Nucleotide sequence analysis of the A2 and B subunits of Vibrio cholerae enterotoxin

Caco-2/HT29-MTX co-cultured cells as a model for studying physiological properties and toxin-induced effects on intestinal cells

Infectious gastrointestinal diseases are frequently caused by toxins secreted by pathogens which may impair physiological functions of the intestines, for instance by cholera toxin or by heat-labile enterotoxin. To obtain a functional model of the human intestinal epithelium for studying toxin-induced disease mechanisms, differentiated enterocyte-like Caco-2 cells were co-cultured with goblet cell-like HT29-MTX cells. These co-cultures formed a functional epithelial barrier, as characterized by a high electrical resistance and the presence of physiological intestinal properties such as glucose transport and chloride secretion which could be demonstrated electrophysiologically and by measuring protein expression. When the tissues were exposed to cholera toxin or heat-labile enterotoxin in the Ussing chamber, cholera toxin incubation resulted in an increase in short-circuit currents, indicating an increase in apical chloride secretion. This is in line with typical cholera toxin-induced secretory diarrhea in humans, while heat-labile enterotoxin only showed an increase in short-circuit-current in Caco-2 cells. This study characterizes for the first time the simultaneous measurement of physiological properties on a functional and structural level combined with the epithelial responses to bacterial toxins. In conclusion, using this model, physiological responses of the intestine to bacterial toxins can be investigated and characterized. Therefore, this model can serve as an alternative to the use of laboratory animals for characterizing pathophysiological mechanisms of enterotoxins at the intestinal level.

Development of a dry reagent-based triplex PCR for the detection of toxigenic and non-toxigenic Vibrio cholerae

Vibrio cholerae has caused severe outbreaks of cholera worldwide with thousands of recorded deaths annually. Molecular diagnosis for cholera has become increasingly important for rapid detection of cholera as the conventional methods are time-consuming and labour intensive. However, traditional PCR tests still require cold-chain transportation and storage as well as trained personnel to perform, which makes them user-unfriendly. The aim of this study was to develop a thermostabilized triplex PCR test for cholera which is in a ready-to-use form and requires no cold chain. The PCR test specifically detects both toxigenic and non-toxigenic strains of V. cholerae based on the cholera toxin A (ctxA) and outer-membrane lipoprotein (lolB) genes. The thermostabilized triplex PCR also incorporates an internal amplification control that helps to check for PCR inhibitors in samples. PCR reagents and the specific primers were lyophilized into a pellet form in the presence of trehalose, which acts as an enzyme stabilizer. The triplex PCR was validated with 174 bacteria-spiked stool specimens and was found to be 100 % sensitive and specific. The stability of the thermostabilized PCR was evaluated using the Q10 method and it was found to be stable for approximately 7 months at 24 °C. The limit of detection of the thermostabilized triplex PCR assay was 2×104 c.f.u. at the bacterial cell level and 100 pg DNA at the genomic DNA level, comparable to conventional PCR methods. In conclusion, a rapid thermostabilized triplex PCR assay was developed for detecting toxigenic and non-toxigenic V. cholerae which requires minimal pipetting steps and is cold chain-free.

Cholera toxin: A paradigm for multi-functional engagement of cellular mechanisms (Review)

Cholera toxin (Ctx) from Vibrio cholerae and its closely related homologue, heat-labile enterotoxin (Etx) from Escherichia coli have become superb tools for illuminating pathways of cellular trafficking and immune cell function. These bacterial protein toxins should be viewed as conglomerates of highly evolved, multi-functional elements equipped to engage the trafficking and signalling machineries of cells. Ctx and Etx are members of a larger family of A-B toxins of bacterial (and plant) origin that are comprised of structurally and functionally distinct enzymatically active A and receptor-binding B sub-units or domains. Intoxication of mammalian cells by Ctx and Etx involves B pentamer-mediated receptor binding and entry into a vesicular pathway, followed by translocation of the enzymatic A1 domain of the A sub-unit into the target cell cytosol, where covalent modification of intracellular targets leads to activation of adenylate cyclase and a sequence of events culminating in life-threatening diarrhoeal disease. Importantly, Ctx and Etx also have the capacity to induce a wide spectrum of remarkable immunological processes. With respect to the latter, it has been found that these toxins activate signalling pathways that modulate the immune system. This review explores the complexities of the cellular interactions that are engaged by these bacterial protein toxins, and highlights some of the new insights to have recently emerged.

Biochemical and immunological characterization of the plant-derived candidate human immunodeficiency virus type 1 mucosal vaccine CTB-MPR

Plants are potentially the most economical platforms for the large-scale production of recombinant proteins. Thus, plant-based expression of subunit human immunodeficiency virus type 1 (HIV-1) vaccines provides an opportunity for their global use against the acquired immunodeficiency syndrome pandemic. CTB-MPR(649-684)[CTB, cholera toxin B subunit; MPR, membrane proximal (ectodomain) region of gp41] is an HIV-1 vaccine candidate that has been shown previously to induce antibodies that block a pathway of HIV-1 mucosal transmission. In this article, the molecular characterization of CTB-MPR(649-684) expressed in transgenic Nicotiana benthamiana plants is reported. Virtually all of the CTB-MPR(649-684) proteins expressed in the selected line were shown to have assembled into pentameric, GM1 ganglioside-binding complexes. Detailed biochemical analyses on the purified protein revealed that it was N-glycosylated, predominantly with high-mannose-type glycans (more than 75%), as predicted from a consensus asparagine-X-serine/threonine (Asn-X-Ser/Thr) N-glycosylation sequon on the CTB domain and an endoplasmic reticulum retention signal attached at the C-terminus of the fusion protein. Despite this modification, the plant-expressed protein retained the nanomolar affinity to GM1 ganglioside and the critical antigenicity of the MPR(649-684) moiety. Furthermore, the protein induced mucosal and serum anti-MPR(649-684) antibodies in mice after mucosal prime-systemic boost immunization. Our data indicate that plant-based expression can be a viable alternative for the production of this subunit HIV-1 vaccine candidate.

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.

Relationships between Bacteroides 16S rRNA genetic markers and presence of bacterial enteric pathogens and conventional fecal indicators

Occurrence and prevalence of different bacterial enteric pathogens as well as their relationships with conventional (total and fecal coliforms) and alternative fecal indicators (host-specific Bacteroides 16S rRNA genetic markers) were investigated for various water samples taken from different sites with different degrees of fecal contamination. The results showed that a wide range of bacterial pathogens could be detected in both municipal wastewater treatment plant samples and in surface water samples. Logistic regression analysis revealed that total and human-specific Bacteroides 16S rRNA genetic markers showed significant predictive values for the presence of Escheriachia coli O-157, Salmonella, heat-labile enterotoxin (LT) of enterotoxigenic E. coli (ETEC), and heat-stable enterotoxin for human (STh) of ETEC. The probability of occurrence of these pathogenic bacteria became significantly high when the concentrations of human-specific and total Bacteroides 16S rRNA genetic markers exceeded 10(3) and 10(4) copies/100 mL. In contrast, Clostridium perfringens was detected at high frequency regardless of sampling sites and levels of Bacteroides 16S rRNA genetic markers. No genes related to Shigella spp., Staphylococcus aureus and Vibrio cholerae were detected in any samples analyzed in this study. Conventional indicator microorganisms had low levels of correlation with the presence of pathogens as compared with the alternative fecal indicators. These results suggested that real-time PCR-based measurement of alternative Bacteroides 16S rRNA genetic markers was a rapid and sensitive tool to identify host-specific fecal pollution and probably associated bacterial pathogens. However, since one fecal indicator might not represent the relative abundance of all pathogenic bacteria, viruses and protozoa, combined application of alternative indicators with conventional ones could provide more comprehensive pictures of fecal contamination, its source and association with pathogenic microorganisms.

Pathophysiology and impact of enteric bacterial and protozoal infections: new approaches to therapy

Despite numerous scientific advances in the past few years regarding the pathogenesis, diagnostic tools and treatment of infectious enteritis, enteric infections remain a serious threat to health worldwide. With globalization of the food supply, the increase in travel, mass food processing and antibiotic resistance, infectious diarrhea has become a critical concern for both developing and developed countries. Oral rehydration therapy has been cited as the most important medical discovery of the century due to the millions of lives that have been saved. However, statistics concerning diarrhea-induced mortality and the highly underestimated morbidity continue to demonstrate the severity of the problem. A more complete understanding of the pathogenesis of infectious diarrhea and potential new vaccines and effective treatments are badly needed. In addition, public health preventive actions, such as early detection of outbreaks, care with food, water and sanitation and, where relevant, immunization, should be considered a priority. This article provides an overview of the epidemiological impact, pathogenesis and new approaches to the management of enteric infections.

Cholera toxin B-subunit gene enhances mucosal immunoglobulin A, Th1-type, and CD8+ cytotoxic responses when coadministered intradermally with a DNA vaccine

A plasmid vector encoding the cholera toxin B subunit (pCtB) was evaluated as an intradermal genetic adjuvant for a model DNA vaccine expressing the human papillomavirus type 16 L1 capsid gene (p16L1) in mice. p16L1 was coadministered with plasmid pCtB or commercial polypeptide CtB as a positive control. Coadministration of pCtB induced a significant increment of specific anti-L1 immunoglobulin A (IgA) antibodies in cervical secretions (P < 0.05) and fecal extracts (P < 0.005). Additionally, coadministration of pCtB enhanced the production of interleukin-2 and gamma interferon by spleen cells but did not affect the production of interleukin-4, suggesting a Th1-type helper response. Furthermore, improved CD8+ T-cell-mediated cytotoxic activity was observed in mice vaccinated with the DNA vaccine with pCtB as an adjuvant. This adjuvant effect was comparable to that induced by the CtB polypeptide. These results indicate that intradermal coadministration of pCtB is an adequate means to enhance the mucosa-, Th1-, and CD8(+)-mediated cytotoxic responses induced by a DNA vaccine.

Progress toward the development of a bacterial vaccine vector that induces high-titer long-lived broadly neutralizing antibodies against HIV-1

Conformationally constrained HIV-1 Env and gp120 immunogens induce broadly cross-reactive neutralizing antibodies. Thus, it is now feasible to rationally design an HIV-1 vaccine that affords protection through humoral mechanisms. This paper reviews our progress toward the development of an oral bacterial vaccine vector that is capable of delivering an HIV-1 DNA vaccine to host lymphoid tissues and inducing broadly neutralizing antibodies to HIV-1 in the mucosal and systemic immune compartments.

Soft docking an L and a D peptide to an anticholera toxin antibody using internal coordinate mechanics

BACKGROUND

The tremendous increase in sequential and structural information is a challenge for computer-assisted modelling to predict the binding modes of interacting biomolecules. One important area is the structural understanding of protein-peptide interactions, information that is increasingly important for the design of biologically active compounds.

RESULTS

We predicted the three-dimensional structure of a complex between the monoclonal antibody TE33 and its cholera-toxin-derived peptide epitope VPGSQHID. Using the internal coordinate mechanics (ICM) method of flexible docking, the bound conformation of the initially extended peptide epitope to the antibody crystal or modelled structure reproduced the known binding conformation to a root mean square deviation of between 1.9 A and 3.1 A. The predicted complexes are in good agreement with binding data obtained from substitutional analyses in which each epitope residue is replaced by all other amino acids. Furthermore, a de novo prediction of the recently discovered TE33-binding D peptide dwGsqhydp (single-letter amino acid code where D amino acids are represented by lower-case letters) explains results obtained from binding studies with 172 peptide analogues.

CONCLUSIONS

Despite the difficulties arising from the huge conformational space of a peptide, this approach allowed the prediction of the correct binding orientation and the majority of essential binding features of a peptide-antibody complex.

Characterisation of cholera toxin by liquid chromatography--electrospray mass spectrometry

Cholera toxin, one of the toxins that may be generated by various strains of the bacterium Vibrio cholerae, can be considered as a substance possibly used in biological warfare. The possibilities of characterising the toxin by liquid chromatography electrospray mass spectrometry (LC-ES-MS) were investigated. The toxin can be detected by flow-injection (FIA) ES-MS of a dialysed solution and observation of the charge envelope signals of its A-unit and B-chain protein; sufficient information for identification by the molecular mass of either protein could be obtained for quantities in the order of 10 fmol. Confirmatory analysis was carried out by 2-mercaptoethanol reduction and FIA-ES-MS detection of the product proteins or by tryptic digest LC-ES-MS with ion chromatogram detection of most of the tryptic fragments of the A-unit and B-chain from the singly, doubly or triply charged ion signals. The confirmatory tryptic digest LC-ES-MS analysis could be achieved with quantities as low as 1 pmol. Possible biovariations in the toxin can mostly be determined by sequencing, where the amino acid composition of tryptic fragments of the A1-chain, T5 and T15, and of the B-chain, T1, T4 and T5, cover all known biovariations. Partial sequencing of cholera toxin, originating from a classical strain, O1/569B, was achieved by LC-ES-MS/MS of most tryptic fragments larger than three amino acid residues.

The Vibrio cholerae genome contains two unique circular chromosomes

Vibrio cholerae, the etiologic agent of the diarrheal disease cholera, is a Gram-negative bacterium that belongs to the gamma subdivision of the family Proteobacteriaceae. The physical map of the genome has been reported, and the genome has been described as a single 3.2-Mb chromosome [Majumder, R., et al. (1996) J. Bacteriol. 178, 1105-1112]. By using pulsed-field gel electrophoresis of genomic DNA immobilized in agarose plugs and digested with the restriction enzymes I-CeuI, SfiI, and NotI, we have also constructed the physical map of V. cholerae. Our analysis estimates the size of the genome at 4.0 Mb, 25% larger than the physical map reported by others. Our most notable finding is, however, that the V. cholerae chromosome appears to be not the single chromosome reported but two unique and separate circular megareplicons.

Differences among marine and hospital strains of Vibrio cholerae during Peruvian epidemic

During a period of 18 months of an epidemic of Vibrio cholerae, cultures from 450 samples of fish, shellfish and seawater were isolated. The highest frequencies of occurrence observed were 5.2% in fish from inshore waters, 3.9% in marine snails, and 1.8% in mussels and crabs. No incidents were isolated from cultures of fish in the open seas or cultures from frozen shrimp. Cultures of marine origin were compared with cultures from hospitalized patients, and these revealed marked serological and toxigenic differences. Marine strains were mainly non-O1 V. cholerae, non toxigenic. We presume fishing off-shore not to be the cause of this outbreak. However, marine species from contaminated waters could contain toxigenic V. cholerae remaining viable and potentially pathogenic. Methods used were more sensitive and specific for detecting marine strains. In this paper the need to use more specific methods is discussed.

Evaluation of a bivalent (CVD 103-HgR/CVD 111) live oral cholera vaccine in adult volunteers from the United States and Peru

To provide optimum protection against classical and El Tor biotypes of Vibrio cholerae O1, a single-dose, oral cholera vaccine was developed by combining two live, attenuated vaccine strains, CVD 103-HgR (classical, Inaba) and CVD 111 (El Tor, Ogawa). The vaccines were formulated in a double-chamber sachet; one chamber contained lyophilized bacteria, and the other contained buffer. In the first study, 23 U.S. adult volunteers received CVD 103-HgR at 10(8) CFU plus CVD 111 at 10(8), 10(7), or 10(6) CFU, CVD 111 alone at 10(7) CFU, or placebo. In the second study, 275 Peruvian adults were randomized to receive CVD 103-HgR at 10(9) CFU plus CVD 111 at 10(9) or 10(8) CFU, CVD 111 alone at 10(9) CFU, CVD 103-HgR alone at 10(9) CFU, or placebo. Three of 15 U.S. volunteers who received CVD 111 at 10(7) or 10(8) CFU developed mild diarrhea, compared to none of 4 who received CVD 111 at 10(6) CFU and 1 of 4 who received placebo. Twelve (63%) of 19 vaccine recipients shed the El Tor vaccine strain. All but one volunteer developed significant Ogawa and Inaba vibriocidal antibody titers. Volunteers who received CVD 111 at 10(7) CFU had geometric mean Ogawa titers four to five times higher than those of volunteers who received the lower dose. In the second study, all dosage regimens were well tolerated in Peruvians. About 20% of volunteers who received CVD 111 at the high dose excreted the El Tor organism, compared to 7% in the low-dose group. CVD 111 was detected in the stools of two placebo recipients, neither of whom had symptoms or seroconverted. In all vaccine groups, 69 to 76% developed fourfold rises in Inaba vibriocidal antibodies. Among those who received the bivalent vaccine, 53 to 75% also developed significant rises in Ogawa vibriocidal antibodies. We conclude that it is feasible to produce a single-dose, oral bivalent vaccine that is safe and immunogenic against both biotypes (El Tor and classical) and both serotypes (Inaba and Ogawa) of cholera for populations in both developed and developing parts of the world.

Probes and polymerase chain reaction for detection of food-borne bacterial pathogens

DNA-hybridization and the polymerase chain reaction (PCR) are techniques commonly used to detect pathogenic bacteria. In this paper, the use of these techniques for detection of Salmonella, E. coli, V. cholerae, non-O1 Vibrio, Yersinia enterocolitica, Campylobacter, Listeria monocytogenes, Staphylococcus aureus, Bacillus cereus, Clostridium perfringens, and C. botulinum is reviewed with emphasis on application in food microbiology. In food control, DNA-techniques have most often been used in a 'culture confirmation' fashion, i.e. bacteria are enriched and sometimes even purified by traditional culture procedures and thereafter identified by the use of DNA-based methods. The most desirable approach is, however, to detect organisms directly in the food, but major problems remain to be solved before this can be routinely performed.

Characterization of phenotypic, serological, and toxigenic traits of Vibrio cholerae O139 bengal

Biochemical and physiological traits of a collection of strains of Vibrio cholerae O139 Bengal isolated from India, Bangladesh, and Thailand showed that these strains formed a phenotypically homogeneous group with identical characteristics that were essentially similar to those of the O1 serogroup. Resistance to 150 micrograms of the vibriostatic agent O/129 (2,4-diamino-6,7-diisopropylpteridine) and Mukherjee's El Tor phage 5 and classical phage IV and the nonagglutinability of the strains with O1 antiserum were the only discernible differences between the O139 and O1 serogroups. Extensive serological characterization further revealed the O139 serogroup to be distinct from the existing 138 serogroups of V. cholerae. Antiserum raised against the O139 serogroup required absorption with the R reference strain CA385 and with the reference strain representing serogroup O22 to remove cross-reacting agglutinins. All of the 223 representative strains of V. cholerae O139 examined hybridized with DNA probes specific for the cholera toxin (CT) gene, zonula occludens toxin gene, and El Tor hemolysin gene but not with the probe specific for the heat-stable enterotoxin gene. The amount of CT present in stool samples of patients infected with the O139 serogroup was higher than that found in stools of patients infected with O1 El Tor, and this echoed findings that the amount of CT produced by O139 strains in vitro was higher than that produced by the O1 El Tor strains. The nucleotide sequences of the genes encoding the A and B subunits of CT of the O139 serogroup were identical to the sequences reported for the CT gene of O1 El Tor. The CT gene of O139 strains could be amplified by using primers developed for detection of the CT gene of the O1 serogroup by a PCR assay, which could also be used to detect the CT gene in stool samples of patients infected with strains of the O139 serogroup.

Cloning and sequencing of the genes encoding Escherichia coli cytolethal distending toxin

Escherichia coli strains expressing cytolethal distending toxin (CDT) cause elongation of CHO cells at 24 h, followed by progressive cellular distention and death for up to 120 h. Similar distention and cytotoxicity are seen in HeLa, HEp-2, and, to a lesser extent, Vero cells. The initial elongation in CHO cells is indistinguishable from that caused by E. coli heat-labile toxin (LT). In contrast to those from LT strains, supernatants from these strains have no effect on Y-1 adrenal cells. TnphoA was introduced into CDT-positive E. coli E6468/62 (O86:H34), isolated from a child with diarrhea, and 13 CDT-negative transconjugants were identified. DNA probes constructed from DNA flanking the TnphoA insertion sites of CDT-negative mutants were used to identify a CDT-positive clone from an E6468/62 genomic library with a 5.5-kb insert. Exonuclease deletions were created and assayed in CHO cells. In this manner, a 2.3-kb CDT-active region was defined, and the nucleotide sequence was determined. Sequence analysis identified three open reading frames (ORFs), designated cdtA, cdtB, and cdtC. These contain 711, 819, and 570 bp, respectively, and encode polypeptides with predicted molecular masses of 25.5, 29.8, and 20.3 kDa, respectively. Each ORF has a putative signal sequence, and there are 4-bp overlaps between cdtA and cdtB and between cdtB and cdtC. The nucleotide and predicted amino acid sequences have no significant homology with those of any previously reported genes or proteins. By in vitro transcription-translation and an anti-alkaline phosphatase immunoblot, native proteins and/or fusion proteins corresponding to each ORF were identified.

The secretion pathway of IgA protease-type proteins in gram-negative bacteria

The pathogenic, Gram-negative bacteria, Neisseria gonorrhoeae, Neisseria meningitidis and Haemophilus influenzae, secrete immunoglobulin A1 proteases into their extracellular surroundings. An extraordinary feature in the secretory pathway of these putative virulence factors is a self-directed outer membrane transport step allowing the proteins to be secreted autonomously, even from foreign Gram-negative host cells like Escherichia coli. Here we summarize recent achievements in the understanding of IgA protease outer membrane translocation.

Detection of a heat-labile enterotoxin gene in enterotoxigenic Escherichia coli by densitometric evaluation using highly specific enzyme-linked oligonucleotide probes

Two alkaline phosphatase-conjugated 24-mer oligonucleotide probes were developed to detect the heat-labile enterotoxin gene in enterotoxigenic Escherichia coli. Probes were antisense codon sequences, which are transcribed into mRNA, of the heat-labile enterotoxin gene of enterotoxigenic Escherichia coli of human origin. Using dot-blot hybridization, probes were tested with 100 clinical isolates and evaluated by a reflectance-type densitometer. Results agreed very well with those of an immunological test, the Biken test, and a 32P-labelled recombinant DNA probe. The oligonucleotide probes did not react with nucleic acids prepared from other diarrhoeagenic bacterial pathogens. Thus, the alkaline phosphatase-conjugated oligonucleotide probes seem to be highly sensitive and specific for detection of heat-labile enterotoxin-producing enterotoxigenic Escherichia coli. Moreover, the results indicate a potential usefulness for densitometric evaluation of DNA hybridization.

Nucleotide sequence analysis of the CT operon of the Vibrio cholerae classical strain 569B

The complete nucleotide sequence of the Vibrio cholerae classical strain 569B was determined. The results prove the exactness of the amino acid CT B sequence published by Takao et al. (1985, Eur. J. Biochem. 146, 503-508). A comparison is made with already reported CT genes.

Recombinant attenuated Vibrio cholerae strains used as live oral vaccines

Although great strides have been made in the development of recombinant attenuated Vibrio cholerae vaccine strains, the task has not been as simple as once imagined. The initial vaccine candidates proved to be unexpectedly reactogenic but further derivatives, such as CVD103-HgR, are well-tolerated, immunogenic and protective after a single dose. In addition, this strain carries a selectable marker to distinguish it from wild strains and has been evaluated in a practical, lyophilized formulation (Levine et al., 1988b). While CVD103-HgR is being further evaluated in expanded trials, we are also investigating a new secretogenic factor which could possibly explain the diarrhoea seen with the earlier vaccine strains. Hopefully, these studies will achieve the long-sought goal of a safe and effective vaccine for the prevention of cholera.

Expression in Escherichia coli of two mutated genes encoding the cholera toxin B subunit

To allow subsequent genetically mediated fusion of foreign antigens to cholera toxin B subunit (CTB), two mutated CTB encoding genes (ctxB) were constructed and overexpressed in Escherichia coli. The signal peptide coding sequence was deleted and restriction sites were created at both ends of the modified sequence. Both synthesized CTBs contain additional amino acid(s) at the N terminus (one and three). They were purified as insoluble products and refolded into the natural pentameric CTB structure by a denaturation-renaturation cycle. After renaturation, both recombinant proteins recovered CTB antigenicity and the ability to bind to GM1 gangliosides, as shown by in vitro analysis. Preliminary data indicated that both properties were unaltered by fusion of a foreign peptide to the mutated CTBs.

Plasmid vectors for constructing translational fusions to the B subunit of cholera toxin

A family of plasmid cloning vectors has been developed for creating translational fusions to the ctxB gene encoding the B subunit of cholera toxin (CTB) in Escherichia coli. These vectors permit insertion of transcriptionally and translationally competent gene sequences upstream from ctxB. To test the utility of the system, a portion of the glucosyltransferase B (GTF) gene (gtfB) from the cariogenic bacterium Streptococcus mutans GS-5 (Bratthall serotype c), encoding the N-terminal one-third of the protein, was inserted into each vector. E. coli lysates containing the constructs were partially purified by passage over a GM1 ganglioside affinity column. Western blotting analysis of the column retentate from one of the lysates revealed the presence of a novel 58-kDa protein which cross-reacted with antisera to GTF and CTB. These vectors are of general use for making other translational fusions to ctxB. The high binding affinity of CTB can be exploited in purifying large polypeptides fused to this relatively small protein. Moreover, these vectors can be used to create neoantigens with altered immunogenicity for use in polypeptide-based vaccines.

A molecular epidemiological study of Vibrio cholerae in Australia

Two hundred and eight strains of Vibrio cholerae serovar O1 and 454 non-O1 strains, which were isolated within or imported into Australia, were tested for the production of cholera enterotoxin and for hybridization with gene probes for cholera enterotoxin. Genetic relationships among representative isolates were studied by a comparison of the restriction-endonuclease digest patterns of the chromosomal DNA and by Southern-blot analysis of toxigenic strains with the cholera-toxin gene probe. V. cholerae O1 isolates from patients and from the environment were predominantly toxigenic in contrast with the non-O1 isolates. The local O1 isolates were found to be diverse genetically and were unrelated to the imported O1 isolates, to those from other endemic areas and to non-O1 isolates.

Campylobacter jejuni chromosomal sequences that hybridize to Vibrio cholerae and Escherichia coli LT enterotoxin genes

Campylobacter jejuni is one of the main etiologic agents of gastrointestinal illness in developing and developed areas throughout the world. Isolation of enterotoxin-producing C. jejuni has been associated with clinical symptoms of a watery-secretory type of diarrhea. Although physiological and immunological relatedness has been demonstrated between the C. jejuni enterotoxin (CJT), the Vibrio cholerae enterotoxin (CT), and the heat-labile cholera-like Escherichia coli enterotoxin (LT), nucleotide sequence similarity between C. jejuni DNA and either the toxA, toxB, eltA or eltB genes remained to be shown. We found that binding to ganglioside GM1 prevented recognition of CJT by monoclonal antibodies directed to either CT or LT. This indicates antigenic similarity between the three enterotoxins in the ganglioside GM1-binding site. Therefore we searched for corresponding similarities at the DNA level and found, by oligodeoxynucleotide hybridization, C. jejuni chromosomal nucleotide sequences similar to the coding region for a postulated ganglioside GM1-binding site on toxB and eltB.

Cholera toxin and its subunits as potential oral adjuvants

Cholera toxin has been shown to have adjuvant effects in multiple different systems. The dose, timing and genetic background of the recipient all seem to be important variables. The role of the two subunits in both the immunogenicity and the adjuvanticity of this molecule remain unclear. The mechanisms of the adjuvant effect likely involves effects on regulatory T cells; there is evidence that the adjuvant effect is due at least in part to inhibition of suppressor T cells. When KLH is used as a model antigen, the adjuvanticity of cholera toxin appears to be related to its immunogenicity in that both properties occur mainly in mouse strains that are high responders to cholera toxin. The genetic engineering of chimeric neoantigens consisting of cholera toxin subunits coupled to antigens of interest has been shown to be technically possible and is an attractive future approach for the generation of effective oral vaccines.

Molecular cloning of a Salmonella typhi LT-like enterotoxin gene

Diarrhoea is a common event during typhoid fever; nevertheless, the possible participation of a diarrhoea-inducing enterotoxin has not been described (Roy et al., 1985). Recombinant bacteriophage lambda FDC1 was isolated from a genomic library of Salmonella typhi, the causal agent of typhoid fever, by screening with a probe for the B subunit gene of the heat-labile, cholera-like, Escherichia coli enterotoxin (LT). Lambda FDC1 codes for an enterotoxin that causes secretion in rat ileal loops, that elongates Chinese hamster ovary (CHO) cells, that is recognized by antibodies against LT, and does not bind in vitro to ganglioside GM1. These results should allow further studies towards elucidating a possible role for the S. typhi enterotoxin in the pathogenesis of typhoid fever.

Identity of molecular structure of Shiga-like toxin I (VT1) from Escherichia coli O157:H7 with that of Shiga toxin

The primary structures of the A and B subunits of Shiga toxin and of Shiga-like toxin I (VT1), isolated from the culture supernatants of Shigella dysenteriae 1 and Escherichia coli O157:H7, respectively, were analyzed by Edman degradation of intact proteins and peptides in their digests with trypsin or Achromobacter protease I and also by fast atom bombardment mass spectrometry of the digests. The results indicated that the A and B subunits of Shiga toxin and Shiga-like toxin I have the same primary structures. The identity of their primary structures was confirmed by determining the nucleotide sequence of the gene encoding Shiga-like toxin I cloned from a Shiga-like toxin I converting phage. This nucleotide sequence was different from that reported by Jackson et al. (Microbial Pathogenesis 1987; 2: 147-153), by Calderwood et al. (Proc Natl Acad Sci USA 1987; 84: 4364-8) and by Grandis et al. (J Bacteriol 1987; 169: 4313-9) in one base at position 231, which was found to be adenine instead of thymine, which they reported. The amino acid residue at position 45 from the N-terminus of the A subunit of Shiga-like toxin I deduced from the nucleotide sequence determined in this study is threonine, which corresponds with that found by amino acid sequencing, whereas from previous reports by other investigators it is serine. Edman degradation of the intact A subunit of Shiga toxin indicated that the A subunit was nicked between Ala253 and Ser254 to form A1 and A2 fragments linked by a disulfide bond.

Nucleotide sequence of the invasion plasmid antigen B and C genes (ipaB and ipaC) of Shigella flexneri

The nucleotide sequence of a 4.8 kilobase (kb) HindIII fragment from pWR100, the virulence plasmid of Shigella flexneri 5, was determined and analysed. This fragment encodes polypeptides b (62 kilodalton, kD) and c (43 kD) which have already been described as two of the four immunogenic polypeptides of Shigellae. The nucleotide sequence revealed that in addition to the ipaB and ipaC genes encoding polypeptides b and c, a third complete open reading frame was found within the fragment. The gene, named ippI, encoded a 17 kD polypeptide. The deduced amino acids sequence of polypeptides b and c showed no signal peptide but presence of highly hydrophobic domains compatible with a transmembraneous location. The surprising A and T richness of the three genes as compared with the Escherichia coli and Shigella genomes, resulted in a biased codon usage, and raises the question of the origin of the sequences.

Genetics of type IIa heat-labile enterotoxin of Escherichia coli: operon fusions, nucleotide sequence, and hybridization studies

Operon fusions for the Escherichia coli heat-labile enterotoxin type IIa (LT-IIa) operon were isolated and characterized. The LT-IIa genes are organized in a transcriptional unit similar to those of cholera toxin (CT) and the closely related E. coli heat-labile toxin type I (LT-I, with subtypes LTh-I and LTp-I). The nucleotide sequence of the LT-IIa genes was determined and compared with the sequences of LTh-I and CT. The A subunit gene of LT-IIa was found to be 57% homologous with the A subunit gene of LTh-I and 55% homologous with the A gene of CT. Most of the homology derived from the region of the A gene which encodes the A1 fragment. The B gene of LT-IIa was not homologous with the B gene of LTh-I or CT. DNA probes containing various portions of the LT-IIa genes and adjacent sequences were used for hybridization studies with restriction endonuclease fragments of DNA from a collection of LT-II-producing strains. These studies showed that a probe containing much of the A subunit gene hybridized well to DNA from the various strains, but a probe for the B subunit gene did not.

A unique amino acid sequence of the B subunit of a heat-labile enterotoxin isolated from a human enterotoxigenic Escherichia coli

The purified B subunit of heat-labile enterotoxin produced from a human strain, 240-3, of enterotoxigenic Escherichia coli (LTh(240-3] was carboxymethylated, succinylated, digested with chymotrypsin and subjected to high performance liquid chromatography (HPLC), and the amino acid compositions of the peptide peaks from the column were analyzed and compared with the data reported by Yamamoto and Yokota (J. Bacteriol. 155, 728.1983), who deduced the amino acid sequence of LTh(H10407) from the DNA sequence of a human strain H10407. Only one fraction differed in amino acid composition from that reported by them. This fraction was found to consist of peptides with the sequences Arg-Asn-Thr-Gln-Ile-Tyr and Arg-Ile-Ala-Tyr. Yamamoto and Yokota reported the sequence of the latter peptide as Arg-Ile-Thr*-Tyr, which corresponds to the peptide from 73rd to 76th from amino (N-) terminus. Thus amino acid residue 75 from the N-terminus of LTh-B(240-3) is alanine, not threonine. The B subunit of cholera toxin also has alanine at position 75. LTh(240-3) appeared similar to LTh(H10407) in an Ouchterlony test, vascular permeability test and GMI ganglioside ELISA. These data show that substitution of threonine for alanine at position 75 from the N-terminus does not affect the immunological and biological characteristics of LTh.

Evolutionary origin of pathogenic determinants in enterotoxigenic Escherichia coli and Vibrio cholerae O1

Three families of the evolutionarily related pathogenic determinants in enterotoxigenic Escherichia coli and Vibrio cholerae O1, a family of cholera enterotoxin (CT) and heat-labile enterotoxin (LT) including CT, LTh, and LTp, a family of heat-stable enterotoxin I (STI) including STIa and STIb, and a family of K88 enteroadhesion fimbriae including K88ab, K88ac, and K88ad were analyzed for synonymous (silent) nucleotide substitutions by using the gene nucleotide sequences of earlier reports and the LTp gene nucleotide sequence presented in this paper. The data suggested that the divergences between LT and CT and between STIa and STIb occurred in the remote past, whereas those between LTh and LTp and between members of the K88 family occurred very recently. We concluded that the LT gene is a foreign gene that has been acquired by E. coli to form an enteropathogen. This provides evolutionary evidence of species-to-species transfer of pathogenic determinants in procaryotes.

Nucleotide sequence analysis of the structural genes for Shiga-like toxin I encoded by bacteriophage 933J from Escherichia coli

A nucleotide sequence analysis was performed on the Shiga-like toxin I genes previously cloned from the Escherichia coli bacteriophage 933J. Two structural genes designated slt-I A and slt-I B were found to be oriented on a single transcriptional unit with slt-I A preceding slt-I B. A 12 base pair gap separated the in-phase open reading frames of slt-I A and slt-I B. Putative ribosome binding sites were identified 5' to both the slt-I A and slt-I B genes. Translation of the SLT-I nucleotide sequence revealed that both the A and B subunits were synthesized with signal peptides. The molecular weight calculated for the mature A subunit was 32,211 while the molecular weight of the mature B subunit was 7690.

Variation in chemical properties and antigenic determinants among type II heat-labile enterotoxins of Escherichia coli

Type II heat-labile enterotoxin (LT-II) from Escherichia coli 41 was purified and compared with prototype LT-II encoded by genes from E. coli SA53. Both toxins were oligomeric proteins consisting of polypeptides A (Mr, 28,000) and B (Mr, 11,800). The A polypeptides were cleaved by trypsin into fragments A1 (Mr, 21,000) and A2 (Mr, about 7,000). These two toxins were shown to belong to two different subclasses of LT-II. We propose to designate the prototype toxin LT-IIa and the new variant LT-IIb. The pI of LT-IIb was between 5.2 and 5.6, significantly lower than the pI of 6.8 for LT-IIa, and the behavior of LT-IIb during purification differed significantly from that of LT-IIa. The toxic dose of unnicked LT-IIb in the Y1 adrenal-cell assay was 94 pg, but trypsin-treated, nicked LT-IIb was toxic at about 3 pg. In contrast, the toxic dose of LT-IIa was previously shown to be 0.5 to 1 pg for several preparations that varied from unnicked to partially nicked, and treatment with trypsin was not required for full toxicity. The titer of LT-II antiserum in neutralization tests was 100-fold greater against LT-IIa than against LT-IIb. In immunodiffusion tests, LT-IIa and LT-IIb gave a reaction of partial identity. In a radioimmunobinding assay, the titer of LT-IIa antiserum against homologous LT-IIa was approximately 10-fold greater than against LT-IIb. The cholera-E. coli family of heat-labile enterotoxins has been divided into serogroup I, which includes cholera toxin and the antigenic variants of E. coli heat-labile toxin designated LTh-I and LTp-I, and serogroup II, which includes LT-IIa and LT-IIb. The type I and type II toxins do not cross-react in neutralization or immunodiffusion tests. By using very sensitive radioimmunobinding assays, it was possible to demonstrate common antigenic determinants between the type I and type II toxins. However, the titers of antibodies in hyperimmune sera that recognized these common determinants were very low.

Comparison of the nucleotide sequences of the genes for the thermostable direct hemolysin and the thermolabile hemolysin from Vibrio parahaemolyticus

The nucleotide sequences of genes encoding the thermostable direct (TSD) hemolysin and the thermolabile (TL) hemolysin of Vibrio parahaemolyticus were determined. From the nucleotide sequence of the TSD hemolysin gene, it was revealed that the preprotein and the mature protein consisted of 189 amino acids and 165 amino acids, and that the molecular weights were 21.1 kDa or 18.5 kDa, respectively. Our data regarding TSD hemolysin were in complete agreement with previously published data. From the nucleotide sequence of the TL hemolysin gene, it was revealed that the preprotein and the mature protein consisted of 418 amino acids and 398 amino acids, and that the molecular weights were 47.5 kDa and 45.3 kDa, respectively. The GC content of the TSD hemolysin gene was 35.6%, while that of the TL hemolysin gene was 47.6% which is almost the same as that of V. parahaemolyticus genome. Maxicell analysis revealed that the molecular weights of the proteins encoded by the TSD hemolysin gene were 22.0 and 19.5 kDa, and that of the protein encoded by the TL hemolysin gene was 45.5 kDa, and that the promoters of these two hemolysin genes of V. parahaemolyticus were functional in Escherichia coli.

Mapping of a gene that regulates hemolysin production in Vibrio cholerae

A gene that regulates the hemolysin structural gene (hly) was found to be tightly linked to the tox-1000 locus of Vibrio cholerae RJ1 and separated from hly by a large section of the V. cholerae genetic map. This hemolysin regulatory gene was designated hlyR.

Facile identification of protein sequences by mass spectrometry. B subunit of Vibrio cholerae classical biotype Inaba 569B toxin

A mass spectrometric method was applied to the B subunit of Vibrio cholerae classical biotype Inaba 569B toxin to determine its amino acid sequence and to confirm the differences in the amino acid sequences predicted from the nucleotide sequences of the genes of El Tor biotype strains 62746 and 2125 toxins. In this method, the Staphylococcus aureus protease V8 digest of the CNBr-treated B subunit of the classical biotype toxin was examined directly by fast-atom-bombardment mass spectrometry without separation of individual peptides. The values of molecular ion signals observed in the mass spectra were compared with the amino acid sequences of the classical biotype and El Tor biotype toxins. All the observed mass values coincided with those calculated from the published sequences of the B subunit except those of the sequences at positions 12-29 and 69-79. Peptides with these sequences were isolated by high-performance liquid chromatography and analyzed by Edman degradation or by combination of mass spectrometry and enzymatic degradation. The results revealed that the amino acid residues at positions 22 and 70 were Asp instead of Asn in the published sequences of classical biotype toxin. It was also found that Asn at position 44 was partially deaminated to Asp. The amino acid sequence of the classical biotype toxin was found to be different only at positions 18 (His----Tyr), 47 (Thr----Ile) and 54 (Gly----Ser) from that of El Tor biotype toxins.