reached by 50% (MIC50) and 90% (MIC90), and minimum bactericidal concentration for the extract were 2.5, > 5, and 10 mg/ml, respectively. Neem extract showed antisecretory activity on Vibrio cholerae induced fluid secretion in mouse intestine with inhibition values of 27.7%, 41.1%, 43.3%, 57.0%, and 77.9% at doses of 100, 200, 300, 450 and 1800 mg/kg, respectively. Oral administration of the extract inhibited hemorrhage induced by Vibrio cholerae in mouse intestine at a dose > or = 300 mg/kg. The results obtained in this study give some scientific support to the uses of neem employed by the indigenous people in India employed for the treatment of diarrhea and dreadful disease cholera.

Protective role of autophagy against Vibrio cholerae cytolysin, a pore-forming toxin from V. cholerae

Autophagy is the unique, regulated mechanism for the degradation of organelles. This intracellular process acts as a prosurvival pathway during cell starvation or stress and is also involved in cellular response against specific bacterial infections. Vibrio cholerae is a noninvasive intestinal pathogen that has been studied extensively as the causative agent of the human disease cholera. V. cholerae illness is produced primarily through the expression of a potent toxin (cholera toxin) within the human intestine. Besides cholera toxin, this bacterium secretes a hemolytic exotoxin termed V. cholerae cytolysin (VCC) that causes extensive vacuolation in epithelial cells. In this work, we explored the relationship between the vacuolation caused by VCC and the autophagic pathway. Treatment of cells with VCC increased the punctate distribution of LC3, a feature indicative of autophagosome formation. Moreover, VCC-induced vacuoles colocalized with LC3 in several cell lines, including human intestinal Caco-2 cells, indicating the interaction of the large vacuoles with autophagic vesicles. Electron microscopy analysis confirmed that the vacuoles caused by VCC presented hallmarks of autophagosomes. Additionally, biochemical evidence demonstrated the degradative nature of the VCC-generated vacuoles. Interestingly, autophagy inhibition resulted in decreased survival of Caco-2 cells upon VCC intoxication. Also, VCC failed to induce vacuolization in Atg5-/- cells, and the survival response of these cells against the toxin was dramatically impaired. These results demonstrate that autophagy acts as a cellular defense pathway against secreted bacterial toxins.

Pore formation by Vibrio cholerae cytolysin requires cholesterol in both monolayers of the target membrane

Vibrio cholerae cytolysin (VCC) forms oligomeric transmembrane pores in cholesterol-rich membranes. To better understand this process, we used planar bilayer membranes. In symmetric membranes, the rate of the channel formation by VCC has a superlinear dependency on the cholesterol membrane fraction. Thus, more than one cholesterol molecule can facilitate VCC-pore formation. In asymmetric membranes, the rate of pore formation is limited by the leaflet with the lower cholesterol content. Methyl-beta-cyclodextrin, which removes cholesterol from membranes, rapidly inhibits VCC pore formation, even when it is added to the side opposite that of VCC addition. The results suggest that cholesterol in both membrane leaflets aid VCC-pore formation and that either leaflet can function as a kinetic bottleneck with respect to the rate of pore-formation.

A molecular model of the Vibrio cholerae cytolysin transmembrane pore

The cytotoxic activity of some of the most pathogenic strains of Vibrio cholerae is associated with a cytolysin protein (VCC), which forms oligomeric transmembrane pores and changes the permeability of intestinal cells. We present here a model structure of the transmembrane pore of VCC based on sequence comparison with other pore-forming toxins. VCC is suggested to form a transmembrane beta-barrel pore with a relatively large trans vestibule region. Calculations of the electrostatic profile within the pore lumen provide a rationale for the low conductance and selectivity of the VCC ion channel.

The role of bacterial and non-bacterial toxins in the induction of changes in membrane transport: implications for diarrhea

Bacterial toxins induce changes in membrane transport which underlie the loss of electrolyte homeostasis associated with diarrhea. Bacterial- and their secreted toxin-types which have been linked with diarrhea include: (a) Vibrio cholerae (cholera toxin, E1 Tor hemolysin and accessory cholera enterotoxin); (b) Escherichia coli (heat stable enterotoxin, heat-labile enterotoxin and colicins); (c) Shigella dysenteriae (shiga-toxin); (d) Clostridium perfringens (C. perfringens enterotoxin, alpha-toxin, beta-toxin and theta-toxin); (e) Clostridium difficile (toxins A and B); (f) Staphylococcus aureus (alpha-haemolysin); (g) Bacillus cereus (cytotoxin K and haemolysin BL); and (h) Aeromonas hydrophila (aerolysin, heat labile cytotoxins and heat stable cytotoxins). The mechanisms of toxin-induced diarrhea include: (a) direct effects on ion transport in intestinal epithelial cells, i.e. direct toxin interaction with intrinsic ion channels in the membrane and (b) indirect interaction with ion transport in intestinal epithelial cells mediated by toxin binding to a membrane receptor. These effects consequently cause the release of second messengers, e.g. the release of adenosine 3',5'-cyclic monophosphate/guanosine 3',5'-monophosphate, IP(3), Ca2+ and/or changes in second messengers that are the result of toxin-formed Ca2+ and K+ permeable channels, which increase Ca2+ flux and augment changes in Ca2+ homeostasis and cause depolarisation of the membrane potential. Consequently, many voltage-dependent ion transport systems, e.g. voltage-dependent Ca2+ influx, are affected. The toxin-formed ion channels may act as a pathway for loss of fluid and electrolytes. Although most of the diarrhea-causing toxins have been reported to act via cation and anion channel formation, the properties of these channels have not been well studied, and the available biophysical properties that are needed for the characterization of these channels are inadequate.

ompU genes in non-toxigenic Vibrio cholerae associated with aquaculture

AIMS

The study was undertaken with the objective of understanding the virulence-associated genes of the CTX and TCP gene clusters in environmental isolates of Vibrio cholerae, an important human pathogen, isolated from the aquaculture environment. The involvement of the ompU gene in conferring bile resistance in these isolates was also evaluated.

METHODS AND RESULTS

The V. cholerae isolates were tested by PCR and fluorescent antibody test for O1 (Ogawa and Inaba) and O139 serotypes. All isolates were found to be non-toxigenic V. cholerae confirmed by their positive PCR reaction for toxR but negative for ctx, zot and tcp gene. The hlyA gene was detected in 85% of the strains and ompU in 77%. The results on the bactericidal effect of bile salts suggest that ompU may play a role in conferring bile resistance in non-O1/non-O139 strains.

CONCLUSION

The results of the study indicate that most environmental strains lacked the CTX and TCP gene clusters. However, most isolates had the hlyA gene indicating the potential of these environmental strains to cause mild gastroenteritis. It was also observed that strains lacking ompU showed less tolerance to bile salts.

SIGNIFICANCE AND IMPACT OF THE STUDY

Information on virulence factors of V. cholerae associated with aquaculture environment and products would be of value in risk assessment for human health.

Purification and characterization of enterotoxigenic El Tor-like hemolysin produced by Vibrio fluvialis

The halophilic bacterium Vibrio fluvialis is an enteric pathogen that produces an extracellular hemolysin. This hemolysin was purified to homogeneity by using sequential hydrophobic-interaction chromatography with phenyl-Sepharose CL-4B and gel filtration with Sephacryl S-200. It has a molecular weight of 63,000 and an isoelectric point of 4.6, and its hemolytic activity is sensitive to heat, proteases, and preincubation with zinc ions. The hemolysin lyses erythrocytes of the eight different animal species that we tested, is cytotoxic against Chinese hamster ovary cells in tissue culture, and elicits fluid accumulation in suckling mice. Lysis of erythrocytes occurs by a temperature-dependent binding step followed by a temperature- and pH-dependent lytic step. Fourteen of the first 20 N-terminal amino acid residues (Val-Ser-Gly-Gly-Glu-Ala-Asn-Thr-Leu-Pro-His-Val-Ala-Phe-Tyr-Ile-Asn-Val-Asn-Arg) are identical to those of the El Tor hemolysin of Vibrio cholerae and the heat-labile hemolysin of Vibrio mimicus. This homology was further confirmed by PCR analysis using a 5' primer derived from the amino-terminal sequence of the hemolysin and a 3' primer derived from the El Tor hemolysin structural gene. The hemolysin also reacts with antibodies to the El Tor-like hemolysin of non-O1 V. cholerae.

Genetic diversity of Vibrio cholerae O1 in Argentina and emergence of a new variant

The genetic diversity of Vibrio cholerae O1 strains from Argentina was estimated by random amplified polymorphic DNA (RAPD) analysis and pulsed-field gel electrophoresis (PFGE). Twenty-nine isolates carrying the virulence genes ctxA, zot, ace, and tcpA appeared to represent a single clone by both typing methods; while 11 strains lacking these virulence genes exhibited several heterogeneous RAPD and PFGE patterns. Among the last group, a set of isolates from the province Tucumán showed a single RAPD pattern and four closely related PFGE profiles. These strains, isolated from patients with diarrhea, did not produce the major V. cholerae O1 virulence determinants, yet cell supernatants of these isolates caused a heat-labile cytotoxic effect on Vero and Y-1 cells and elicited significant variations on the water flux and short-circuit current in human small intestine mounted in an Ussing chamber. All these effects were completely abolished by incubation with a specific antiserum against El Tor hemolysin, suggesting that this virulence factor was responsible for the toxic activity on both the epithelial cells and the small intestine specimens and may hence be involved in the development of diarrhea. We propose "Tucumán variant" as the designation for this new cluster of cholera toxin-negative V. cholerae O1 strains.

Plasmid-mediated iron uptake and virulence in Vibrio anguillarum

The plasmid pJM1 of Vibrio anguillarum harbors genes encoding proteins that enable the bacterial cell to survive under iron limiting conditions. A subset of these proteins are involved in the biosynthesis of the siderophore anguibactin and in the internalization of the ferric-siderophore into the cell cytosol. We have identified several genes encoding non-ribosomal peptide synthetases that catalyze the synthesis of anguibactin, these genes are: angB/G, angM, angN, angR, and angT. In addition, the genes fatA, fatB, fatC, and fatD are involved in the transport of ferric-anguibactin complexes. These transport genes, together with the biosynthesis genes angR and angT, are included in the iron transport biosynthesis operon (ITBO). Both the biosynthesis and the transport genes are under tight positive as well as negative control. We have identified four regulators; two of them, a chromosomally encoded Fur and a plasmid-mediated antisense RNA, RNAbeta, act in a negative fashion, while positive regulation is facilitated by AngR and TAFr. We also have evidence that the siderophore itself plays a positive role in the regulatory mechanism of the expression of both transport and biosynthesis genes.

The Vibrio cholerae haemolysin anion channel is required for cell vacuolation and death

Several strains of Vibrio cholerae secrete a haemolytic toxin of 63 kDa, termed V. cholerae cytolysin (VCC). This toxin causes extensive vacuolation and death of cells in culture and forms an anion-selective channel in planar lipid bilayers and in cells. Here, we identify inhibitors of the VCC anion channel and show that the formation of the anion channel is necessary for the development of the vacuoles and for the cell death induced by this toxin. Using markers of cell organelles, we show that vacuoles derive from different intracellular compartments and we identify the contribution of late endosomes and of the trans-Golgi network in vacuole biogenesis.

Cloning, expression and characterization of the CHO cell elongating factor (Cef) from Vibrio cholerae O1

CHO cell-elongating factor (Cef) is a recently identified putative virulence factor of Vibrio cholerae. Our previous studies show that this 85 kDa protein elongates CHO cells, causes fluid accumulation in suckling mice and has esterase activity. In this study, the cef gene was cloned in Escherichia coli using a yeast vector and subsequently expressed in the yeast Pichia pastoris. The cef genes from V. cholerae candidate vaccine strains JBK 70 and CVD 103-HgR were sequenced and found to be nearly identical (100 and 99.9% respectively) with an open reading frame (ORF) from the published sequence of V. cholerae N16961. Cloned toxin was purified to homogeneity in 3 steps using anion exchange, hydrophobic interaction and gel filtration chromatography. The size of cloned Cef on SDS-PAGE gels was 114 kDa. The increased size was probably due to glycosylation by the yeast since cloned protein reacted strongly with a glycoprotein stain. The cloned protein could not be directly sequenced, but when treated with trypsin, yielded a protein fragment with an amino acid sequence that matched the sequence predicted for the Cef protein. The purified cloned protein had esterase and CHO cell activity, but no suckling mouse activity.

Hemolytic and hemoxidative activities in Mycoplasma penetrans

Mycoplasma penetrans is a newly isolated Mollicute from the urine of patients infected with human immunodeficiency virus that demonstrates the capacity to adhere to and invade human cells. A previous report, based on assays with mouse red blood cells (RBCs), indicated that M. penetrans lacked hemolytic activity. In our studies, we incubated different isolates of M. penetrans with various RBC species and observed hemolytic zones surrounding individual mycoplasma colonies. All M. penetrans strains displayed hemolysis after 2 to 3 days of incubation. Hemolytic activity diffused from single colonies, eventually causing complete lysis. Hemolysis was most pronounced with sheep RBCs, followed by horse, chicken, and human cells. Furthermore, hemolytic activity was demonstrable in both intact mycoplasma cell preparations and spent culture supernatant. However, unlike intact mycoplasmas, the hemolytic activity in the supernatant was dependent on the reducing agent, cysteine. In addition to hemolysis, a brown precipitate was closely associated with mycoplasma colonies, suggesting oxidation of hemoglobin. Absorption spectra indicated that hemoglobin was oxidized to methemoglobin, and the addition of catalase demonstrated H(2)O(2)-mediated hemoxidation. Other experiments suggested that hemoxidation enhanced total hemolysis, providing the first evidence of both hemolytic and hemoxidative activities in M. penetrans.

Purification and characterization of a cytotonic protein expressed In vitro by the live cholera vaccine candidate CVD 103-HgR

Cholera vaccines developed by the deletion of CTX genes from Vibrio cholerae induce a residual reactogenicity in up to 10% of vaccinees. A novel cytotonic agent named secreted CHO cell elongating protein (S-CEP) was purified from culture supernatants of CVD 103-HgR (Levine et al., Lancet ii:467-470, 1988). Five fractionation steps yielded electrophoretically pure S-CEP with an M(r) of 79,000. A partially purified preparation caused fluid accumulation in the sealed infant mouse model. The amino terminus bore a unique sequence with strong homology to a cytotonic toxin of El Tor V. cholerae.

Identification of a CHO cell-elongating factor produced by Vibrio cholerae O1

Vibrio cholerae strains with all known toxin genes deleted or inactivated still cause diarrhoea in some volunteers, suggesting the presence of an unknown virulence factor or factors. Lysozyme-EDTA treated cells of JBK70, a genetically manipulated cholera toxin negative strain of Vibrio cholerae O1, biotype El Tor, release a factor that causes elongation of Chinese hamster ovary (CHO) cells. CHO cell-elongating toxin (Cef) was purified by FPLC chromatography (anion exchange; Q Sepharose High Performance) followed by 2D electrophoresis (isoelectric focusing gel, IEF; pH 3-9 and SDS-PAGE, 8-25% gradient gel). Partly purified toxin (anion exchange or IEF-eluted concentrate) caused fluid accumulation in sealed infant mice suggesting that Cef shows some properties of an enterotoxin. On SDS-PAGE (8-25%) and IEF (pH 2.5-5.0) gels, CHO cell activity was associated with a single band at 85 kDa and a pI of 3.8, respectively. A unique amino terminal sequence, XGDETNSSGASTEVVYESYIQQ, was determined by automated Edman degradation of gel-purified protein. The unique molecular mass, N-terminal sequence and activity on CHO cells indicate that this factor is not zonula occludens toxin (Zot) or accessory cholera enterotoxin (Ace) or the Hly A haemolysin. Partly purified Cef did not increase cyclic AMP or prostaglandin E(2)levels in CHO cells which suggests that its mechanism of action differs from that of cholera toxin.

Cytotoxic cell vacuolating activity from Vibrio cholerae hemolysin

A Vibrio cholerae cytotoxin, designated VcVac, was found to cause vacuolation in Vero cells. It was originally detected in the pathogenic O1 Amazonia variant of V. cholerae and later shown to be produced in environmental strains and some El Tor strains. Comparison of VcVac production in various strains suggested that hemolysin was responsible for the vacuolating phenotype. Genetic experiments established a firm correlation between vacuolation and hemolysin production. The mammalian cell vacuolating activity of the V. cholerae hemolysin is a new property of this protein and points to a previously unknown type of interaction between V. cholerae and its host.

Purification and characterization of novel toxin produced by Vibrio cholerae O1

Vibrio cholerae WO7 (serogroup O1) isolated from patients with diarrhea produces an extracellular toxin despite the absence of ctx, zot, and ace genes from its genome. The toxin elongates Chinese hamster ovary cells, produces fluid accumulation in ligated rabbit ileal loops, and agglutinates freshly isolated rabbit erythrocytes. Maximal production of this toxin (WO7 toxin) was seen in AKI medium with the pH adjusted to 8.5 at 37 degrees C under shaking conditions. We purified this toxin to homogeneity by sequential ammonium sulfate precipitation, affinity chromatography using a fetuin-Sepharose CL-4B column, and gel filtration chromatography, which increased the specific activity of the toxin by 1.6 x 10(6)-fold. The toxin is heat labile and sensitive to proteases and has a subunit structure consisting of two subunits with molecular masses of about 58 and 40 kDa as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Agglutination of GM1-coated sheep erythrocytes by toxin suggests that GM1 might be the physiologic receptor for WO7 toxin on the enterocytes. An immunodiffusion test between the antiserum raised against the purified WO7 toxin and the purified toxin gave a well-defined precipitation band. In the immunoblot assay, two bands were observed in the 58- and 40-kDa region. At the same time, antiserum against WO7 toxin failed to show any cross-reactivity with cholera toxin or Escherichia coli heat-labile toxin (LT1) in an immunodiffusion test or immunoblot assay. The enterotoxic activity of WO7 toxin could be inhibited by antiserum against purified WO7 toxin. Our results indicate that WO7 toxin is structurally and functionally distinct from other cholera toxins and that the enterotoxic activities expressed by WO7 toxin appear to contribute to the pathogenesis of disease associated with V. cholerae O1 strains.

Analysis of receptor for Vibrio cholerae El tor hemolysin with a monoclonal antibody that recognizes glycophorin B of human erythrocyte membrane

El Tor hemolysin (ETH), a pore-forming toxin secreted by Vibrio cholerae O1 biotype El Tor and most Vibrio cholerae non-O1 isolates, is able to lyse erythrocytes and other mammalian cells. To study the receptor for this toxin or the related molecule(s) on erythrocyte, we first isolated a monoclonal antibody, B1, against human erythrocyte membrane, which not only blocks the binding of ETH to human erythrocyte but also inhibits the hemolytic activity of ETH. Biochemical characterization and immunoblotting revealed that this antibody recognized an epitope on the extracellular domain of glycophorin B, a sialoglycoprotein of erythrocyte membrane. Erythrocytes lacking glycophorin B but not glycophorin A were less sensitive to the toxin than were normal human erythrocytes. These results indicate that glycophorin B is a receptor for ETH or at least an associated molecule of the receptor for ETH on human erythrocytes.

Disappearance of glyceraldehyde 3-phosphate dehydrogenase from erythrocyte membrane by hemolysis with thermostable direct hemolysin of Vibrio parahaemolyticus or Vibrio cholerae El Tor hemolysin

It is believed that the thermostable direct hemolysin (TDH) of Vibrio parahaemolyticus and El Tor hemolysin (ETH) of Vibrio cholerae damage erythrocytes and other cells by acting as pore-forming toxins. In this study, we found that a protein band with a molecular weight of 37,000 daltons specifically disappeared from erythrocyte membrane after hemolysis by TDH and ETH, but not after hypotonic hemolysis. The 37 kDa band was identified as glyceraldehyde 3-phosphate dehydrogenase (G3PD), a glycolytic enzyme, based on N-terminal 14 amino acid sequencing. The role of G3PD in hemolysis is discussed.

Identification of a vibrio cholerae RTX toxin gene cluster that is tightly linked to the cholera toxin prophage

We identify and characterize a gene cluster in El Tor Vibrio cholerae that encodes a cytotoxic activity for HEp-2 cells in vitro. This gene cluster contains four genes and is physically linked to the cholera toxin (CTX) element in the V. cholerae genome. We demonstrate by using insertional mutagenesis that this gene cluster is required for the cytotoxic activity. The toxin, RtxA, resembles members of the RTX (repeats in toxin) toxin family in that it contains a GD-rich repeated motif. Like other RTX toxins, its activity depends on an activator, RtxC, and an associated ABC transporter system, RtxB and RtxD. In V. cholerae strains of the classical biotype, a deletion within the gene cluster removes rtxC and eliminates cytotoxic activity. Other strains, including those of the current cholera pandemic, contain a functional gene cluster and display cytotoxic activity. Thus, the RTX gene cluster in El Tor O1 and O139 strains might have contributed significantly to their emergence. Furthermore, the RTX toxin of V. cholerae may be associated with residual adverse properties displayed by certain live, attenuated cholera vaccines.

Two forms of Vibrio cholerae O1 El Tor hemolysin derived from identical precursor protein

Vibrio cholerae O1 grown in heart infusion broth produces two forms of El Tor hemolysin (ETH) monomers of 65 and 50 kDa. These monomers form several different sizes of mixed oligomers ranging from 180 to 280 kDa in the liposomal membranes. We found that the N-terminal amino acid sequences, NH2-Trp-Pro-Ala-Pro-Ala-Asn-Ser-Glu, of both the 65- and 50-kDa toxins were identical. We assumed, therefore, that the 65- and 50-kDa toxins were derivatives of the identical precursor protein and the 50-kDa protein was a truncated derivative of 65-kDa ETH. To substantiate this assumption, we treated the 260-kDa oligomer with trypsin and obtained a 190-kDa oligomer. This 190-kDa oligomer consisted of only the 50-kDa subunits. Both 260- and 190-kDa oligomers formed ion channels indistinguishable from each other in planar lipid bilayers. These results suggest that the essential part of the ETH in forming the membrane-damaging aggregate is a 50-kDa protein.

Use of monoclonal antibodies to identify phospholipase C as the enterotoxic factor of the bifunctional hemolysin-phospholipase C molecule of Vibrio cholerae O139

Two hybrid clones producing monoclonal antibodies (MAbs) raised against the purified enterotoxic hemolysin-phospholipase C (HlyPC) bifunctional molecule of a Vibrio cholerae O139 strain were used to study its enterotoxicity in relation to its hemolytic and enzymatic activities. Fab fragments of MAbs from ascites produced by the two hybrids neutralized the hemolytic activity of HlyPC, leaving the enzymatic activity unaffected. In ligated rabbit ileal loop and infant mouse intestine, the Fab fragments of the MAbs were not able to neutralize the enterotoxicity of HlyPC, suggesting that PC rather than Hly is the enterotoxic moiety of the molecule. The enterotoxicity of the purified PC molecule isolated from an Hly- spontaneous mutant of the HlyPC-producing parent strain further confirms this contention. The Hly molecule isolated from a PC- mutant was not diarrheagenic.