Enterotoxins in acute infective diarrhoea

Enterotoxigenic Escherichia coli Heat-Stable Toxin and Ebola Virus Delta Peptide: Similarities and Differences

Enterotoxigenic Escherichia coli (ETEC) STb toxin exhibits striking structural similarity to Ebola virus (EBOV) delta peptide. Both ETEC and EBOV delta peptide are enterotoxins. Comparison of the structural and functional similarities and differences of these two toxins illuminates features that are important in induction of pathogenesis by a bacterial and viral pathogen.

Development of a loop-mediated isothermal amplification (LAMP) for the detection of F5 fimbriae gene in enterotoxigenic Escherichia coli (ETEC)

The objective of this study was to establish a loop-mediated isothermal amplification (LAMP) method for the detection of F5 fimbriae gene in Enterotoxigenic Escherichia coli. A set of four primers were designed based on the conservative sequence of coding F5 fimbriae. Temperature and time condition, specificity test, and sensitivity test were performed with the DNA of Escherichia coli (F5+). The results showed that the optimal reaction condition for LAMP was achieved at 61 °C for 45 min in a water bath. Ladder-like products were produced with those F5-positive samples by LAMP, while no product was generated with other negative samples. The assay of LAMP had a detection limit equivalent to 72 cfu/tube, which was more sensitive than PCR (7.2 × 10² cfu/tube). The agreement rate between LAMP and PCR was 100 % in detecting simulation samples. Thus, the LAMP assay may be a new method for rapid detection of F5 fimbriae gene of ETEC.

Longus, a type IV pilus of enterotoxigenic Escherichia coli, is involved in adherence to intestinal epithelial cells

Enterotoxigenic Escherichia coli (ETEC) is the leading bacterial cause of diarrhea in the developing world, as well as the most common cause of traveler's diarrhea. The main hallmarks of this type of bacteria are the expression of one or more enterotoxins and fimbriae used for attachment to host intestinal cells. Longus is a pilus produced by ETEC. These bacteria grown in pleuropneumonia-like organism (PPLO) broth at 37 degrees C and in 5% CO(2) produced longus, showing that the assembly and expression of the pili depend on growth conditions and composition of the medium. To explore the role of longus in the adherence to epithelial cells, quantitative and qualitative analyses were done, and similar levels of adherence were observed, with values of 111.44 x 10(4) CFU/ml in HT-29, 101.33 x 10(4) CFU/ml in Caco-2, and 107.11 x 10(4) CFU/ml in T84 cells. In addition, the E9034A Delta lngA strain showed a significant reduction in longus adherence of 32% in HT-29, 22.28% in Caco-2, and 21.68% in T84 cells compared to the wild-type strain. In experiments performed with nonintestinal cells (HeLa and HEp-2 cells), significant differences were not observed in adherence between E9034A and derivative strains. Interestingly, the E9034A and E9034A Delta lngA(pLngA) strains were 30 to 35% more adherent in intestinal cells than in nonintestinal cells. Twitching motility experiments were performed, showing that ETEC strains E9034A and E9034A Delta lngA(pLngA) had the capacity to form spreading zones while ETEC E9034A Delta lngA does not. In addition, our data suggest that longus from ETEC participates in the colonization of human colonic cells.

Outbreaks of cholera-like diarrhoea caused by enterotoxigenic Escherichia coli in the Brazilian Amazon Rainforest

The relationship between enteropathogens and severe diarrhoea in the Brazilian Amazon is poorly understood. In 1998, outbreaks of acute diarrhoea clinically diagnosed as cholera occurred in two small villages localized far from the main cholera route in the Brazilian rainforest. PCR was performed on some enteropathogens and heat-labile (LT) and/or heat-stable (STh) toxin genes, the virulence determinants of enterotoxigenic Escherichia coli (ETEC), were detected. Further characterization of ETEC isolates revealed the presence of two clones, one from each outbreak. One presenting serotype O167:H5 harboured LT-I and STh toxin genes and expressed the CS5CS6 colonization factor. The other, a non-typeable serotype, was positive for the LT-I gene and expressed the CS7 colonization factor. The current study demonstrates the importance of molecular diagnosis in regions such as the Amazon basin, where the enormous distances and local support conditions make standard laboratory diagnosis difficult. Here we also show that the mis-identified cholera cases were in fact associated with ETEC strains. This is the first report of ETEC, molecularly characterized as the aetiological agent of severe diarrhoea in children and adults in the Brazilian Amazon Rainforest.

Region of heat-stable enterotoxin II of Escherichia coli involved in translocation across the outer membrane

Heat-stable enterotoxin II of Escherichia coli (STII) is synthesized as a precursor form consisting of pre- and mature regions. The pre-region is cleaved off from the mature region during translocation across the inner membrane, and the mature region emerges in the periplasm. The mature region, composed of 48 amino acid residues, is processed in the periplasm by DsbA to form an intramolecular disulfide bond between Cys-10 and Cys-48 and between Cys-21 and Cys-36. STII formed with these disulfide bonds is efficiently secreted out of the cell through the secretory system, including TolC. However, it remains unknown which regions of STII are involved in interaction with TolC. In this study, we mutated the STII gene and examined the secretion of these STIIs into the culture supernatant. A deletion of the part covering from amino acid residue 37 to the carboxy terminal end did not markedly reduce the efficiency of secretion of STII into the culture supernatant. On the other hand, the efficiency of secretion of the peptide covering from the amino terminal end to position 18 to the culture supernatant was significantly low. These observations indicated that the central region of STII from amino acid residue 19 to that at position 36 is involved in the secretion of STII into the milieu. The experiment using a dsbA-deficient strain of E. coli showed that the disulfide bond between Cys-21 and Cys-36 by DsbA is necessary for STII to adapt to the structure that can cross the outer membrane.

Mutation of aromatic amino acid residues located at the amino- and carboxy-termini of Escherichia coli heat-stable enterotoxin Ip reduces the efficiency of the toxin to cross the outer membrane

Heat-stable enterotoxin Ip (STIp) of Escherichia coli is synthesized as a precursor form consisting of pre- (amino acid residues 1 to 19), pro- (amino acid residues 20 to 54) and mature (amino acid residues 55 to 72) regions. Mature STIp (bioactive STIp) is formed in the periplasmic space after the precursor is proteolytically processed and the mature STIp translocates across the outer membrane through the secretory system including TolC, an outer membrane protein of E. coli. However, it remains unknown how the mature STIp is recognized by this secretory system. In this study, we investigated the amino acid residues of STIp involved in its translocation across the outer membrane. We prepared mutant STIp genes by site-directed mutagenesis and analyzed translocation of the mutant STIps across the outer membrane. Deletion of the Phe or Tyr residue at position 3 or 18, respectively, decreased the efficiency of translocation of STIp across the outer membrane. To confirm the involvement of these amino acid residues, we further mutated the codons for these amino acid residues to that for Gly. These mutations also decreased the efficiency of extracellular secretion of STIp. In contrast, substitution of Phe-3 and Syr-18 with Tyr and Phe, respectively, did not affect the efficiency of translocation of the toxin. These results indicated that the aromatic amino acid residues at positions 3 and 18 in the mature region are important for the ability of STIp to cross the outer membrane.

Enterotoxigenic Escherichia coli--an overview

Enterotoxigenic Escherichia coli is an important cause of traveler's diarrhea and diarrheal illnesses in children in the developing world. In this presentation we will focus on the main virulence attributes of this pathogenic category of E. coli, and discuss the evolution of studies conducted in our laboratory.

Full capacity of recombinant Escherichia coli heat-stable enterotoxin fusion proteins for extracellular secretion, antigenicity, disulfide bond formation, and activity

We have successfully used the major subunit ClpG of Escherichia coli CS31A fimbriae as an antigenic and immunogenic exposure-delivery vector for various heterologous peptides varying in nature and length. However, the ability of ClpG as a carrier to maintain in vitro and in vivo the native biological properties of passenger peptide has not yet been reported. To address this possibility, we genetically fused peptides containing all or part of the E. coli human heat-stable enterotoxin (STh) sequence to the amino or carboxyl ends of ClpG. Using antibodies to the ClpG and STh portions for detecting the hybrids; AMS (4-acetamido-4'-maleimidylstilbene-2, 2'-disulfonate), a potent free thiol-trapping reagent, for determining the redox state of STh in the fusion; and the suckling mouse assay for enterotoxicity, we demonstrated that all ClpG-STh proteins were secreted in vitro and in vivo outside the E. coli cells in a heat-stable active oxidized (disulfide-bonded) form. Indeed, in contrast to many earlier studies, blocking the natural NH(2) or COOH extremities of STh had, in all cases, no drastic effect on cell release and toxin activity. Only antigenicity of STh C-terminally extended with ClpG was strongly affected in a conformation-dependent manner. These results suggest that the STh activity was not altered by the chimeric structure, and therefore that, like the natural toxin, STh in the fusion had a spatial structure flexible enough to be compatible with secretion and enterotoxicity (folding and STh receptor recognition). Our study also indicates that disulfide bonds were essential for enterotoxicity but not for release, that spontaneous oxidation by molecular oxygen occurred in vitro in the medium, and that the E. coli cell-bound toxin activity in vivo resulted from an effective export processing of hybrids and not cell lysis. None of the ClpG-STh subunits formed hybrid CS31A-STh fimbriae at the cell surface of E. coli, and a strong decrease in the toxin activity was observed in the absence of CS31A helper proteins. In fact, chimeras translocated across the outer membrane as a free folded monomer once they were guided into the periplasm by the ClpG leader peptide through the CS31A-dependent secretory pathway. In summary, ClpG appears highly attractive as a carrier reporter protein for basic and applied research through the engineering of E. coli for culture supernatant delivery of an active cysteine-containing protein, such as the heat-stable enterotoxin.

Extracellular DsbA-insensitive folding of Escherichia coli heat-stable enterotoxin STa in vitro

To study the folding of human Escherichia coli heat-stable enterotoxin STh, we used the major protein subunit of CS31A fimbriae (ClpG) as a marker of STh secretion and a provider of a signal peptide. We established that STh genetically fused to the N or C terminus of ClpG was able to mobilize ClpG to the culture supernatant while still retaining full enterotoxicity. These features indicate that the STh activity was not altered by the chimeric structure and suggest that spatial conformation of STh in the fusion is close to that of the native toxin, thus permitting recognition and activation of the intestinal STh receptor in vivo. In contrast to other studies, we showed that disulfide bond formation did not occur in the periplasm through the DsbA pathway and that there was no correlation between DsbA and secretion, folding, or activity. This discrepancy was not attributable to the chimeric nature of STh since there was no effect of dsbA or dsbB mutations on secretion and activity of recombinant STh from which ClpG had been deleted. Periplasmic and lysate fractions of dsbA(+) and dsbA(-) cells did not have any STh activity. In addition, the STh chimera was exclusively found in an inactive reduced form intracellularly and in an active oxidized form extracellularly, irrespective of the dsbA background. Subsequently, a time course experiment in regard to the secretion of STh from both dsbA(+) and dsbA(-) cells indicated that the enterotoxin activity (proper folding) in the extracellular milieu increased with time. Overall, these findings provide evidence that STa toxins can be cell-released in an unfolded state before being completely disulfide-bonded outside the cell.

Human disease associated with Clostridium perfringens enterotoxin

Clostridium perfringens continues to be a common cause of food-borne disease. Characteristics of this organism that contribute to its ability to cause food-borne illness include the formation of heat-resistant spores that survive normal cooking/heating temperatures, a rapid growth rate in warm food, and the production of enterotoxin (CPE) in the human gut. Time and temperature abuse associated with food preparation contributes to the majority of outbreaks of C. perfringens food-borne disease. CPE-induced diarrhea has been reported in the absence of a defined food vehicle. These cases have been typically associated with the elderly and following a course of antibiotic therapy. The incidence of CPE-induced diarrhea may be expected to increase with the growing population of immunocompromised (disease-, treatment-, or age-induced) individuals. Clostridium perfringens has been implicated as a possible contributor to the development of SIDS in susceptible individuals. Specifically, it has been hypothesized that CPE acts as a triggering agent, initiating the events associated with the development of SIDS. Continued refinement of both immunoassays and molecular methods for toxin and gene detection, respectively, will facilitate their eventual availability as commercial kits, providing rapid and simplified methods for the detection of C. perfringens isolates that produce or have the capacity to produce CPE as well as other toxins associated with this organism.

Amino acid residues in the pro region of Escherichia coli heat-stable enterotoxin I that affect efficiency of translocation across the inner membrane

Escherichia coli heat-stable enterotoxin Ip (STIp), which is a typical extracellular toxin consisting of 18 amino acid residues, is synthesized as a precursor consisting of pre (amino acid residues 1 to 19), pro (amino acid residues 20 to 54), and mature (amino acid residues 55 to 72) regions. Though the pre region functions as a conventional leader peptide that guides the following region to cross the inner membrane, the role of the pro region in the maturation pathway remains to be elucidated. We previously indicated that the sequence from residues 29 to 38 in the pro region increases the efficiency of STI translocation across the inner membrane (H. Yamanaka, Y. Fuke, S. Hitotsubashi, Y. Fujii, and K. Okamoto, Microbiol. Immunol. 37:195-205, 1993). We therefore examined the amino acid residues in the sequence that are responsible for this function. We substituted several amino acid residues in the sequence by means of oligonucleotide-directed site-specific mutagenesis. We then evaluated the effect of the substitution on the efficiency of STI translocation across the inner membrane by determining the enterotoxic activity of the culture supernatant, the amount of a fusion protein consisting of STI and nuclease A released into the periplasm, and the amount of the labeled ST released into the periplasm after pulse-labeling with [35S]cysteine. Substitution of the charged amino acid residues at positions 29 to 31 (K-E-K) with hydrophobic (I-V-L, F-W-F, or F-W-Q) or basic (K-K-K) residues significantly reduced these values in every assay. In contrast, the substitution of these amino acid residues with acidic amino acid residues (E-E-E) increased these values in all assays. This means that the negative charge near position 30 is important for STI to translocate efficiently across the inner membrane. A similar substitution of lysine residues at positions 37 and 38 showed that they are not involved in the translocation of STI across the inner membrane.

Necrotizing enterocolitis: the search for a unifying pathogenic theory leading to prevention

During the past two decades, necrotizing enterocolitis has emerged as a major cause of mortality and morbidity in premature infants. The specific cause of the disease remains enigmatic, but several putative risk factors provide clues to a pathophysiology that seems to be multifactorial. With the use of newly developed scientific tools, an understanding of the basic pathophysiologic cascade that causes necrotizing enterocolitis is emerging, providing hope for improved treatment and prevention.

Electrolyte fluxes in the gut and oral rehydration solutions

This article provides a brief overview of the normal physiology of water and electrolyte fluxes across the gut as a prerequisite for understanding the pathologic disturbances occurring with diarrheal illnesses. In turn, the rationale for the use of oral rehydration solutions in diarrheal disorders is explored.

Fimbriae extracts from enterotoxigenic Escherichia coli strains of bovine and porcine origin with K99 and/or F41 antigens

Fimbriae extracts obtained using the thermal shock method, from bovine and porcine enterotoxigenic Escherichia coli strains with K99 and/ or F41 antigens, were analyzed by SDS-PAGE, immunoblotting and haemagglutinating activity. Three major protein bands with molecular weights 17 kDa, 29.3 kDa and 30.9 kDa were detected depending on the strain assayed. A 17 kDa band was identified as the fimbrial subunit for K99 fimbriae and was detected in strains of bovine and porcine origin. The 30.9 kDa band was identified as the fimbriae subunit for F41 fimbriae and was detected in all porcine strains with F41 antigen and only in the bovine strains of serogroups O9 and 0101 that proved positive for F41 antigen. The 29.3 kDa band was shown to be antigenically related to F41 and K88, and was only detected in bovine strains of serogroups O8 (5 strains) and O20 (a single strain). We speculate that the 29.3 kDa band may be related to the CS31A antigen.

A survey of bacterial toxins involved in food poisoning: a suggestion for bacterial food poisoning toxin nomenclature

There is at present no accepted nomenclature for bacterial protein toxins, although there have been several attempts at dividing them into groups by their mode of action. In this paper we will not try to describe all known bacterial protein toxins, but concentrate on the toxins involved in food poisoning. Although most of these toxins are enterotoxins (protein exotoxins with the site of action on the mucosal cells of the intestinal tract) there are also other toxins involved in food poisoning, like the neurotoxins. In Table 1 the most important food pathogens in Europe are listed. For most, but not all, of these food pathogens, toxins are virulence factors. Generally, we divide food poisoning into infections and intoxications, where Salmonella spp. and Shigella spp. are typical examples of infections and Clostridium botulinum and Staphylococcus aureus for intoxications. We consider it better to make four different groups of food pathogenic bacteria, according to Table 2. Today the first three groups are all defined as infections, although for both group 2 and 3 the bacterium itself does not harm the host directly. The bacterium in such locations is like an 'enterotoxin factory'. The bacteria belonging to group 3 do not even interact with the epithelial cells in the intestine, while the bacteria of group 2 must colonise the epithelial cells prior to enterotoxin production.

Host-microbe interaction in the gastrointestinal tract

In order for an infection to occur, the target organ must come in contact with sufficient microbes, the microbe must possess specific virulence factors, these virulence factors must be expressed, and the defenses of the organ system must be overcome. This dynamic process, which is ongoing in all living entities, can be described by the following relationship: [formula: see text] The establishment of infection first occurs in a particular organ. This phenomenon is known as tissue trophism and the association of microbes with organ systems governs the practice of clinical microbiology and infectious disease. With some microbes (e.g., Giardia, Cryptosporidium) the interaction with the particular organ is so specific that infections are almost always confined to one site; with others (e.g., Salmonella, enterovirus) the microbe has the potential to become systemic. When attempting to establish health risk assessment from microbes by contact with food and drinking water, one must therefore consider that the gastrointestinal tract is a complex organ system with a variety of specific host defense mechanisms. It is only when the microbe has particular virulence factors for sites in gastrointestinal tract, and the specific host defense mechanisms in the gastrointestinal tract are breached, that infection of this organ system occurs. Therefore, the general terms "immunosuppression" or "immunocompromise" are meaningless unless the specific immune defect is known. A description of the microbial virulence factors active against the gastrointestinal tract and the defense mechanisms of this organ system are reviewed to provide a biological basis health risk assessment and future food and drinking water regulations.

Acute infectious diarrhea

Diarrhea continues to be a major cause of mortality and morbidity in third world countries as well as a major symptomatic complaint in the primary care setting in the United States. The etiologic pathogen depends on an exposure history to include recent travel to foreign countries, consuming fecally contaminated water or food, prior use of antibiotics, or homosexual behavior. A careful history from patients directed at attempting to identify particular risk factors may help in making a diagnosis. Not all patients require a diagnostic workup. A large number of patients may only require oral rehydration, careful observation over time with or without use of antimotility agents. In toxic appearing patients or patients with fever, however, bloody stools, abdominal pain or tenesmus, a selective diagnostic workup is indicated. Antimicrobial treatments are not always required, some pathogens clearly call for treatment while some have less clear indications and other pathogens are not responsive to antimicrobial agents at all. Finally, one needs to remember that the differential diagnosis of acute diarrhea includes many noninfectious origins.

Functional properties of pro region of Escherichia coli heat-stable enterotoxin

Escherichia coli heat-stable enterotoxin Ip (STp) is synthesized as the 72-amino-acid residue precursor consisting of three regions: pre region (amino acid residues 1 to 19), pro region (amino acid residues 20 to 54), and mature ST (mST) region (amino acid residues 55 to 72). We examined the role of the pro sequence of STp in enterotoxigenicity of a strain by deleting the gene fragment encoding amino acids 22 to 57. This deletion caused a remarkable reduction of its enterotoxic activity of culture supernatant. In order to analyze the sequence responsible for the function of the pro region, two additional deletion mutants were made. The deletion of the sequence covering amino acids 29 to 38, which is conserved in all sequences of ST reported, brought about a significant reduction of enterotoxic activity but the deletion of the non-conserved sequence (amino acids 40 to 53) did not. This result shows that conserved sequence is mainly responsible for the function. Subsequently, to examine the mechanism of action of the pro region, plasmids carrying DNA sequences of hybrid proteins consisting of pre-pro-nuclease, pre-mST-nuclease, pre-pro-mST-nuclease and pre-pro-nuclease-mST were constructed. Amino acid sequence determination and SDS-polyacrylamide gel analysis revealed that these fusion proteins were cleaved between pre sequence and pro sequence during secretion and the cleaved fusion proteins were accumulated in periplasmic space. But the amount of hybrid protein accumulated in the periplasmic space varied among the strains. That is, the amount of the pre-pro-nuclease gene product that accumulated in the periplasmic space was the highest of all fusion gene products. These results indicate that the existence of the mST region strongly interferes with the translocation of the gene product into the periplasmic space and that the pro region functions to guide the mST region into the periplasmic space.