Purification and partial characterization of heat-stable enterotoxin of enterotoxigenic Escherichia coli

BEC, a novel enterotoxin of Clostridium perfringens found in human clinical isolates from acute gastroenteritis outbreaks

Clostridium perfringens is a causative agent of food-borne gastroenteritis for which C. perfringens enterotoxin (CPE) has been considered an essential factor. Recently, we experienced two outbreaks of food-borne gastroenteritis in which non-CPE producers of C. perfringens were strongly suspected to be the cause. Here, we report a novel enterotoxin produced by C. perfringens isolates, BEC (binary enterotoxin of C. perfringens). Culture supernatants of the C. perfringens strains showed fluid-accumulating activity in rabbit ileal loop and suckling mouse assays. Purification of the enterotoxic substance in the supernatants and high-throughput sequencing of genomic DNA of the strains revealed BEC, composed of BECa and BECb. BECa and BECb displayed limited amino acid sequence similarity to other binary toxin family members, such as the C. perfringens iota toxin. The becAB genes were located on 54.5-kb pCP13-like plasmids. Recombinant BECb (rBECb) alone had fluid-accumulating activity in the suckling mouse assay. Although rBECa alone did not show enterotoxic activity, rBECa enhanced the enterotoxicity of rBECb when simultaneously administered in suckling mice. The entertoxicity of the mutant in which the becB gene was disrupted was dramatically decreased compared to that of the parental strain. rBECa showed an ADP-ribosylating activity on purified actin. Although we have not directly evaluated whether BECb delivers BECa into cells, rounding of Vero cells occurred only when cells were treated with both rBECa and rBECb. These results suggest that BEC is a novel enterotoxin of C. perfringens distinct from CPE, and that BEC-producing C. perfringens strains can be causative agents of acute gastroenteritis in humans. Additionally, the presence of becAB on nearly identical plasmids in distinct lineages of C. perfringens isolates suggests the involvement of horizontal gene transfer in the acquisition of the toxin genes.

A second generation of double mutant cholera toxin adjuvants: enhanced immunity without intracellular trafficking

Nasal application of native cholera toxin (nCT) as a mucosal adjuvant has potential toxicity for the CNS through binding to GM1 gangliosides in the olfactory nerves. Although mutants of cholera toxin (mCTs) have been developed that show mucosal adjuvant activity without toxicity, it still remains unclear whether these mCTs will induce CNS damage. To help overcome these concerns, in this study we created new double mutant CTs (dmCTs) that have two amino acid substitutions in the ADP-ribosyltransferase active center (E112K) and COOH-terminal KDEL (E112K/KDEV or E112K/KDGL). Confocal microscopic analysis showed that intracellular localization of dmCTs differed from that of mCTs and nCTs in intestinal epithelial T84 cells. Furthermore, both dmCTs exhibited very low toxicity in the Y1 cell assay and mouse ileal loop tests. When mucosal adjuvanticity was examined, both dmCTs induced enhanced OVA-specific immune responses in both mucosal and systemic lymphoid tissues. Interestingly, although both dmCT E112K/KDEV and dmCT E112K/KDGL showed high Th2-type and significant Th1-type cytokine responses by OVA-specific CD4+ T cells, dmCT E112K/KDEV exhibited significantly lower Th1-type cytokine responses than did nCT and dmCT E112K/KDGL. These results show that newly developed dmCTs retain strong biological adjuvant activity without CNS toxicity.

Isolation and characterization of a cytotoxin produced byPlesiomonas shigelloidesP-1 strain

In order to clarify the enteropathogenicity of Plesiomonas shigelloides, we investigated a cytotoxin produced by the P-1 strain isolated from patients suffering from diarrhea. The cytotoxicity of the culture filtrate of the strain reached a maximum in culture at 37 degrees C after 12 h shaken in BHI medium. The cytotoxin in the cultures was purified by (NH4)2SO4 precipitation, and Sephacryl S-100, Mono Q HR, and Superdex 200 HR column chromatographies. An approximate 340-fold purification was achieved, with a recovery of about 1.4%, from the culture supernatant. The cytotoxin is heat-stable, and is a complex of three major proteins (LPS-binding proteins with molecular weights of 32, 40, and 48 kDa), with lipopolysaccharide (LPS) giving a total a molecular weight of more than 600 kDa. The ratio of protein to LPS in the cytotoxin was 6-5. The cytotoxic activity was reduced by about 80% by proteinase K treatment or when incubated with anti-cholera toxin antibody (Anti-CT). Western blotting of the cytotoxin with Anti-CT demonstrated the presence of two anti-cholera toxin-reactive protein (ACRP) bands with molecular weights of 40 kDa (a major single protein band) and 48 kDa. The N-terminal amino acid sequence (20 residues) of the 40 kDa protein was 75% identical to Pasteurella multocida cell membrane proteins. The cytotoxin gave a positive reaction in the suckling mouse assay whereas LPS alone hardly exhibited any cytotoxic or enterotoxigenic activity. In conclusion, P. shigelloides produces a cytotoxin that consists of a complex of protein and LPS with the former component exhibiting both cytotoxicity and enteropathogenicity. This cytotoxin has the potential to have an important role in the enteropathogenicity of P. shigelloides.

Effectiveness and safety of mutant Escherichia coli heat-labile enterotoxin (LT H44A) as an adjuvant for nasal influenza vaccine

The effectiveness and safety of mutant Escherichia coli heat-labile enterotoxin, LT H44A (His to Arg substitution at position 44 from the N-terminus of the A1 fragment of the A subunit) as an adjuvant for nasal influenza vaccine were examined. (1) When 0.2 microg of LT H44A, together with 0.2 microg of influenza A/PR/8/34 virus (PR8, H1N1) vaccine, was administered intranasally into BALB/c mice (twice, 4 weeks apart), anti-PR8 hemagglutinin (HA) IgA and IgG antibody (Ab) responses were induced at levels that were sufficient to provide either complete protection against infection with a small volume of PR8 virus suspension or partial protection against infection with a lethal dose of the suspension. The dose of the mutant LT and vaccine used here (0.2 microg/ 20 g doses mouse) corresponded to the estimated dose per person, i.e. 0.1 mg/10 kg body weight. (2) Using these vaccination conditions, no additional total IgE Ab responses were induced. (3) The mutant was confirmed to be less toxic than the native LT when the toxicity was analyzed either using Y1 adrenal cells in vitro (1/483 EC(50)) or by an ileal loop test. (4) One hundred micrograms of the mutant, administered intranasally or intraperitoneally into guinea-pigs (Heartley strain, 0.3-0.4 kg), caused no body-weight changes 7 days after administration, although 100 microg of the native LT administered intraperitoneally caused death in all guinea-pigs due to diarrhea within 2 days. The intranasal administration of 100 microg of the mutant resulted in almost no pathological changes in the nasal mucosa 3 days after administration. These results suggest that LT H44A, which can be produced in high yields in an E. coli culture (about 5 mg/l), could be used as one of the effective and safe adjuvants for nasal influenza vaccine in humans.

Serodiagnosis of infection with verocytotoxin-producing Escherichia coli

Human sera (167) were screened for antibodies to lipopolysaccharide (LPS) prepared from strains of Verocytotoxin-producing Escherichia coli (VTEC) belonging to a range of serogroups, secreted proteins expressed by attaching and effacing VTEC, enterohaemolysin and H = 7 flagellar proteins. Twelve sera (about 7%) contained antibodies to the LPS of E. coli 05 (one), 026 (two), 0115 (two), 0145 (one), 0163 (one) and 0165 (five). Sera containing antibodies to the LPS of E. coli O26 and O145 also contained antibodies to secreted proteins of 100 and 40 kDa. An additional 34 sera, known to contain antibodies to the lipopolysaccharide of E. coli O157, were examined for antibodies to enterohaemolysin, H = 7 flagellar antigens and bacterial cell surface-associated proteins of 5, 6 and 22 kDa. Three sera contained antibodies to enterohaemolysin and one serum contained antibodies to flagellar proteins. Antibodies to membrane-associated proteins were not detected. It was concluded that enterohaemolysin, H = 7 flagellar proteins and the cell surface-associated proteins were unsuitable for use in immunoassays for providing evidence of infection with VTEC.

On the structural stability of a small bioactive peptide of potential use in biotechnology

A tridecapeptide with the sequence CCEICCNPACFGC has been synthesized to reproduce the active moiety of a heat stable enterotoxin from Vibrio cholerae. The proton NMR analysis indicates, for the active synthetic fragment, a rigid secondary structure stabilised by three disulfide bridges. Such a rigid peptide, suitably detoxified and activated, could be a good candidate to be used as a carrier for linear bioactive peptides or other functional groups.

Characterization of a highly toxic, large molecular size heat-stable enterotoxin produced by a clinical isolate of Yersinia enterocolitica

A novel heat-stable enterotoxin (ST) designated as Y-STc was purified to homogeneity from the culture supernatant of a pathogenic strain of Yersinia enterocolitica serotype O3 and its amino acid sequence was determined. The mature Y-STc was found to consist of 53 amino acid residues, which includes the putative pro-sequence. The molecular weight of Y-STc was 5638 and constituted the largest molecular size in the family of currently known STs. The minimum effective dose of purified Y-STc in the suckling mouse assay was 0.6 ng (0.0 pmol), indicating that, despite the long sequence, Y-STc is the most toxic in the ST family.

Purification and sequence determination of heat-stable enterotoxin elaborated by a cholera toxin-producing strain of Vibrio cholerae O1

Four molecular species of heat-stable enterotoxins elaborated by a cholera toxin-producing strain of Vibrio cholerae O1 were isolated from its culture supernatant. The amino acid sequence of one of the enterotoxins was determined to be Phe-Ile-Lys-Gln-Val-Asp-Glu-Asn-Gly-Asn-Leu-Ile-Asp-Cys-Cys-Glu-Ile-Cys- Cys-Asn-Pro-Ala-Cys-Phe-Gly-Cys-Leu-Asn with three intramolecular disulfide linkages. The other enterotoxins had shorter amino acid sequences in the N-terminal regions, but possessed the same sequence in their C-terminal regions including the three disulfide linkages. The enterotoxins with the shorter N-terminal sequences showed more potent toxicities, and the minimum effective dose of the longest one with 28 amino acid residues was 10-folds of that of the shortest one.

Production of heat-stable enterotoxin II by chicken clinical isolates of Escherichia coli

Enterotoxigenic Escherichia coli isolated from diarrhea stools of chickens were examined for production of heat-stable enterotoxin II which is considered to be implicated only in diarrhea of pigs. Seven out of 38 strains examined were found to contain heat-stable enterotoxin II gene, determined by colony hybridization and the polymerase chain reaction. The culture supernatants of these strains caused fluid accumulation in the mouse intestinal loop test. This fluid accumulation activity was not lost by heating at 100 degrees C and was neutralized by anti-heat-stable enterotoxin II antiserum. Purified heat-stable enterotoxin II caused fluid accumulation in the chicken intestinal loop assay. These results indicate that STII-producing E. coli is implicated in chicken diarrhea.

Identification and characterization of heat-stable enterotoxin II-producing Escherichia coli from patients with diarrhea

Stock strains of Escherichia coli isolated from patients with traveller's diarrhea were examined for production of heat-stable enterotoxin II (STII). Of 400 strains examined, 3 were found to produce STII. The nucleotide sequence of the STII gene of these human strains was shown to be identical to that of porcine strains. Cultured cells of these strains induced fluid accumulation in ligated mouse intestinal loops and the activity was neutralized by anti-STII antiserum. These results suggest that STII-producing enterotoxigenic E. coli can cause human diarrhea.

Heterogeneity in the molecular species of heat-stable enterotoxin of Vibrio cholerae non-O1 expressed by Escherichia coli carrying the cloned toxin gene

The biological activity of the heat-stable enterotoxin of Vibrio cholerae non-O1 (NAG-ST) was found to be predominantly associated with the periplasmic extract (about four-fold higher than the culture supernatant) of a recombinant E. coli (JM109) strain carrying the NAG-ST toxin gene. Four molecular species of NAG-ST, two each from the periplasmic extract and culture supernatant of JM109, were purified. Amino acid sequence analysis of the four NAG-ST peptides isolated by HPLC revealed that they all differed from that of the mature 17-amino acid residue NAG-ST released by V. cholerae non-O1. The M(r)-values of the peptides obtained from the periplasmic extract were 4331 and 2785, while those recovered from the culture supernatant were 3154 and 2785. It thus appears that V. cholerae NAG-ST is synthesized as larger molecules in the recombinant E. coli strain. The differences in sizes of the exported NAG-ST molecule could relate to differences in the enzyme cleavage system between E. coli and V. cholerae.

Epitope mapping and characterization of antigenic determinants of heat-stable enterotoxin (STh) of enterotoxigenic Escherichia coli by using monoclonal antibodies

A panel of monoclonal antibodies (MAbs) specific for the heat-stable enterotoxin (STh) of enterotoxigenic Escherichia coli was produced. All four MAbs (8G7, 53-4, 11C, and SH1) bound to native STh in an enzyme-linked immunosorbent assay to various degrees, with clone SH1 showing the best affinity. The MAbs were screened for neutralizing and guanylate cyclase-inhibiting activities by the suckling mouse assay and the cyclic GMP assay using T84 cells, respectively. The contact amino acid residues governing the reactivity of the four MAbs were precisely determined by using several chemically synthesized analogs of the various heat-stable enterotoxins (STa's). Three distinct antigenic sites of STh sufficiently removed from each other, one near the N terminus, another in the core functional region of the toxin, and the third in the C-terminal region, were recognized by the different MAbs. MAb SH1, which recognized Asn at position 4 and Tyr at position 5 from the N terminus was 100 times more potent in neutralizing the bioactivity of STh in the suckling mouse assay than was MAb 11C, which recognized Thr at position 16 and Tyr at position 19 from the N terminus of the STh molecule. The MAbs which recognized Leu at position 9 from the N terminus (MAb 53-4) and Tyr at position 19 from the N terminus (MAb 8G7) showed intermediate activities in the neutralization assay. The guanylate cyclase-inhibiting activities of SH1 and 11C essentially paralleled the results for the neutralization of bioactivity, while MAbs 53-4 and 8G7 exhibited reverse activity. These results indicate that MAbs that recognize the N-terminal residues which have been shown not to be essential for toxic activity have a potent protective capacity. None of the MAbs reacted with reduced and carboxy-methylated native STh. This suggests that all of the MAbs mediate their effect by reacting with conformation-dependent antigenic determinants.

Glycoprotein receptors for a heat-stable enterotoxin (STh) produced by enterotoxigenic Escherichia coli

Glycoprotein receptors for heat-stable enterotoxin STh of enterotoxigenic Escherichia coli in the rat intestinal cell membrane were identified and characterized. Incubation of rat intestinal cell membranes with radioiodinated N-5-azidonitrobenzoyl-STh[5-19] (125I-ANB-STh[5-19]) followed by photolysis resulted in specific radiolabeling of two distinct proteins with M(r)s of 200,000 (designated STR-200A and STR-200B). STR-200A was found to be composed of two molecules of a protein with an M(r) of 70,000 (70-kDa protein), whereas STR-200B was composed of two different protein molecules with M(r)s of 53,000 (53-kDa protein) and 77,000 (77-kDa protein). These proteins showed no guanylate cyclase activity. The 70-kDa protein was labeled most with 125I-ANB-STh[5-19], suggesting that STR-200A is the main receptor protein in the rat intestinal cell membrane. The carbohydrate moieties of STR-200A and STR-200B were examined by enzymatic deglycosylation. The 70-kDa protein of STR-200A was found to contain N-linked high-mannose-type and/or hybrid-type oligosaccharides, and results suggested that it possesses at least three N glycosylation sites. The 53-kDa protein of STR-200B was found to have an N-linked complex-type oligosaccharide side chain. The deglycosylated 70-kDa protein retained activity for binding to STh, suggesting that the carbohydrate moieties of these receptor proteins are not important for binding with STh.

Reassessment of the prevalence of heat-stable enterotoxin (NAG-ST) among environmental Vibrio cholerae non-O1 strains isolated from Calcutta, India, by using a NAG-ST DNA probe

A collection of 521 environmental isolates of Vibrio cholerae which were previously examined by the suckling mouse assay and found to be negative for the heat-stable enterotoxin NAG-ST were reassessed by a recently developed DNA probe for NAG-ST. A total of 12 (2.3%) of the isolates hybridized with the NAG-ST probe. By using a cholera toxin (CT) DNA probe, the CT gene was detected in six of the strains in the collection, although none of the isolates of V. cholerae non-O1 hybridized with both of the toxin probes. All of the NAG-ST and CT probe-positive strains were hemolysin positive. Thirty-fold-concentrated supernatants of the three representative NAG-ST DNA probe-positive V. cholerae non-O1 strains gave positive fluid accumulation ratios in the suckling mouse assay even after heating (100 degrees C for 5 min) and also inhibited the binding of a NAG-ST monoclonal antibody to the bound NAG-ST in a competitive enzyme-linked immunosorbent assay (ELISA). Likewise, all six CT probe-positive V. cholerae non-O1 strains produced in vitro CT when examined by the CT bead ELISA. HindIII digest patterns of chromosomal DNA from the representative NAG-ST gene-positive strains were visually indistinguishable. Between the groups of NAG-ST probe-positive strains examined, there was a variation in the hybridizable fragments, with one group of strains exhibiting a hybridizable fragment similar to that of the NRT 36 reference strain; a smaller HindIII fragment hybridized with the NAG-ST probe in the other group of strains.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of the bead enzyme-linked immunosorbent assay for detection of cholera toxin directly from stool specimens

A highly sensitive bead enzyme-linked immunosorbent assay (bead ELISA) for detection of cholera toxin (CT) was evaluated for direct detection of CT from stool specimens of patients with acute secretory diarrhea. Of the 75 stool samples examined, 59 yielded biochemically, and serologically confirmed strains of Vibrio cholerae O1. The bead ELISA was positive for CT in stool supernatants in 50 (84.7%) of the 59 samples from which V. cholerae O1 was isolated. In addition, the bead ELISA was positive for three stool specimens which were negative by culture. The free CT present in 48 of the 50 stool samples positive by culture for V. cholerae O1 and for CT by bead ELISA was completely absorbed by anti-CT immunoglobulin G. All of the 59 strains of V. cholerae O1 biotype eltor isolated in this study produced in vitro CT. The concentration of CT present in the bead ELISA-positive stool samples ranged between 26 pg/ml and greater than 100 ng/ml. This evaluation study demonstrates that the bead ELISA is a sensitive and simple method for direct detection of CT in nonsterile stool samples, and we recommend routine use of this assay for detection of CT in stool samples and culture supernatants in clinical and reference laboratories.

Escherichia coli heat-stable enterotoxin (STa)-biotin conjugates for the titration of STa antisera by an enzyme-linked immunosorbent assay

The development of a new approach to the diagnosis of infectious diarrhoea, caused by Escherichia coli heat-stable enterotoxin (ST), was preceded by a preliminary study. The purpose of the latter was to establish whether three preparations of ST produced by a human isolate of enterotoxigenic E. coli (STa), obtained at different steps of the purification procedure (involving Amberlite XAD2 resin chromatography (P3), a gel filtration chromatography on a Biogel P4 (P2) or a disc-gel electrophoresis (P1)), could be employed to titrate antisera to STa using an ST-biotin enzyme-linked immunosorbent assay (ELISA). The solid-phase STa was obtained by first coupling the toxin to biotinyl-N-hydroxysuccinimide and then binding this conjugate to avidin adsorbed to flat-bottomed polystyrene microtitre plates. Using these reagents, the assay conditions were examined. Checkerboard tests determined optimal biotin-P3, P2 or P1 toxin conjugate concentrations to be used as the immunosorbent for P3, P2 and P1 antiserum titration. The immunosorbent prepared with STa purified only on Amberlite XAD2 resin was unable to differentiate significantly between P3, P2 or P1 antisera. Immunosorbent prepared with P2 or P1 detected widely differing titres between the three antisera and gave more sensitive results. Only small but questionable differences were observed between P2 and P1 toxin preparations.

Purification and characterization of a new heat-stable enterotoxin produced by Vibrio cholerae non-O1 serogroup Hakata

The possible production of a heat-stable enterotoxin (Vc-H-ST) by Vibrio cholerae non-O1 serogroup Hakata was investigated, and the purified Vc-H-ST was characterized. It has a unique amino acid sequence, LIDCCEICCNPACFGCLN. This sequence is quite similar to that of the heat-stable enterotoxin (NAG-ST) produced by V. cholerae non-O1 except for one amino acid (leucine) residue excess at the N terminus. Other characteristics, including biological activity, are compatible with those of NAG-ST.

Synthesis and biological properties of carba-analogs of heat-stable enterotoxin (ST) produced by enterotoxigenic Escherichia coli

Analogs of a heat-stable enterotoxin (ST) that have a CH2-S linkage instead of an S-S linkage in the molecule were synthesized by conventional methods. The synthetic peptides showed toxicity, assayed as induction of fluid secretion in suckling mice, although their toxicities were hundredth that of native ST. This finding implies that ST is not recognized by its receptor protein through an exchange reaction between its disulfide linkages and thiol-groups of its receptor protein(s), but through hydrophobic or electrostatic interactions.

The 106-kilobase plasmid of Salmonella braenderup and the 100-kilobase plasmid of Salmonella typhimurium are not necessary for the pathogenicity in experimental models

Among 1.041 clinical isolates (77 serovars) of Salmonella which had been derived from cases with acute enterocolitis, 601 (58%) contained one or more plasmids. Large serovar-specific plasmids were seen in 95 of 307 isolates (31%) of Salmonella typhimurium, in 34 of 34 isolates (100%) of Salmonella enteritidis and in 36 of 38 isolates (94.7%) of Salmonella braenderup: the sizes of which were 100, 60 and 106 kilobases (kb), respectively. In order to determine the role of these plasmids in pathogenicity for enterocolitis, the plasmids were eliminated from some strains of S. braenderup and S. typhimurium and the pathogenicity of the plasmid-less strains was compared with that of the parent strains by invasiveness to HeLa cells, fluid accumulation in the rabbit ligated ileal loop, lesion of mucosal tissue and the Sereny test. The virulence of all the plasmid-less strains was as strong as that of the plasmid-bearing strains in these pathogenicity assay systems. We therefore concluded that the 106-kb plasmid of S. braenderup and the 100-kb plasmid of S. typhimurium are not necessary for their pathogenicity in the experimental models: invasiveness to HeLa cells, fluid accumulation in the rabbit ligated ileal loop, and Sereny test.

Stimulation of phosphorylation of rat brush-border membrane proteins by Escherichia coli heat-stable enterotoxin, cholera enterotoxin and cyclic nucleotides, and its inhibition by protein kinase inhibitors, isoquinolinesulfonamides

Stimulation of phosphorylation of rat brush-border membrane proteins by the heat-stable enterotoxin of Escherichia coli (STh), cholera enterotoxin, cGMP and cAMP was demonstrated. Among at least 14 proteins examined, a protein with a molecular mass of 81,000 Da (81 kDa protein) was phosphorylated most in the presence of both STh and cholera enterotoxin, as well as in the presence of cGMP and cAMP. This phosphorylation was inhibited by N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide (H-8) or N-(2-aminoethyl)-5-isoquinoline-sulfonamide (H-9), which suggests that the phosphorylation occurs through cGMP- and cAMP-dependent protein kinases.

Demonstration of enterotoxigenic Escherichia coli in diarrheic broiler chicks

An investigation was made to survey the possible presence of enterotoxigenic Escherichia coli (ETEC) in the stools of diarrheal chicks. We analyzed two outbreaks of diarrhea in broiler chicks at two independent farms in the Philippines, from which no pathogens other than Escherichia coli were found. In one outbreak at Farm #1, all 42 isolates produced heat-labile enterotoxin (LT), with 3 of these isolates also producing heat-stable enterotoxin (ST). The O serotypes of 15 strains tested randomly could not be identified as any known serotype (0-antigen; 1-170). In another outbreak at Farm #2, 7 out of 52 isolates produced only LT, their subtypes being identified as O-149 or O-8, common serotypes in pig ETEC. Strains from Farm #1 did not produce any pili usually found in human ETEC. We believe this to be the first isolation of ETEC from diarrheal chicks.

Inhibition by the protein kinase inhibitors, isoquinolinesulfonamides, of fluid accumulation induced by Escherichia coli heat-stable enterotoxin, 8-bromo-cGMP and 8-bromo-cAMP in suckling mice

The effects of isoquinolinesulfonamides, which inhibit protein kinase, on fluid accumulations induced by heat-stable enterotoxin of enterotoxigenic Escherichia coli (STh), 8-bromo-cGMP and 8-bromo-cAMP in suckling mice were studied. Both N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide (H-8) and N-(2-aminoethyl)-5-isoquinolinesulfonamide (H-9) inhibited the fluid accumulation induced by STh. Fluid accumulation induced by four mouse units of STh (four-fold the minimum effective dose, 10 ng) was completely inhibited by 0.4 mumol of H-8 or H-9. H-8 and H-9 also inhibited fluid accumulation induced by 8-bromo-cGMP. On the other hand, H-8 and H-9 only partially inhibited fluid accumulation in suckling mice induced by 8-bromo-cAMP, probably because their affinities to cAMP-dependent protein kinase were lower than their affinities to cGMP-dependent protein kinase. From these results, it is concluded that the activation of cGMP-dependent protein kinase by increase in cGMP by ST or by 8-bromo-cGMP, and very probably the activation of cAMP-dependent protein kinase by increase in cAMP by cholera enterotoxin and heat-labile enterotoxin of enterotoxigenic E. coli and by 8-bromo-cAMP are necessary steps in signal transduction following increases in concentrations of cGMP and cAMP in intestinal brush border cells.

Erythrocyte receptors for cholera and heat-labile enterotoxins of Escherichia coli

Many serological reactions using red blood cells (RBC) such as radial immune haemolysis (RIH) and indirect haemagglutination (IH) tests have often been used for the detection of cholera toxin (CT) and heat-labile (LT) enterotoxin produced by porcine and human Escherichia coli strains. In these tests, the enterotoxins bind to sheep, bovine and guinea-pig RBC without any ligand. We studied several factors which might interfere with such binding, as well as the nature of the receptors involved. Treatment of erythrocytes with different enzymes revealed that proteolytic enzymes had no effect on the adsorption of enterotoxins to RBC. Conversely, treatment with neuraminidase increased the adsorption. Experiments carried out with delipidized RBC revealed that none of the enterotoxins under study bound to the cells thus treated. Pre-incubation of ganglioside fractions with the enterotoxins blocked RIH and IH reactions and the biological effect of them on Vero cells. Assaying RBC ganglioside fractions by thin-layer chromatography revealed the presence of GM1. Our results suggest that the receptors for GT and LT enterotoxins in sheep, bovine and guinea pig RBC are gangliosides: mainly GM1.

A long-acting heat-stable enterotoxin analog of enterotoxigenic Escherichia coli with a single D-amino acid

A heat-stable enterotoxin (STp) consisting of 18 amino acid residues including 6 half-cystine residues is produced by a porcine strain of enterotoxigenic Escherichia coli. Analogs of STp with replacements of single residues at each from positions 5 to 17 by the corresponding D-amino acid residue were synthesized by a solid-phase method. Of these analogs, [D-Cys5]-STp[5-17] showed the same biological properties as STp[5-17]. Moreover, its activity to cause fluid accumulation in suckling mouse lasts more than 24 hours, whereas the activity of STp[5-17] decreases after 6-10 hours. These results indicate that the action of the analog [D-Cys5]-STp[5-17] is strongly agonistic to that of the native ST.

Toxin profiles of Vibrio cholerae non-O1 from environmental sources in Calcutta, India

A collection of Vibrio cholerae non-O1 isolated from the aquatic environs of Calcutta, a cholera-hyperendemic area, were examined for the production of cholera toxin (CT), Shiga-like toxins (Vero toxins), heat-stable enterotoxin, and hemolysins. Two (0.5%) V. cholerae non-O1 isolates produced CT. The DNA from both these isolates also hybridized with a DNA probe containing sequences encoding the A subunit of CT. None of the strains produced Shiga-like toxins or heat-stable enterotoxin. Hemolytic activity was observed in 89.7% of the strains, of which 36.1% exhibited biological activity in the suckling mouse. However, none of them produced a hemolysin that cross-reacted with the thermostable direct hemolysin of Vibrio parahaemolyticus. It appears from this study that a small percentage of environmental V. cholerae non-O1 strains do possess the potential for causing cholera-like diarrhea.

Production and partial purification of a fluid-accumulating factor of non-O1 Vibrio cholerae

A fluid-accumulating factor (FAF in the ligated rabbit ileal loop test) from a strain of non-O1 Vibrio cholerae not producing cholera toxin-like enterotoxin (CTLT) was partially purified by ammonium sulfate precipitation, gel filtration with Sephadex G-100, and DEAE cellulose column chromatography. The preparation thus obtained showed collagenolytic, cytolytic, necrotic, and hemorrhagic activities, but was not lethal to mice nor hemolytic to sheep erythrocytes. Desquamation of epithelial cells, inflammatory edema, and hemorrhage were observed in sections of rabbit intestine after inoculation of partially purified FAF (PPFAF). Biological and enzymatic activities of FAF were completely neutralized with anti-PPFAF rabbit serum. More than 70% of non-O1 V. cholerae strains from human diarrheal feces produced FAF in the shake culture of heart infusion broth (Difco). A fluid-accumulating factor immunologically similar to FAF of non-O1 V. cholerae was also produced by V. mimicus strains isolated from human diarrheal feces. These results indicate that the FAF produced by CTLT-negative non-O1 V. cholerae strains is an entity closely related to a cytolytic and hemorrhagic substance or the like, and that this FAF may play a role in the enteropathogenicity of CTLT-negative strains.

Molecular analysis of enterotoxin plasmids of enterotoxigenic Escherichia coli of 14 different O serotypes

A total of 104 isolates of enterotoxigenic Escherichia coli derived from diarrheal patients from more than 10 countries were examined for serotype and toxigenicity. The transferability and molecular structure of the enterotoxin plasmids from each isolate were also examined. Enterotoxin plasmids from serotypes such as O6, O25, O27, O126, O128, and O159, which are frequently associated with E. coli diarrhea (classical strains) generally did not transfer by conjugation from clinical isolates, whereas those from serotypes such as O7, O17, O80, O98, O139, O150, and O153, which are rarely associated with diarrhea (rare strains) transferred almost always from the clinical isolates by conjugation. Analyses of enterotoxin plasmids by restriction endonucleases and DNA-DNA hybridization with the enterotoxin probes revealed that the strains with the same O serotype and toxigenicity carry closely related enterotoxin plasmids. These results suggest that classical strains resulted from the dissemination of ancestral clones which received enterotoxin plasmids long ago, while the rare strains acquired the enterotoxin plasmids recently by conjugation and have not yet been spread to the same degree as the ancestral clones.

Modulating action of Escherichia coli heat-stable enterotoxin (STa) on the humoral immune response

The effect of Escherichia coli enterotoxin STa on the primary and secondary immune response in F1 (CBA x C57 B1/10) mice immunized against sheep red blood cells (SRBC) was investigated. Modulating action on the IgM and IgG response was found to be dependent on the dose-time administration of the toxin. Immunosuppression of the primary response on the 4th day after immunization was observed when the toxin was injected 15 min before the SRBC, followed by immunostimulation on the 6th day after antigen (Ag) injection. Moreover, toxin administration 48 h before SRBC caused immunosuppression of the primary immune response on the 4th and 6th days. On the other hand, the IgM and IgG secondary immune response, determined 6 days after boosting, was greatly enhanced by toxin administration 15 min before priming (day 0) or boosting (day 26) and 48 h before priming. The same response was suppressed by toxin administration 48 h before booster antigen injection.

Enteropathogenic and enterotoxigenic Escherichia coli as aetiological factors of infantile diarrhoea in rural and urban Ghana

There are 4 recognized classes of Escherichia coli that cause diarrhoeal disease in humans: enteropathogenic (EPEC), enterotoxigenic (ETEC), enteroinvasive (EIEC), and enterohaemorrhagic E. coli (EHEC). Preliminary analysis of enterotoxin production in a rural community in Ghana showed a prevalence of 11.0% LT-ST+, 9.5% LT+ST- and 7.5% EPEC. (LT = heat-labile, and ST = heat-stable, enterotoxin). The results of a similar study in an urban community in Accra, Ghana, showed 10.9% LT-ST+, 5.9% LT+ST-, 1.6% LT+ST+ and 6.5% EPEC. 14 different serotypes of EPEC were isolated in the urban area, whereas 6 serotypes and two untypable strains were isolated in the rural area. The most common serotype isolated in Accra was 0126:K71 and that from the rural area was 0128:K67. Serotypes 0143:KXI and 0155:K59 are reported for the first time in Ghana.

Evaluation of a nonisotopically labeled oligonucleotide probe to detect the heat-stable enterotoxin gene of Escherichia coli by the DNA colony hybridization test

A commercially available, alkaline-phosphatase-conjugated oligonucleotide probe for detecting the heat-stable enterotoxin gene of Escherichia coli was compared with cloned gene probes by examining E. coli isolates from traveler's diarrhea by DNA colony hybridization tests. The oligonucleotide probe was useful in specifically identifying the so-called STh gene. No deproteinization of sample was necessary to prepare the colony blots.

Detection of enterotoxigenic Escherichia coli by colony hybridization with biotinylated enterotoxin probes

By using biotinylated enterotoxin DNA probes, a method to detect enterotoxigenic Escherichia coli by colony hybridization was developed. The treatment of colonies on nitrocellulose membrane filters with proteinase K and Triton X-100 was essential for obtaining the specific hybridization. A total of 200 E. coli strains isolated from travelers with diarrhea were tested for colony hybridization by using a probe encoding heat-labile toxin (LT) type h. All strains (86 of 86) that produced LT, but none of the non-LT producers, hybridized with 32P-labeled and biotinylated LT type h probes. A total of 36 strains chosen randomly from the 200 isolates were tested for colony hybridization by using heat-stable enterotoxin (ST) probes. All but two strains that hybridized with the 32P-labeled ST type Ia probe also hybridized with the biotinylated ST type Ia probe. All strains that hybridized with the 32P-labeled ST type Ib probe also hybridized with the biotinylated ST type Ib probe. Thus, almost all E. coli strains tested were judged to be the same by colony hybridization with biotinylated or 32P-labeled enterotoxin probes. These results demonstrate that the biotinylated enterotoxin probes are useful in the diagnosis of enterotoxigenic E. coli strains by colony hybridization.

Analysis of the plasmids of Escherichia coli O148:H28 from travellers with diarrhea

98 Escherichia coli strains of serotype O148:H28 isolated from diarrheal patients from 10 Asian countries and Mexico at Osaka Airport Quarantine were analyzed for enterotoxigenicity and plasmid profile. They were classified into three groups. The first group contained 44 strains that were non-enterotoxigenic and carried 3.9 kb and 50 kb non-enterotoxin plasmids. The second group contained 9 strains that produced LT and ST. They carried a 45 kb enterotoxin plasmid, and 4.6 kb and 9.2 kb non-enterotoxin plasmids. The third group contained 45 strains that produced ST. They carried a 40 kb enterotoxin plasmid, and non-enterotoxin plasmids other than 3.9 kb, 4.6 kb, 9.2 kb and 50 kb. Southern blot hybridization demonstrated that all the non-enterotoxin or enterotoxin plasmids carried by the strains of the same group were identical or similar. These results suggested that the 98 E. coli strains with O148:H28 serotype were derived from three clones, and that the individual strains among each group were derived from a single clonal strain.

Alternative procedure for the purification of the heat-stable enterotoxin of enterotoxigenic Escherichia coli pathogenic for calves

A method for purification of the heat-stable enterotoxin (ST) of enterotoxigenic Escherichia coli (ETEC) strains (C1444 and B41) pathogenic for calves and some physiochemical properties of the ST are described. The method involved ultrafiltration on PM-10 and UM-2 Diaflo membranes, acetone fractionation, ion-exchange chromatography on AG 1-X2, chromatofocusing and a combination of hydrophobic interaction chromatography on octyl-Sepharose CL-4B and gel-permeation on Bio-Gel P-2. Polyacrylamide gel electrophoresis in sodium dodecyl sulphate of fluorescamine-labeled purified, reduced and alkylated ST preparations revealed a single band with approximate molecular masses of 2500 and 2200 for the C1444 and B41 STs, respectively. For the C1444 ST, the final purification achieved was approximately 27,000-fold on the basis of absorbance at 280 nm per mouse effective dose. However, it was 2000-fold when calculated on the basis of mg protein per effective dose (5 ng). Amino acid composition of the C1444 ST was found to be different from that of the B41 ST suggesting that the ST produced by bovine isolates may be heterogeneous in their structure.

Mode of disulfide bond formation of a heat-stable enterotoxin (STh) produced by a human strain of enterotoxigenic Escherichia coli

To determine the modes of three disulfide linkages in the heat-stable enterotoxin (STh) produced by a human strain of enterotoxigenic Escherichia coli, we synthesized STh(6-18), which consists of 13 amino acid residues and has the same intramolecular disulfide linkages as native STh [(1985) FEBS Lett. 181, 138-142], by stepwise and selective formation of disulfide bonds using different types of removable protecting groups for the Cys residues. Synthesis of the peptide with different modes of disulfide bond formation provided three peptides consistent with standard STh(6-18) in their physicochemical and biological properties, thereby indicating that the disulfide bonds in STh(6-18) are Cys-Cys-Glu-Leu-Cys-Cys-Asn-Pro-Ala-Cys-Thr-Gly-Cys.

Enterotoxicity of El Tor-like hemolysin of non-O1 Vibrio cholerae

The enterotoxicity of an El Tor-like hemolysin purified from non-O1 Vibrio cholerae was investigated. Fluid accumulation was induced by injection of purified hemolysin into the ligated intestinal loops in adult rabbits (De test), intraintestinal administration in infant rabbits (Dutta test), and oral inoculation in suckling mice. The accumulated fluid was invariably mucous and bloody, and a histological change in the mucosa was observed. These results suggest that the hemolysin is an enterotoxic factor that is responsible for non-O1 V. cholerae gastroenteritis.

Production of Escherichia coli STa-like heat-stable enterotoxin by Citrobacter freundii isolated from humans

Citrobacter species are often present in the stools of children and are generally considered a normal component of the intestinal microflora. Previous reports suggested that they might act as enteric pathogens. Aiming at defining the role of Citrobacter species in inducing diarrhea, we looked for their presence in the stools of 328 children with diarrhea and in 108 controls. Citrobacter strains were isolated from 46 patients (14%) and 7 controls (6.5%) (P less than 0.05). All isolates were tested for heat-stable (ST) and heat-labile (LT) enterotoxin. No LT-producing organisms were found. Three C. freundii strains, all isolated from children with diarrhea, elaborated an enterotoxin detected by the suckling mouse assay. A crude extract was prepared by acetone precipitation and a sequential ultrafiltration technique. The enterotoxin was heat stable, and its estimated molecular weight was between 2,000 and 10,000. Citrobacter enterotoxin was soluble in methanol and stable at acid and neutral pHs but not above pH 8, and its activity was destroyed by treatment with 2-mercaptoethanol. Citrobacter enterotoxin was inactive in the 18-h rabbit ileal loop test. All these characteristics closely resemble STa produced by Escherichia coli. The time course of Citrobacter enterotoxin-induced intestinal secretion in suckling mice was similar to that of E. coli STa. The enterotoxin produced by C. freundii cross-reacted with monoclonal antibodies raised against E. coli STa. These results suggest that C. freundii is capable of inducing diarrhea through the production of an E. coli-like STa, and its presence in the stools of patients with diarrhea should be considered as that of a possible etiologic agent.

Purification and characterization of Vibrio cholerae non-O1 heat-stable enterotoxin

A toxin which causes rapid fluid accumulation in a suckling mouse assay and which was produced by Vibrio cholerae non-O1 was investigated. The toxin was purified from the culture supernatant of V. cholerae non-O1 (strain A-5) by ammonium sulfate fractionation, hydroxyapatite treatment, ethanol extraction, column chromatographies on SP-Sephadex C-50 and DEAE-Sephadex A-25, and high-pressure liquid chromatography on a Lichrosorb RP-8 column. About 1.4 X 10(5)-fold purification was achieved, with a recovery of about 12%. Although the crude preparation was heat labile, the purified toxin was heat stable. The minimum effective dose of purified toxin was about 5 ng in the suckling mouse assay. The amino acid composition of the purified toxin was determined to be Asp(3), Glu(1), Ala(1), half-Cys(6), Ile(2), Leu(1), Phe(1), and Pro(1). These data show the production of a new type of heat-stable enterotoxin (NAG-ST) by V. cholerae non-O1.

Characterization of intestinal brush border guanylate cyclase activation by Escherichia coli heat-stable enterotoxin

Intestinal brush border guanylate cyclase was previously reported to be activated by the Escherichia coli enterotoxin (STa). This system was reexamined in order to develop a hypothesis for the mechanism of activation. The extent of activation was previously underestimated, since by using sodium azide to inhibit competing reactions and ethylene glycol bis(beta-aminoethyl ether) N,N-tetraacetic acid to chelate Ca2+, which is inhibitory, maximal activations of 30- to 50-fold were obtained. Ca2+ inhibition was only partially relieved by the calmodulin inhibitor calmidazolium. Inhibitors of the O2-dependent activation of soluble guanylate cyclase had no effect on STa activation; hence, it was concluded that STa activation did not involve arachidonate release and oxidation. STa was able to further increase activity already elevated by the nonionic detergent Lubrol PX. The membrane-active agent filipin, which was previously reported to inhibit both basal and agonist-stimulated adenylate cyclase, did not inhibit STa activation of guanylate cyclase. Digitonin, another cholesterol binder, inhibited STa activation at low concentrations, which disappeared at higher concentrations. Both of these agents stimulated basal activity. Dimethyl sulfoxide produced a concentration-dependent inhibition of STa activation, while increasing basal activity 7-fold. Ethanol inhibited both basal and STa-stimulated activity, with the former being more affected. Benzyl alcohol, like ethanol, a "fluidizer" of cell membranes, also inhibited both basal and activated enzymes. We concluded that STa directly activates this guanylate cyclase and, because of the differential effects of inhibitors on basal and STa-stimulated activity, propose a receptor-mediated mechanism.

Preparative purification of Escherichia coli heat-stable enterotoxin

Heat-stable enterotoxin (STa) isolated from bovine Escherichia coli strains was purified to homogeneity by growing the bacterial strains in a chemically defined medium, desalting, and concentrating the culture filtrate by batch adsorption chromatography on Amberlite XAD-2 resin, batch adsorption chromatography on reversed-phase silica, and preparative reversed-phase high-performance liquid chromatography. This rapid preparative purification scheme gave high recovery yields of pure STa which exhibited biochemical homology to STa purified by more complicated procedures.