Recombinant nontoxinogenic Vibrio cholerae strains as attenuated cholera vaccine candidates

Emerging Concepts in Cholera Vaccine Design

Cholera is a severe diarrheal disease caused by the bacterium Vibrio cholerae and constitutes a significant public health threat in many areas of the world. V. cholerae infection elicits potent and long-lasting immunity, and efforts to develop cholera vaccines have been ongoing for more than a century. Currently available inactivated two-dose oral cholera vaccines are increasingly deployed to both prevent and actively curb cholera outbreaks, and they are key components of the global effort to eradicate cholera. However, these killed whole-cell vaccines have several limitations, and a variety of new oral and nonoral cholera vaccine platforms have recently been developed. Here, we review emerging concepts in cholera vaccine design and implementation that have been driven by insights from human and animal studies. As a prototypical vaccine-preventable disease, cholera continues to be an excellent target for the development and application of cutting-edge technologies and platforms that may transform vaccinology. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The organosulfur compound dimethylsulfoniopropionate (DMSP) is utilized as an osmoprotectant by Vibrio species

Dimethylsulfoniopropionate (DMSP), a key component of the global geochemical sulfur cycle, is a secondary metabolite produced in large quantities by marine phytoplankton and utilized as an osmoprotectant, thermoprotectant and antioxidant. Marine bacteria can use two pathways to degrade and catabolize DMSP, a demethylation pathway and a cleavage pathway that produces the climate active gas dimethylsulfide (DMS). Whether marine bacteria can also accumulate DMSP as an osmoprotectant to maintain the turgor pressure of the cell in response to changes in external osmolarity has received little attention. The marine halophile Vibrio parahaemolyticus, contains at least six osmolyte transporters, four betaine carnitine choline transport (BCCT) carriers BccT1-BccT4 and two ABC-family ProU transporters. In this study, we showed that DMSP is used as an osmoprotectant by V. parahaemolyticus and several other Vibrio species including V. cholerae and V. vulnificus Using a V. parahaemolyticus proU double mutant, we demonstrated that these ABC transporters are not required for DMSP uptake. However, a bccT null mutant lacking all four BCCTs had a growth defect compared to wild type in high salinity media supplemented with DMSP. Using mutants possessing only one functional BCCT in growth pattern assays, we identified two BCCT-family transporters, BccT1 and BccT2, which are carriers of DMSP. The only V. parahaemolyticus BccT homolog that V. cholerae and V. vulnificus possess is BccT3 and functional complementation in Escherichia coli MKH13 showed V. cholerae VcBccT3 could transport DMSP. In V. vulnificus strains, we identified and characterized an additional BCCT family transporter, which we named BccT5 that was also a carrier for DMSP.Importance DMSP is present in the marine environment, produced in large quantities by marine phytoplankton as an osmoprotectant, and is an important component of the global geochemical sulfur cycle. This algal osmolyte has not been previously investigated for its role in marine heterotrophic bacterial osmotic stress response. Vibrionaceae are marine species, many of which are halophiles exemplified by V. parahaemolyticus, a species that possesses at least six transporters for the uptake of osmolytes. Here, we demonstrated that V. parahaemolyticus and other Vibrio species can accumulate DMSP as an osmoprotectant and show that several BCCT family transporters uptake DMSP. These studies suggest that DMSP is a significant bacterial osmoprotectant, which may be important for understanding the fate of DMSP in the environment. DMSP is produced and present in coral mucus and Vibrio species form part of the microbial communities associated with them. The function of DMSP in these interactions is unclear, but could be an important driver for these associations allowing Vibrio proliferation. This work suggests that DMSP likely has an important role in heterotrophic bacteria ecology than previously appreciated.

Investigations of Dimethylglycine, Glycine Betaine, and Ectoine Uptake by a Betaine-Carnitine-Choline Transporter Family Transporter with Diverse Substrate Specificity in Vibrio Species

Fluctuations in osmolarity are one of the most prevalent stresses to which bacteria must adapt, both hypo- and hyperosmotic conditions. Most bacteria cope with high osmolarity by accumulating compatible solutes (osmolytes) in the cytoplasm to maintain the turgor pressure of the cell. Vibrio parahaemolyticus, a halophile, utilizes at least six compatible solute transporters for the uptake of osmolytes: two ABC family ProU transporters and four betaine-carnitine-choline transporter (BCCT) family transporters. The full range of compatible solutes transported by this species has yet to be determined. Using an osmolyte phenotypic microarray plate for growth analyses, we expanded the known osmolytes used by V. parahaemolyticus to include N,N-dimethylglycine (DMG), among others. Growth pattern analysis of four triple-bccT mutants, possessing only one functional BCCT, indicated that BccT1 (VP1456), BccT2 (VP1723), and BccT3 (VP1905) transported DMG. BccT1 was unusual in that it could take up both compounds with methylated head groups (glycine betaine [GB], choline, and DMG) and cyclic compounds (ectoine and proline). Bioinformatics analysis identified the four coordinating amino acid residues for GB in the BccT1 protein. In silico modeling analysis demonstrated that GB, DMG, and ectoine docked in the same binding pocket in BccT1. Using site-directed mutagenesis, we showed that a strain with all four residues mutated resulted in the loss of uptake of GB, DMG, and ectoine. We showed that three of the four residues were essential for ectoine uptake, whereas only one of the residues was important for GB uptake. Overall, we have demonstrated that DMG is a highly effective compatible solute for Vibrio species and have elucidated the amino acid residues in BccT1 that are important for the coordination of GB, DMG, and ectoine transport.IMPORTANCE Vibrio parahaemolyticus possesses at least six osmolyte transporters, which allow the bacterium to adapt to high-salinity conditions. In this study, we identified several additional osmolytes that were utilized by V. parahaemolyticus We demonstrated that the compound DMG, which is present in the marine environment, was a highly effective osmolyte for Vibrio species. We determined that DMG is transported via BCCT family carriers, which have not been shown previously to take up this compound. BccT1 was a carrier for GB, DMG, and ectoine, and we identified the amino acid residues essential for the coordination of these compounds. The data suggest that for BccT1, GB is more easily accommodated than ectoine in the transporter binding pocket.

Deciphering the possible role of ctxB7 allele on higher production of cholera toxin by Haitian variant Vibrio cholerae O1

Cholera continues to be an important public health concern in developing countries where proper hygiene and sanitation are compromised. This severe diarrheal disease is caused by the Gram-negative pathogen Vibrio cholerae belonging to serogroups O1 and O139. Cholera toxin (CT) is the prime virulence factor and is directly responsible for the disease manifestation. The ctxB gene encodes cholera toxin B subunit (CTB) whereas the A subunit (CTA) is the product of ctxA gene. Enzymatic action of CT depends on binding of B pentamers to the lipid-based receptor ganglioside GM1. In recent years, emergence of V. cholerae Haitian variant strains with ctxB7 allele and their rapid spread throughout the globe has been linked to various cholera outbreaks in Africa and Asia. These strains produce classical type (WT) CTB except for an additional mutation in the signal sequence region where an asparagine (N) residue replaces a histidine (H) at the 20th amino acid position (H20N) of CTB precursor (pre-CTB). Here we report that Haitian variant V. cholerae O1 strains isolated in Kolkata produced higher amount of CT compared to contemporary O1 El Tor variant strains under in vitro virulence inducing conditions. We observed that the ctxB7 allele, itself plays a pivotal role in higher CT production. Based on our in silico analysis, we hypothesized that higher accumulation of toxin subunits from ctxB7 allele might be attributed to the structural alteration at the CTB signal peptide region of pre-H20N CTB. Overall, this study provides plausible explanation regarding the hypertoxigenic phenotype of the Haitian variant strains which have spread globally, possibly through positive selection for increased pathogenic traits.

THE AER2 RECEPTOR FROM VIBRIO CHOLERAE IS A DUAL PAS-HEME OXYGEN SENSOR

The diarrheal pathogen Vibrio cholerae navigates complex environments using three chemosensory systems and 44-45 chemoreceptors. Chemosensory cluster II modulates chemotaxis, whereas clusters I and III have unknown functions. Ligands have been identified for only five V. cholerae chemoreceptors. Here we report that the cluster III receptor, VcAer2, binds and responds to O₂ . VcAer2 is an ortholog of Pseudomonas aeruginosa Aer2 (PaAer2), but differs in that VcAer2 has two, rather than one, N-terminal PAS domain. We have determined that both PAS1 and PAS2 form homodimers and bind penta-coordinate b-type heme via an Eη-His residue. Heme binding to PAS1 required the entire PAS core, but receptor function also required the N-terminal cap. PAS2 functioned as an O₂ -sensor [K_d(O2) , 19 μM], utilizing the same Iβ Trp (W276) as PaAer2 to stabilize O₂ . The crystal structure of PAS2-W276L was similar to that of PaAer2-PAS, but resided in an active conformation mimicking the ligand-bound state, consistent with its signal-on phenotype. PAS1 also bound O₂ [K_d(O2), 12 μM], although O₂ binding was stabilized by either a Trp or Tyr residue. Moreover, PAS1 appeared to function as a signal modulator, regulating O₂ -mediated signaling from PAS2, and resulting in activation of the cluster III chemosensory pathway. This article is protected by copyright. All rights reserved.

CRISPR-Cas and Contact-Dependent Secretion Systems Present on Excisable Pathogenicity Islands with Conserved Recombination Modules

Pathogenicity islands (PAIs) are mobile integrated genetic elements that contain a diverse range of virulence factors. PAIs integrate into the host chromosome at a tRNA locus that contains their specific bacterial attachment site, attB, via integrase-mediated site-specific recombination generating attL and attR sites. We identified conserved recombination modules (integrases and att sites) previously described in choleragenic Vibrio cholerae PAIs but with novel cargo genes. Clustered regularly interspaced short palindromic repeat (CRISPR)-associated proteins (Cas proteins) and a type VI secretion system (T6SS) gene cluster were identified at the Vibrio pathogenicity island 1 (VPI-1) insertion site in 19 V. cholerae strains and contained the same recombination module. Two divergent type I-F CRISPR-Cas systems were identified, which differed in Cas protein homology and content. The CRISPR repeat sequence was identical among all V. cholerae strains, but the CRISPR spacer sequences and the number of spacers varied. In silico analysis suggests that the CRISPR-Cas systems were active against phages and plasmids. A type III secretion system (T3SS) was present in 12 V. cholerae strains on a 68-kb island inserted at the same tRNA-serine insertion site as VPI-2 and contained the same recombination module. Bioinformatics analysis showed that two divergent T3SSs exist among the strains examined. Both the CRISPR and T3SS islands excised site specifically from the bacterial chromosome as complete units, and the cognate integrases were essential for this excision. These data demonstrated that identical recombination modules that catalyze integration and excision from the chromosome can acquire diverse cargo genes, signifying a novel method of acquisition for both CRISPR-Cas systems and T3SSs.IMPORTANCE This work demonstrated the presence of CRISPR-Cas systems and T3SSs on PAIs. Our work showed that similar recombination modules can associate with different cargo genes and catalyze their incorporation into bacterial chromosomes, which could convert a strain into a pathogen with very different disease pathologies. Each island had the ability to excise from the chromosome as distinct, complete units for possible transfer. Evolutionary analysis of these regions indicates that they were acquired by horizontal transfer and that PAIs are the units of transfer. Similar to the case for phage evolution, PAIs have a modular structure where different functional regions are acquired by identical recombination modules.

The Live Attenuated Cholera Vaccine CVD 103-HgR Primes Responses to the Toxin-Coregulated Pilus Antigen TcpA in Subjects Challenged with Wild-Type Vibrio cholerae

One potential advantage of live attenuated bacterial vaccines is the ability to stimulate responses to antigens which are only expressed during the course of infection. To determine whether the live attenuated cholera vaccine CVD 103-HgR (Vaxchora) results in antibody responses to the in vivo-induced toxin-coregulated pilus antigen TcpA, we measured IgA and IgG responses to Vibrio cholerae O1 El Tor TcpA in a subset of participants in a recently reported experimental challenge study. Participants were challenged with V. cholerae O1 El Tor Inaba N16961 either 10 days or 90 days after receiving the vaccine or a placebo. Neither vaccination nor experimental infection with V. cholerae alone resulted in a robust TcpA IgG or IgA response, but each did elicit a strong response to cholera toxin. However, compared to placebo recipients, vaccinees had a marked increase in IgG TcpA antibodies following the 90-day challenge, suggesting that vaccination with CVD 103-HgR resulted in priming for a subsequent response to TcpA. No such difference between vaccine and placebo recipients was observed for volunteers challenged 10 days after vaccination, indicating that this was insufficient time for vaccine-induced priming of the TcpA response. The priming of the response to TcpA and potentially other antigens expressed in vivo by attenuated V. cholerae may have relevance to the maintenance of immunity in areas where cholera is endemic.

CTXφ Replication Depends on the Histone-Like HU Protein and the UvrD Helicase

The Vibrio cholerae bacterium is the agent of cholera. The capacity to produce the cholera toxin, which is responsible for the deadly diarrhea associated with cholera epidemics, is encoded in the genome of a filamentous phage, CTXφ. Rolling-circle replication (RCR) is central to the life cycle of CTXφ because amplification of the phage genome permits its efficient integration into the genome and its packaging into new viral particles. A single phage-encoded HUH endonuclease initiates RCR of the proto-typical filamentous phages of enterobacteriaceae by introducing a nick at a specific position of the double stranded DNA form of the phage genome. The rest of the process is driven by host factors that are either essential or crucial for the replication of the host genome, such as the Rep SF1 helicase. In contrast, we show here that the histone-like HU protein of V. cholerae is necessary for the introduction of a nick by the HUH endonuclease of CTXφ. We further show that CTXφ RCR depends on a SF1 helicase normally implicated in DNA repair, UvrD, rather than Rep. In addition to CTXφ, we show that VGJφ, a representative member of a second family of vibrio integrative filamentous phages, requires UvrD and HU for RCR while TLCφ, a satellite phage, depends on Rep and is independent from HU.

One of the major strategies to prevent Cholera epidemics is the development of oral vaccines based on live attenuated Vibrio cholerae strains. The most promising vaccine strains have been obtained by deletion of the cholera toxin genes, which are harboured in the genome of an integrated phage, CTXϕ. However, they can re-acquire the cholera toxin genes when re-infected by CTXϕ or by hybrid phages between CTXϕ and other vibrio phages, which raised safety concerns about their use. Here, we developed a screening strategy to identify non-essential host factors implicated in CTXϕ replication. We show that the histone-like HU protein and the UvrD helicase are both absolutely required for its replication. We further show that they are essential for the replication of VGJϕ, a representative member of a family of phages that can form hybrids with CTXϕ. Accordingly, we demonstrate that the disruption of the two subunits of HU and/or of UvrD prevents infection of the V. cholerae by CTXϕ and VGJϕ. In addition, we show that it limits CTXϕ horizontal transmission. Taken together, these results indicate that HU^- and/or UvrD^- cells are promising candidates for the development of safer live attenuated cholera vaccine.

Live-attenuatedSalmonellaas a prototype vaccine vector for passenger immunogens in humans: are we there yet?

It has been nearly 20 years since the first Phase I clinical trial of a live-attenuated bacterial vaccine was created by recombinant DNA methods, opening the door to the use of these organisms as mucosal delivery vehicles for passenger antigens. Over this time, a number of animal studies have indicated the feasibility of this approach. These include studies showing that bacteria can deliver antigens expressed by the bacterium itself and that bacteria can deliver DNA vaccines to be expressed in target eukaryotic cells. Concomitant studies have identified a number of attenuating mutations that render the bacterial vectors both safe and immunogenic in humans. Both avenues of research indicate the significant promise of this approach to mucosal vaccine development; however, this promise remains largely unrealized at the level of human clinical trials. This review sketches the history of this problem and points toward possible solutions using Salmonella vaccine vectors as the prototypes.

In-silico structural and functional characterization of a V. cholerae O395 hypothetical protein containing a PDZ1 and an uncommon protease domain

Vibrio cholerae, the causative agent of epidemic cholera, has been a constant source of concern for decades. It has constantly evolved itself in order to survive the changing environment. Acquisition of new genetic elements through genomic islands has played a major role in its evolutionary process. In this present study a hypothetical protein was identified which was present in one of the predicted genomic island regions of the large chromosome of V. cholerae O395 showing a strong homology with a conserved phage encoded protein. In-silico physicochemical analysis revealed that the hypothetical protein was a periplasmic protein. Homology modeling study indicated that the hypothetical protein was an unconventional and atypical serine protease belonging to HtrA protein family. The predicted 3D-model of the hypothetical protein revealed a catalytic centre serine utilizing a single catalytic residue for proteolysis. The predicted catalytic triad may help to deduce the active site for the recruitment of the substrate for proteolysis. The active site arrangements of this predicted serine protease homologue with atypical catalytic triad is expected to allow these proteases to work in different environments of the host.

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.

Construction and characterization of rtxA and rtxC mutants of auxotrophic O139 Vibrio cholerae

Vibrio cholerae is a Gram-negative bacterium that causes diarrheal disease. V. cholerae O1 and O139 serogroups are toxigenic and are known to cause epidemic cholera. These serogroups produce cholera toxin and other accessory toxins such as accessory cholera enterotoxin, zonula occludens toxin, and multifunctional, autoprocessing repeat in toxin (MARTX). In the present study, we incorporated mutated rtxA and rtxC genes that encode MARTX toxin into the existing aminolevulinic acid (ALA) auxotrophic vaccine candidate VCUSM2 of V. cholerae O139 serogroup. The rtxC mutant was named VCUSM9 and the rtxC/rtxA mutant was named VCUSM10. VCUSM9 and VCUSM10 were able to colonize intestinal cells well, compared with the parent vaccine strain, and produced no fluid accumulation in a rabbit ileal loop model. Cell rounding and western blotting assays indicated that mutation of the rtxC gene alone (VCUSM9 strain) did not abolish MARTX toxicity. However mutation of both the rtxA and rtxC genes (VCUSM10) completely abolished MARTX toxicity. Thus we have produced a new, less reactogenic, auxotrophic rtxC/rtxA mutated vaccine candidate against O139 V. cholerae.

Metalloprotease vsm is the major determinant of toxicity for extracellular products of Vibrio splendidus

Genomic data combined with reverse genetic approaches have contributed to the characterization of major virulence factors of Vibrio species; however, these studies have targeted primarily human pathogens. Here, we investigate virulence factors in the oyster pathogen Vibrio splendidus LGP32 and show that toxicity is correlated to the presence of a metalloprotease and its corresponding vsm gene. Comparative genomics showed that an avirulent strain closely related to LGP32 lacked the metalloprotease. The toxicity of LGP32 metalloprotease was confirmed by exposing mollusk and mouse fibroblastic cell lines to extracellular products (ECPs) of the wild type (wt) and a vsm deletion mutant (Deltavsm mutant). The ECPs of the wt induced a strong cytopathic effect whose severity was cell type dependent, while those of the Deltavsm mutant were much less toxic, and exposure to purified protein demonstrated the direct toxicity of the Vsm metalloprotease. Finally, to investigate Vsm molecular targets, a proteomic analysis of the ECPs of both LGP32 and the Deltavsm mutant was performed, revealing a number of differentially expressed and/or processed proteins. One of these, the VSA1062 metalloprotease, was found to have significant identity to the immune inhibitor A precursor, a virulence factor of Bacillus thuringiensis. Deletion mutants corresponding to several of the major proteins were constructed by allelic exchange, and the ECPs of these mutants proved to be toxic to both cell cultures and animals. Taken together, these data demonstrate that Vsm is the major toxicity factor in the ECPs of V. splendidus.

Structural organization of the transfer RNA operon I of Vibrio cholerae: differences between classical and El Tor strains

Nine major transfer RNA (tRNA) gene clusters were analysed in various Vibrio cholerae strains. Of these, only the tRNA operon I was found to differ significantly in V. cholerae classical (sixth pandemic) and El Tor (seventh pandemic) strains. Amongst the sixteen tRNA genes contained in this operon, genes for tRNA Gln3 (CAA) and tRNA Leu6 (CUA) were absent in classical strains as compared to El Tor strains. The observation strongly supported the view that the above two pandemic strains constitute two different clones.

Detection and transformation of genome segments that differ within a coastal population of Vibrio cholerae strains

Vibrio cholerae is an autochthonous member of diverse aquatic ecosystems around the globe. Collectively, the genomes of environmental V. cholerae strains comprise a large repository of encoded functions which can be acquired by individual V. cholerae lineages through uptake and recombination. To characterize the genomic diversity of environmental V. cholerae, we used comparative genome hybridization to study 41 environmental strains isolated from diverse habitats along the central California coast, a region free of endemic cholera. These data were used to classify genes of the epidemic V. cholerae O1 sequenced strain N16961 as conserved, variably present, or absent from the isolates. For the most part, absent genes were restricted to large mobile elements and have known functions in pathogenesis. Conversely, genes present in some, but not all, California isolates were in smaller contiguous clusters and were less likely to be near genes with functions in DNA mobility. Two such clusters of variable genes encoding different selectable metabolic phenotypes (mannose and diglucosamine utilization) were transformed into the genomes of environmental isolates by chitin-dependent competence, indicating that this mechanism of general genetic exchange is conserved among V. cholerae. The transformed DNA had an average size of 22.7 kbp, demonstrating that natural competence can mediate the movement of large chromosome fragments. Thus, whether variable genes arise through the acquisition of new sequences by horizontal gene transfer or by the loss of preexisting DNA though deletion, natural transformation provides a mechanism by which V. cholerae clones can gain access to the V. cholerae pan-genome.

Biological safety concepts of genetically modified live bacterial vaccines

Live vaccines possess the advantage of having access to induce cell-mediated and antibody-mediated immunity; thus in certain cases they are able to prevent infection, and not only disease. Furthermore, live vaccines, particularly bacterial live vaccines, are relatively cheap to produce and easy to apply. Hence they are suitable to immunize large communities or herds. The induction of both cell-mediated immunity as well as antibody-mediated immunity, which is particularly beneficial in inducing mucosal immune responses, is obtained by the vaccine-strain's ability to colonize and multiply in the host without causing disease. For this reason, live vaccines require attenuation of virulence of the bacterium to which immunity must be induced. Traditionally attenuation was achieved simply by multiple passages of the microorganism on growth medium, in animals, eggs or cell cultures or by chemical or physical mutagenesis, which resulted in random mutations that lead to attenuation. In contrast, novel molecular methods enable the development of genetically modified organisms (GMOs) targeted to specific genes that are particularly suited to induce attenuation or to reduce undesirable effects in the tissue in which the vaccine strains can multiply and survive. Since live vaccine strains (attenuated by natural selection or genetic engineering) are potentially released into the environment by the vaccinees, safety issues concerning the medical as well as environmental aspects must be considered. These involve (i) changes in cell, tissue and host tropism, (ii) virulence of the carrier through the incorporation of foreign genes, (iii) reversion to virulence by acquisition of complementation genes, (iv) exchange of genetic information with other vaccine or wild-type strains of the carrier organism and (v) spread of undesired genes such as antibiotic resistance genes. Before live vaccines are applied, the safety issues must be thoroughly evaluated case-by-case. Safety assessment includes knowledge of the precise function and genetic location of the genes to be mutated, their genetic stability, potential reversion mechanisms, possible recombination events with dormant genes, gene transfer to other organisms as well as gene acquisition from other organisms by phage transduction, transposition or plasmid transfer and cis- or trans-complementation. For this, GMOs that are constructed with modern techniques of genetic engineering display a significant advantage over random mutagenesis derived live organisms. The selection of suitable GMO candidate strains can be made under in vitro conditions using basic knowledge on molecular mechanisms of pathogenicity of the corresponding bacterial species rather than by in vivo testing of large numbers of random mutants. This leads to a more targeted safety testing on volunteers and to a reduction in the use of animal experimentation.

Novel surface polypeptides of Campylobacter jejuni as traveller's diarrhoea vaccine candidates discovered by proteomics

Campylobacter jejuni is one of the most common causes of traveller's diarrhoea and food poisoning, therefore development of a vaccine is important. Using biochemical fractionation and mass spectrometry analysis, we identified more than 110 surface polypeptides. Eight C. jejuni identified surface proteins were expressed in Escherichia coli and purified. Mice were immunized with different doses of these purified proteins and challenged orally with C. jejuni strains ML1 and ML53. The degree of protection of mice was tested by intestinal colonization. At least two groups of mice vaccinated with purified proteins clear the infection faster than control mice. Here, we present the use of a proteomics based approach for the identification of novel protein based C. jejuni vaccines for the first time.

Molecular phylogenetic analysis ofVibrio cholerae O1 El Tor strains isolated before, during and after the O139 outbreak based on the intergenomic heterogeneity of the 16S-23S rRNA intergenic spacer regions

We have cloned, sequenced and analysed all the five classes of the intergenic (16S-23S rRNA) spacer region (ISR) associated with the eight rrn operons (rrna-rrnh) of Vibrio cholerae serogroup O1 El Tor strains isolated before, during and after the O 139 outbreak. ISR classes 'a' and 'g' were found to be invariant, ISR-B (ISRb and ISRe) exhibited very little variation, whereas ISR-C (ISRc, ISRd, and ISRf) and ISRh showed the maximum variation. Phylogenetic analysis conducted with all three ISR classes (ISR-B, ISR-C and ISRh) showed that the pre-O 139 serogroup and post-O 139 serogroup O1 El Tor strains arose out of two independent clones, which was congruent with the observation made by earlier workers suggesting that analyses of ISR-C and ISR-h, instead of all five ISR classes, could be successfully used to study phylogeny in this organism.

Transcriptional responses of intestinal epithelial cells to infection with Vibrio cholerae

Vibrio cholerae is a noninvasive enteric bacterium that causes the severe diarrheal disease cholera. Candidate cholera vaccines have been engineered by deleting genes encoding known virulence factors in V. cholerae; however, many of these attenuated strains were still reactogenic in human volunteers. In this study, DNA arrays were utilized to monitor the transcriptional responses of human intestinal epithelial cells (T84) to eight strains of V. cholerae, including attenuated, toxigenic, and environmental isolates. cDNA probes generated from host RNA samples were hybridized against low- and high-density gene arrays. V. cholerae induced the transcription of a variety of host genes and repressed the expression of a lower number of genes. Expression patterns were confirmed for certain genes by reverse transcriptase PCR and enzyme-linked immunosorbent assays. A core subset of genes was found to be differentially regulated in all experiments. These genes included genes involved in innate mucosal immunity, intracellular signaling, and cellular proliferation. Reactogenic vaccine strains induced greater expression of genes for certain proinflammatory cytokines than nonreactogenic strains. Wild-type and attenuated derivatives induced and repressed many genes in common, although there were differences in the transcription profiles. These results indicate that the types of host genes modulated by attenuated V. cholerae, and the extent of their induction, may mediate the symptoms seen with reactogenic cholera vaccine strains.

Biosafety aspects of the recombinant live oral Vibrio cholerae vaccine strain CVD 103-HgR

The development of live attenuated vaccines, allowing for the safe and effective immunisation at mucosal surfaces, is a strategy of great interest for vaccinologists. The main advantage of this approach over conventional parenteral vaccines is the induction of strong mucosal immune responses, allowing targeting of the pathogen at the initial point of contact with the host. Further advantages include the ease of administration, high acceptance by vaccines, and relatively low production costs. Finally, well-characterised, safe and immunogenic vaccine strains are well suited as vectors for the mucosal delivery of foreign vaccine antigens and of DNA vaccines. However, such vaccines, when based on or containing genetically modified organisms (GMOs), are facing new and specific regulatory hurdles, particularly regarding the potential risks for humans and the environment. In this contribution we address selected aspects of the risk assessment of live attenuated bacterial vaccines covered in the course of the registration of vaccine strain CVD 103-HgR as a recombinant live oral vaccine against cholera.

Induction of interleukin-8 in T84 cells by Vibrio cholerae

The induction of interleukin-8 (IL-8) in vitro has been suggested to correlate with the reactogenicity of Vibrio cholerae vaccine candidates. V. cholerae vaccine candidate 638, a hemagglutinin protease/hap-defective strain, was recently reported to be well tolerated in human volunteers, suggesting a role for Hap in reactogenicity. We examined the role of hap in the induction of IL-8 in intestinal epithelial T84 cells. Wild-type V. cholerae strains 3038 and C7258 and a vaccine candidate strain, JBK70, induced levels of IL-8 similar to those of their isogenic hap mutants. Supernatant containing Hap did not stimulate IL-8 production at a variety of concentrations tested, suggesting that Hap itself does not induce IL-8 production. Furthermore, supernatant from CVD115, which had deletions of hap and rtxA (encoding repeats in toxin) and was derived from a reactogenic strain, CVD110, induced IL-8 production in T84 cells in a dose-dependent manner. The IL-8-stimulating activity of CVD115 culture supernatants was growth phase dependent and was strongest in stationary phase cultures. This IL-8 stimulator(s) was resistant to heat treatment but sensitive to proteinase. Protease activity in vitro did not correlate with the reactogenicity of V. cholerae vaccine candidates. Our data suggest that Hap is not an IL-8 inducer in T84 cells and that the IL-8 stimulator in the supernatant of V. cholerae culture may play a role in reactogenicity.

The intestinal zonula occludens toxin (ZOT) receptor recognises non-native ZOT conformers and localises to the intercellular contacts

A preliminary structural analysis of Vibrio cholerae zonula occludens toxin (ZOT) was made by equilibrium denaturation and circular dichroism. ZOT is a structurally unstable protein in aqueous solution (DeltaG((H2O)) 3.82 kcal/mol), the putative intra- and extracellular domains unfold co-operatively, with complete denaturation via observed conformational intermediates. Refolding of denatured ZOT is not dependent on disulphide bridge formation. Partial refolding of a maltose binding protein-ZOT fusion did not prevent its specific binding to the ZOT receptor on Caco-2 cells. Immuno-gold labelling showed that the ZOT receptor localises to the intercellular contacts between cells in a confluent monolayer.

The vibrio pathogenicity island-encoded mop protein modulates the pathogenesis and reactogenicity of epidemic vibrio cholerae

Epidemic Vibrio cholerae possess the VPI (Vibrio pathogenicity island) essential virulence gene cluster. The VPI is 41.2 kb in size and encodes 29 potential proteins, several of which have no known function. We show that the VPI-encoded Orf4 is a predicted 34-kDa periplasmic protein containing a zinc metalloprotease motif. V. cholerae seventh-pandemic (El Tor) strain N16961 carrying an orf4 mutation showed no obvious difference relative to its parent in the production of cholera toxin and the toxin-coregulated pilus, motility, azocasein digestion, and colonization of infant mice. However, analysis of rabbit ileal loops revealed that the N16961 orf4 mutant is hypervirulent, causing increased serosal hemorrhage and reactogenicity compared to its parent. Histology revealed a widening of submucosa, with an increase in inflammatory cells, diffuse lymphatic vessel dilatation, edema, endothelial cell hypertrophy of blood vessels, blunting of villi, and lacteal dilatation with lymphocytes and polymorphonuclear leukocytes. The mutant could be complemented in vivo with an orf4 gene on a plasmid but not with an orf4 gene containing a site-directed mutation in the putative zinc metalloprotease motif. Although its mechanism of its action is being studied further, our results suggest that the Orf4 protein is a zinc metalloprotease that modulates the pathogenesis and reactogenicity of epidemic V. cholerae. Based on our findings, we name this VPI-encoded protein Mop (for modulation of pathogenesis).

Mobilization of the Vibrio pathogenicity island between Vibrio cholerae isolates mediated by CP-T1 generalized transduction

Pathogenicity islands are large chromosomal regions encoding virulence genes that were acquired by horizontal gene transfer and are found in a wide range of pathogenic bacteria. In toxigenic Vibrio cholerae isolates the receptor for the cholera toxin encoding filamentous phage CTXphi, the toxin-coregulated pilus, is part of the Vibrio pathogenicity island (VPI). In this paper, we show that the VPI can be transferred between O1 serogroup strains, the predominant cause of epidemic cholera, via a generalized transducing phage CP-T1.

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.

Flagellum-independent surface migration of Vibrio cholerae and Escherichia coli

Surface translocation has been described in a large variety of microorganisms, including some gram-negative enteric bacteria. Here, we describe the novel observation of the flagellum-independent migration of Vibrio cholerae and Escherichia coli on semisolid surfaces with remarkable speeds. Important aspects of this motility are the form of inoculation, the medium composition, and the use of agarose rather than agar. Mutations in several known regulatory or surface structure proteins, such as ToxR, ToxT, TCP, and PilA, did not affect migration, whereas a defect in lipopolysaccharide biosynthesis prevented translocation. We propose that the observed surface migration is an active process, since heat, protease, or chloramphenicol treatments of the cells have strong negative effects on this phenotype. Furthermore, several V. cholerae strains strongly expressing the hemagglutinin/protease but not their isogenic hap-negative mutants, lacked the ability of surface motility, and the treatment of migrating strains with culture supernatants from hap strains but not hap-null strains prevented surface translocation.

Differential interleukin-8 response of intestinal epithelial cell line to reactogenic and nonreactogenic candidate vaccine strains of Vibrio cholerae

In this study, we analyzed whether attachment of Vibrio cholerae vaccine strains to human intestinal epithelial cells can induce an interleukin-8 (IL-8) response. The IL-8 transcripts were detected by PCR amplification of reverse-transcribed mRNA, and the gene product secretion was measured by an enzyme-linked immunosorbent assay. Infection of monolayers of the undifferentiated HT29-18N2 cell line with reactogenic (JBK70 and 81) and nonreactogenic (CVD103HgR and 638) vaccine strains of V. cholerae resulted in markedly higher IL-8 expression by epithelial cells exposed to reactogenic strains than by cells exposed to the nonreactogenic strains. Additionally, epithelial cells produced IL-8 transcripts following stimulation with cholera vaccine strains in a concentration-dependent manner. These results represent a new insight into the inflammatory component of reactogenicity and could be used as a predictive marker of vaccine reactogenicity prior to human testing.

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

Enteric pathogens often export toxins that elicit diarrhea as a part of the etiology of disease, including toxins that affect cytoskeletal structure. Recently, we discovered that the intestinal pathogen Vibrio cholerae elicits rounding of epithelial cells that is dependent upon a gene we designated rtxA. Here we investigate the association of rtxA with the cell-rounding effect. We find that V. cholerae exports a large toxin, RTX (repeats-in-toxin) toxin, to culture supernatant fluids and that this toxin is responsible for cell rounding. Furthermore, we find that cell rounding is not due to necrosis, suggesting that RTX toxin is not a typical member of the RTX family of pore-forming toxins. Rather, RTX toxin causes depolymerization of actin stress fibers and covalent cross-linking of cellular actin into dimers, trimers and higher multimers. This RTX toxin-specific cross-linking occurs in cells previously rounded with cytochalasin D, indicating that G-actin is the toxin target. Although several models explain our observations, our simultaneous detection of actin cross-linking and depolymerization points toward a novel mechanism of action for RTX toxin, distinguishing it from all other known toxins.

Molecular analyses of a putative CTXphi precursor and evidence for independent acquisition of distinct CTX(phi)s by toxigenic Vibrio cholerae

The genes encoding cholera toxin (ctxA and ctxB) are encoded in the genome of CTXphi, a filamentous phage that infects Vibrio cholerae. To study the evolutionary history of CTXphi, we examined genome diversity in CTX(phi)s derived from a variety of epidemic and nonepidemic Vibrio sp. natural isolates. Among these were three V. cholerae strains that contained CTX prophage sequences but not the ctxA and ctxB genes. These prophages each gave rise to a plasmid form whose genomic organization was very similar to that of the CTXphi replicative form, with the exception of missing ctxAB. Sequence analysis of these three plasmids revealed that they lacked the upstream control region normally found 5' of ctxA, as well as the ctxAB promoter region and coding sequences. These findings are consistent with the hypothesis that a CTXphi precursor that lacked ctxAB simultaneously acquired the toxin genes and their regulatory sequences. To assess the evolutionary relationships among additional CTX(phi)s, two CTXphi-encoded genes, orfU and zot, were sequenced from 13 V. cholerae and 4 V. mimicus isolates. Comparative nucleotide sequence analyses revealed that the CTX(phi)s derived from classical and El Tor V. cholerae isolates comprise two distinct lineages within otherwise nearly identical chromosomal backgrounds (based on mdh sequences). These findings suggest that nontoxigenic precursors of the two V. cholerae O1 biotypes independently acquired distinct CTX(phi)s.

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.

Enteroaggregative Escherichia coli expresses a novel flagellin that causes IL-8 release from intestinal epithelial cells

Enteroaggregative Escherichia coli (EAEC) is an emerging cause of acute and persistent diarrhea worldwide. EAEC infections are associated with intestinal inflammation and growth impairment in infected children, even in the absence of diarrhea. We previously reported that prototype EAEC strains rapidly induce IL-8 production by Caco-2 intestinal epithelial cells, and that this effect is mediated by a soluble, heat-stable factor released by these bacteria in culture. We herein report the cloning, sequencing, and expression of this biologically active IL-8-releasing factor from EAEC, and its identification as a flagellin that is unique among known expressed proteins. Flagella purified from EAEC 042 and several other EAEC isolates potently release IL-8 from Caco-2 cells; an engineered aflagellar mutant of 042 does not release IL-8. Finally, cloned EAEC flagellin expressed in nonpathogenic E. coli as a polyhistidine-tagged fusion protein maintains its proinflammatory activity. These findings demonstrate a major new means by which EAEC may cause intestinal inflammation, persistent diarrhea, and growth impairment that characterize human infection with these organisms. Furthermore, they open new approaches for diagnosis and vaccine development. This novel pathogenic mechanism of EAEC extends an emerging paradigm of bacterial flagella as inflammatory stimuli.

Cutaneous vaccination: the skin as an immunologically active tissue and the challenge of antigen delivery

Vaccination is one of the major achievements of modern medicine. As a result of vaccination, diseases such as polio and measles have been controlled and small pox has been eradicated. However, despite these successes there are still many microbial diseases that cause tremendous suffering because there is no vaccine or the vaccines available are inadequate. In addition, even if vaccines were available for all infectious diseases there is no guarantee that people would use them routinely. One of the major impediments to ensuring vaccine efficacy and compliance is that of delivery. Presently most vaccines are given by intramuscular administration. Unfortunately this is often traumatic, especially in infants. Thus, if it was possible to replace intramuscular immunization by mucosal (oral/intranasal) or transdermal delivery it may be possible to both enhance mucosal immunity as well as improve overall compliance rates. The transdermal route has been used by the pharmaceutical industry for the delivery of various low molecular weight drugs. Some of the approaches used for smaller compounds may also have potential for delivery of either protein or polynucleotide vaccines. However, there is a greater challenge to delivering large molecular weight molecules through the skin due to size, charge and other physicochemical properties. This review will describe the recent advances that have been made in dermal and topical delivery as related to vaccines.

Infectious CTXPhi and the vibrio pathogenicity island prophage in Vibrio mimicus: evidence for recent horizontal transfer between V. mimicus and V. cholerae

Vibrio mimicus differs from Vibrio cholerae in a number of genotypic and phenotypic traits but like V. cholerae can give rise to diarrheal disease. We examined clinical isolates of V. mimicus for the presence of CTXPhi, the lysogenic filamentous bacteriophage that carries the cholera toxin genes in epidemic V. cholerae strains. Four V. mimicus isolates were found to contain complete copies of CTXPhi. Southern blot analyses revealed that V. mimicus strain PT5 contains two CTX prophages integrated at different sites within the V. mimicus genome whereas V. mimicus strains PT48, 523-80, and 9583 each contain tandemly arranged copies of CTXPhi. We detected the replicative form of CTXPhi, pCTX, in all four of these V. mimicus isolates. The CTX prophage in strain PT5 was found to produce infectious CTXPhi particles. The nucleotide sequences of CTXPhi genes orfU and zot from V. mimicus strain PT5 and V. cholerae strain N16961 were identical, indicating contemporary horizontal transfer of CTXPhi between these two species. The receptor for CTXPhi, the toxin-coregulated pilus, which is encoded by another lysogenic filamentous bacteriophage, VPIPhi, was also present in the CTXPhi-positive V. mimicus isolates. The nucleotide sequences of VPIPhi genes aldA and toxT from V. mimicus strain PT5 and V. cholerae N16961 were identical, suggesting recent horizontal transfer of this phage between V. mimicus and V. cholerae. In V. mimicus, the vibrio pathogenicity island prophage was integrated in the same chromosomal attachment site as in V. cholerae. These results suggest that V. mimicus may be a significant reservoir for both CTXPhi and VPIPhi and may play an important role in the emergence of new toxigenic V. cholerae isolates.

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.

Vibrio cholerae intestinal population dynamics in the suckling mouse model of infection

The suckling mouse has been used as a model to identify Vibrio cholerae intestinal colonization factors for over two decades, yet little is known about the location of recoverable organisms along the gastrointestinal (GI) tract following intragastric inoculation. In the present study, we determined the population dynamics of wild-type and avirulent mutant derivatives of both classical and El Tor biotype strains throughout the entire suckling mouse GI tract at various times after intragastric inoculation. Wild-type strains preferentially colonized the middle small bowel with a sharp demarcation between more proximal segments which had manyfold-fewer recoverable cells. Surprisingly, large and stable populations of viable cells were also recovered from the cecum and large bowel. Strains lacking toxin-coregulated pili (TCP(-)) were cleared from the small bowel; however, an El Tor TCP(-) strain colonized the cecum and large bowel almost as well as the wild-type strain. Strains lacking lipopolysaccharide O antigen (OA(-)) were efficiently cleared from the small bowel at early times but then showed net growth for the remainder of the infections. Moreover, large populations of the OA(-) strains were maintained in the large bowel. These results show that for the El Tor biotype neither TCP nor OA is required for colonization of the suckling mouse large bowel. Finally, similar percent recoveries of wild-type, TCP(-), and OA(-) strains from the small bowel at an early time after infection suggest that TCP and OA are not required for strains of either biotype to resist bactericidal mechanisms in the suckling mouse GI tract.

Site-specific integration of the conjugal Vibrio cholerae SXT element into prfC

Vibrio cholerae O139, the first non-O1 serogroup of V. cholerae to give rise to epidemic cholera, is characteristically resistant to the antibiotics sulphamethoxazole, trimethoprim, chloramphenicol and streptomycin. Resistances to these antibiotics are encoded by a 62 kb self-transmissible, conjugative, chromosomally integrating element designated the 'SXT element'. We found that the SXT element integrates site specifically into both V. cholerae and Escherichia coli K-12 into the 5' end of prfC, the gene encoding peptide chain release factor 3. Integration of the SXT element interrupts the chromosomal prfC gene, but the element encodes a new 5' end of prfC that restores the reading frame of this gene. The recombinant of prfC allele created upon element integration is functional. The integration and excision mechanism of the SXT element shares many features with site-specific recombination found in lambdoid phages. First, like lambda, the SXT element forms a circular extrachromosomal intermediate through specific recombination of the left and right ends of the integrated element. Second, chromosomal integration of the element occurs via site-specific recombination in a 17 bp sequence found in the circular form of the SXT element and a similar 17 bp sequence in prfC. Third, both chromosomal integration and excision of the SXT element were found to require an element-encoded int gene with strong similarities to the lambda integrase family. Based on the properties of the SXT element, we propose to classify this element as a CONSTIN, an acronym for a conjugative, self-transmissible, integrating element.

Structural basis for the differential toxicity of cholera toxin and Escherichia coli heat-labile enterotoxin. Construction of hybrid toxins identifies the A2-domain as the determinant of differential toxicity

Cholera toxin (Ctx) and E. coli heat-labile enterotoxin (Etx) are structurally and functionally similar AB5 toxins with over 80% sequence identity. When their action in polarized human epithelial (T84) cells was monitored by measuring toxin-induced Cl- ion secretion, Ctx was found to be the more potent of the two toxins. Here, we examine the structural basis for this difference in toxicity by engineering a set of mutant and hybrid toxins and testing their activity in T84 cells. This revealed that the differential toxicity of Ctx and Etx was (i) not due to differences in the A-subunit's C-terminal KDEL targeting motif (which is RDEL in Etx), as a KDEL to RDEL substitution had no effect on cholera toxin activity; (ii) not attributable to the enzymatically active A1-fragment, as hybrid toxins in which the A1-fragment in Ctx was substituted for that of Etx (and vice versa) did not alter relative toxicity; and (iii) not due to the B-subunit, as the replacement of the B-subunit in Ctx for that of Etx caused no alteration in toxicity, thus excluding the possibility that the broader receptor specificity of EtxB is responsible for reduced activity. Remarkably, the difference in toxicity could be mapped to a 10-amino acid segment of the A2-fragment that penetrates the central pore of the B-subunit pentamer. A comparison of the in vitro stability of two hybrid toxins, differing only in this 10-amino acid segment, revealed that the Ctx A2-segment conferred a greater stability to the interaction between the A- and B-subunits than the corresponding segment from Etx A2. This suggests that the reason for the relative potency of Ctx compared with Etx stems from the increased ability of the A2-fragment of Ctx to maintain holotoxin stability during uptake and transport into intestinal epithelia.

Endochitinase is transported to the extracellular milieu by the eps-encoded general secretory pathway of Vibrio cholerae

The chiA gene of Vibrio cholerae encodes a polypeptide which degrades chitin, a homopolymer of N-acetylglucosamine (GlcNAc) found in cell walls of fungi and in the integuments of insects and crustaceans. chiA has a coding capacity corresponding to a polypeptide of 846 amino acids having a predicted molecular mass of 88.7 kDa. A 52-bp region with promoter activity was found immediately upstream of the chiA open reading frame. Insertional inactivation of the chromosomal copy of the gene confirmed that expression of chitinase activity by V. cholerae required chiA. Fluorescent analogues were used to demonstrate that the enzymatic activity of ChiA was specific for beta,1-4 glycosidic bonds located between GlcNAc monomers in chitin. Antibodies against ChiA were obtained by immunization of a rabbit with a MalE-ChiA hybrid protein. Polypeptides with antigenic similarity to ChiA were expressed by classical and El Tor biotypes of V. cholerae and by the closely related bacterium Aeromonas hydrophila. Immunoblotting experiments using the wild-type strain 569B and the secretion mutant M14 confirmed that ChiA is an extracellular protein which is secreted by the eps system. The eps system is also responsible for secreting cholera toxin, an oligomeric protein with no amino acid homology to ChiA. These results indicate that ChiA and cholera toxin have functionally similar extracellular transport signals that are essential for eps-dependent secretion.

Replication and integration of a Vibrio cholerae cryptic plasmid linked to the CTX prophage

We identified a 4.7kb cryptic plasmid in all ctxAB+ Vibrio cholerae strains we tested. An isolate of the V. cholerae classical biotype strain 0395 that harbours the cryptic plasmid at high copy number was found. Hybridization analysis demonstrated that sequences highly related or identical to this plasmid exist in all toxigenic strains of V. cholerae but were notably absent in all non-toxigenic environmental isolates that lacked the genes for toxin-co-regulated pili and the filamentous CTX prophage. Accordingly, we have named the cryptic plasmid pTLC for toxin-linked cryptic. The complete nucleotide sequence of pTLC from the high-copy-number isolate was determined. The largest open reading frame in the plasmid is predicted to encode a protein similar to the replication initiation protein (pII) of Escherichia coli F-specific filamentous phages. The nucleotide sequence of pTLC also facilitated the structural characterization of the DNA homologous to pTLC in other strains of V. cholerae. pTLC-related DNA exists in these strains as both low-copy-number, covalently closed circular DNA and tandemly duplicated, chromosomally integrated DNA. Remarkably, the chromosomally integrated form of pTLC is adjacent to the CTX prophage. The strain distribution, chromosomal location and DNA sequence of pTLC suggests that it may be a genetic element that plays some role in the biology of CTXphi, perhaps facilitating either its acquisition or its replication.

HlyA hemolysin of Vibrio cholerae O1 biotype E1 Tor. Identification of the hemolytic complex and evidence for the formation of anion-selective ion-permeable channels

Hemolysin (HlyA) was concentrated from supernatants of different Vibrio cholerae O1 biotype E1 Tor strains by ammonium sulfate precipitation. The concentration of the toxin in the supernatants and in the precipitates was quantified using its hemolytic activity. The toxin formed a high molecular-mass band (about 220 kDa) on SDS/PAGE while the toxin monomer had a molecular mass of 60 kDa when it was heated. The addition of the E1 Tor hemolysin oligomers, but not that of the monomers, to the aqueous phase bathing lipid bilayer membranes resulted in the formation of ion-permeable channels, which had long lifetimes at small voltages. The hemolysin channel had a single-channel conductance of 350 pS in 1 M KCl. These results defined hemolysin (HlyA) from V. cholerae as a channel-forming component with properties similar to other cytolytic toxins. The long lifetime of the channel suggested that the channel-forming oligomer did not show a rapid association/dissociation reaction. At voltages larger than 50 mV, the hemolysin channel was voltage dependent in an asymmetric fashion dependent on the side of its addition. The single-channel conductance of the hemolysin (HlyA) from V. cholerae O1 biotype E1 Tor channel was a linear function of the bulk aqueous conductance, which suggested that the toxin forms aqueous channels with an estimated minimum diameter of about 0.7 nm. The hemolysin channel of V. cholerae was found to be moderately anion-selective. The pore-forming properties of hemolysin (HlyA) from V. cholerae O1 biotype E1 Tor were compared with those of aerolysin of Aeromonas sobria and alpha-toxin from Staphylococcus aureus. All these cytolytic toxins must probably oligomerize for activity in biological and artificial membranes and form anion-selective channels.

Synthesis of hybrid molecules between heat-labile enterotoxin and cholera toxin B subunits: potential for use in a broad-spectrum vaccine

Three variants of the cholera toxin B subunit (CTB) were generated by site-specific mutagenesis in which regions of the mature protein were altered to the composition found at the corresponding positions of the closely related B subunit of the heat-labile enterotoxin of enterotoxigenic Escherichia coli (LTB). The mutant proteins were expressed in Vibrio cholerae and purified from the growth medium. In the first of the mutant proteins, the first 25 amino acids corresponded to the sequence found in LTB, and in the second, changes were made at positions 94 and 95 of the mature protein. The third mutant protein combined the changes made in the first two. Analysis of the immunological properties of these novel proteins by using monoclonal antibodies and absorbed polyclonal antiserum demonstrated that they had acquired LTB-specific epitopes. Immunizations with the mutant proteins resulted in antisera containing LTB-specific as well as CTB-specific and cross-reactive antibodies. The sera were also found to be more strongly cross-reactive in the in vitro neutralization of both cholera toxin and heat-labile enterotoxin than were antisera raised against either CTB or LTB. The results suggest that such hybrid CTB-LTB proteins may be useful in a broad-spectrum vaccine against enterotoxin-induced diarrhea.

Immunologic characteristics of a Streptococcus mutans glucosyltransferase B sucrose-binding site peptide-cholera toxin B-subunit chimeric protein

Glucosyltranferases (Gtfs) produced by the mutans streptococci are recognized as virulence factors in dental caries, and the inhibition of Gtfs by secretory immunoglobulin A is predicted to provide protection against this disease. The basis of such mucosal immunity is linked to the ability to reliably stimulate production of secretory immunoglobulin A against Gtfs. In this regard, we are exploring the immunogenicities of various Gtf peptides genetically fused to the B subunit of cholera toxin (CTB), a known mucosal adjuvant. In this work, we have created a gene fusion linking the GtfB active-site (AS) peptide DANFDSIRVDAVDNVDADLLQIA to the amino terminus of CTB. This sequence, deduced from the nucleotide sequence of gtfB from Streptococcus mutans GS5, has been found to be strongly conserved in Gtfs from several mutans streptococci. We have purified this recombinant protein (AS:CTB) from Escherichia coli carrying the fusion gene under the control of the lactose operon promoter. This protein was immunogenic in rabbits and produced specific serum antibodies against both the Gtf peptide and the CTB moiety. The antiserum was tested for its ability to inhibit GtfB activity obtained from a mutant of S. mutans able to make only this enzyme and none of the other usual Gtfs or fructosyltransferase. Approximately 50% of the GtfB activity was inhibited in such assays. These results suggest that the AS of this enzyme is accessible to antibody binding and that this region of the protein may be considered a vulnerable target for vaccine design and development. The AS:CTB was able to bind GM1, ganglioside in enzyme-linked immunosorbent assays, indicating that the recombinant protein retained this property, which is though to be critical to the mucosal immunoadjuvant properties of CTB. Thus, this protein may be promising as a candidate anticaries vaccinogen alone or in combination with other Gtf peptides or conjugates.

Initial studies of the structural signal for extracellular transport of cholera toxin and other proteins recognized by Vibrio cholerae

The specificity of the pathway used by Vibrio cholerae for extracellular transport of cholera toxin (CT) and other proteins was examined in several different ways. First, V. cholerae was tested for the ability to secrete the B polypeptides of the type II heat-labile enterotoxins of Escherichia coli. Genes encoding the B polypeptide of LT-IIb in pBluescriptKS- phagemids were introduced into V. cholerae by electroporation. Culture supernatants and periplasmic extracts were collected from cultures of the V. cholerae transformants, and the enterotoxin B subunits were measured by an enzyme-linked immunosorbent assay. Results confirmed that the B polypeptides of both LT-IIa and LT-IIb were secreted by V. cholerae with efficiencies comparable to that measured for secretion of CT. Second, the plasmid clones were introduced into strain M14, an epsE mutant of V. cholerae. M14 failed to transport the B polypeptides of LT-IIa and LT-IIb to the extracellular medium, demonstrating that secretion of type II enterotoxins by V. cholerae proceeds by the same pathway used for extracellular transport of CT. These data suggest that an extracellular transport signal recognized by the secretory machinery of V. cholerae is present in LT-IIa and LT-IIb. Furthermore, since the B polypeptide of CT has little, if any, primary amino acid sequence homology with the B polypeptide of LT-IIa or LT-IIb, the transport signal is likely to be a conformation-dependent motif. Third, a mutant of the B subunit of CT (CT-B) with lysine substituted for glutamate at amino acid position 11 was shown to be secreted poorly by V. cholerae, although it exhibited immunoreactivity and ganglioside GM1-binding activity comparable to that of wild-type CT-B. These findings suggest that Glu-11 may be within or near the extracellular transport motif of CT-B. Finally, the genetic lesion in the epsE allele of V. cholerae M14 was determined by nucleotide sequence analysis.

Zonula occludens toxin modulates tight junctions through protein kinase C-dependent actin reorganization, in vitro

The intracellular signaling involved in the mechanism of action of zonula occludens toxin (ZOT) was studied using several in vitro and ex vivo models. ZOT showed a selective effect among various cell lines tested, suggesting that it may interact with a specific receptor, whose surface expression on various cells differs. When tested in IEC6 cell monolayers, ZOT-containing supernatants induced a redistribution of the F-actin cytoskeleton. Similar results were obtained with rabbit ileal mucosa, where the reorganization of F-actin paralleled the increase in tissue permeability. In endothelial cells, the cytoskeletal rearrangement involved a decrease of the soluble G-actin pool (-27%) and a reciprocal increase in the filamentous F-actin pool (+22%). This actin polymerization was time- and dose-dependent, and was reversible. Pretreatment with a specific protein kinase C inhibitor, CGP41251, completely abolished the ZOT effects on both tissue permeability and actin polymerization. In IEC6 cells ZOT induced a peak increment of the PKC-alpha isoform after 3 min incubation. Taken together, these results suggest that ZOT activates a complex intracellular cascade of events that regulate tight junction permeability, probably mimicking the effect of physiologic modulator(s) of epithelial barrier function.

Cholera

Despite more than a century of study, cholera still presents challenges and surprises to us. Throughout most of the 20th century, cholera was caused by Vibrio cholerae of the O1 serogroup and the disease was largely confined to Asia and Africa. However, the last decade of the 20th century has witnessed two major developments in the history of this disease. In 1991, a massive outbreak of cholera started in South America, the one continent previously untouched by cholera in this century. In 1992, an apparently new pandemic caused by a previously unknown serogroup of V. cholerae (O139) began in India and Bangladesh. The O139 epidemic has been occurring in populations assumed to be largely immune to V. cholerae O1 and has rapidly spread to many countries including the United States. In this review, we discuss all aspects of cholera, including the clinical microbiology, epidemiology, pathogenesis, and clinical features of the disease. Special attention will be paid to the extraordinary advances that have been made in recent years in unravelling the molecular pathogenesis of this infection and in the development of new generations of vaccines to prevent it.