The role of the eae gene of enterohemorrhagic Escherichia coli in intimate attachment in vitro and in a porcine model.

The eaeA gene of enteropathogenic Escherichia coli (EPEC) is necessary for intimate attachment to epithelial cells in vitro. Enterohemorrhagic E. coli (EHEC) strains also possess an eae gene and are capable of intimate attachment and microvillus effacement in vitro and in animal models. To assess the role of the EHEC eae gene in intimate attachment, we constructed an eae deletion/insertion mutation in wild-type EHEC O157:H7 strain 86-24 by using linear electroporation of a recombinant allele. The mutant obtained was deficient in inducing f-actin accumulation in HEp-2 cells and was incapable of attaching intimately to colonic epithelial cells in a newborn piglet model of infection. Intimate attachment in vivo was restored when the EHEC eae gene or the eaeA gene of EPEC was introduced into the mutant on a plasmid. These results indicate that the eae gene is necessary for intimate attachment of EHEC in vivo. In addition, the complementation achieved by the EPEC locus indicates that the eae gene of EHEC and the eaeA gene of EPEC are functionally homologous.

Role of the eaeA gene in experimental enteropathogenic Escherichia coli infection.

Enteropathogenic Escherichia coli (EPEC) infections are a leading cause of infant diarrhea in developing countries. Recently eaeA, a gene necessary for the characteristic intimate attachment of EPEC to epithelial cells in tissue culture, was described. We conducted a randomized, double-blind study to determine the role of the eaeA gene in human EPEC infection. 11 adult volunteers ingested 2 x 10(10) colony-forming units of O127:H6 EPEC strain E2348/69, and an equal number received the same dose of an isogenic eaeA deletion mutant constructed from E2348/69. Volunteers were monitored for the development of diarrhea, fever, and systemic and gastrointestinal complaints. Diarrhea developed in all 11 volunteers who received E2348/69 and in 4 of 11 who received the mutant (P = 0.002). Fever was more common in recipients of the wild-type strain (P = 0.024). Stool volumes were lower in recipients of the mutant. All volunteers seroconverted to E2348/69 LPS, but the geometric mean peak titers of serum IgG and IgA in recipients of the mutant were lower than those of recipients of the wild-type strain. IgA against LPS was detected in the jejunal fluid of six of six recipients of E2348/69 and 5/6 recipients of the mutant. This study unambiguously assigns a role for eaeA as an EPEC virulence gene, but the residual diarrhea seen in recipients of the mutant indicates that other factors are involved.

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

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

Enterohemorrhagic Escherichia coli.

Enterohemorrhagic Escherichia coli has been responsible for an increasing number of large food-borne outbreaks of bloody diarrhea and hemolytic uremic syndrome. Recent developments in our understanding of the pathogenesis of disease due to enterhemorrhagic E. coli include the description of a pathogenicity island, a type III secretion system and potential plasmid-encoded virulence factors. Recent developments in our understanding of the epidemiology include a recognition of a widening spectrum of vehicles.

Interactions between enteropathogenic Escherichia coli and host epithelial cells.

The pathogenesis of enteropathogenic Escherichia coli (EPEC) infection is emerging as a paradigm for a multistage microorganism-host cell interaction. Both type IV fimbriae and a type III secretion apparatus play principal roles in interactions between the bacteria and host cells. Recent data suggest that bacteria-induced signal transduction activates the receptor that allows tenacious adherence of the bacteria to the host cell surface.

Longus pilus of enterotoxigenic Escherichia coli and its relatedness to other type-4 pili--a minireview.

Longus is a long pilus produced by human enterotoxigenic Escherichia coli (ETEC) which shares significant structural and biochemical features with class-B type-4 pili. These pili include the toxin-coregulated pilus (TCP) of Vibrio cholerae, the bundle-forming pilus (BFP) of enteropathogenic E. coli and both longus and the colonization factor antigen III (CFA/III) of ETEC. These pili are produced under defined growth conditions indicating that they are under the control of different regulatory elements. While TCP is chromosomally encoded, the remaining pili are encoded on large virulence plasmids. Longus and CFA/III are closely related pili although certain DNA and protein differences also exist between them. This may account for the differences in the regulation, surface presentation, antigenicity, and prevalence of these two pilins among ETEC. Neighboring lngA, a second open reading frame termed lngB was found which encodes a protein with significant homology to proteins which are part of a type-II secretory system such as XcpV, OutC, and PulO of Pseudomonas aeruginosa, Erwinia chrysanthemi, and Klebsiella pneumoniae, respectively. This suggests that lngB may be an accessory gene involved in biogenesis of longus.

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.

Patterns of adherence of diarrheagenic Escherichia coli to HEp-2 cells.

A total of 516 Escherichia coli strains randomly isolated from coprocultures of 154 Chilean children with diarrhea and 66 controls were examined with DNA probes and tested for adherence to HEp-2 cells. Three adherence patterns were distinguished, localized, true diffuse and "aggregative." Enteropathogenic E. coli (EPEC) were detected by EPEC adherence factor probe among 86 of the 372 isolates (23%) from patients with diarrhea vs. 14 of 144 (10%) strains from controls (P less than 0.0002). Of 95 strains that manifested localized adherence, 97% were EPEC adherence factor probe-positive; thus the HEp-2 assay may serve as an alternative to the probe in identifying EPEC adherence factor-positive EPEC. True diffuse adherence was not associated with diarrhea. In contrast the aggregative pattern appears to signify a new, distinct class of diarrheagenic E. coli (enteroadherent-aggregative E. coli). The aggregative pattern was found in only 3 of 27 enterotoxigenic, 0 of 4 enteroinvasive, 0 of 2 enterohemorrhagic and 2 of 86 EPEC strains but in 84 of 253 probe-negative strains (P less than 0.00001) from patients with diarrhea; in comparison only 20 of 134 probe-negative strains from controls were aggregative E. coli (P less than 0.00001 vs. probe-negative strains from diarrhea patients).

Volunteer studies investigating the safety and efficacy of live oral El Tor Vibrio cholerae O1 vaccine strain CVD 111.

A live oral cholera vaccine should ideally protect against both classical and El Tor biotypes of Vibrio cholerae O1. An El Tor biotype vaccine strain, therefore, would complement classical cholera vaccine strain CVD 103-HgR, a strain already in use in some countries. In this study, 25 healthy adult volunteers received a single dose of 10s colony-forming units of El Tor vaccine strain CVD 111, a derivative of El Tor Ogawa strain N16117 deleted in the virulence cassette. Three (12%) volunteers developed mild diarrhea (mean stool volume = 813 ml) but no systemic symptoms; 23 (92%) of the 25 volunteers developed serum vibriocidal antibodies (geometric mean titer = 1:2,291). Five weeks after vaccination, 18 vaccines and eight uninimunized control volunteers underwent wild-type challenge with El Tor Ogawa strain 3008. Three (16.7%) of 18 vaccinees and seven (87.5%) of eight controls developed diarrhea (P = 0.001) (vaccine efficacy = 80.9%). Further studies are underway to determine a dosage of CVD 111 that will be more clinically acceptable but equally immunogenic and protective.

Persistence of cholera in the United States.

In 1973, 1978, and 1981, cases of cholera were acquired along the Gulf Coast of the United States. The isolates from all of the cases were toxigenic Vibrio cholerae O-group 1, biotype El Tor, serotype Inaba, hemolytic, and of the same phage sensitivity pattern, and all had the same restriction endonuclease pattern by molecular genetic analysis. The strain from one of the two 1981 cases differed from the others in having a small plasmid and a negative Voges-Proskauer reaction. Multiple importations, chronic carriers, and continuous occurrence of undetected cases are unlikely explanations for these findings, which suggest that toxigenic V. cholerae 01 can multiply and persist for years in some environments, making eradication of cholera a formidable task.

Effects of DNase production, plasmid size, and restriction barriers on transformation of Vibrio cholerae by electroporation and osmotic shock.

Attempts to transform wild type strains of V. cholerae with plasmid DNA by traditional osmotic shock methods were not successful. A mutant of V. cholerae that was deficient in extracellular DNase was transformed with plasmid DNA by osmotic shock, demonstrating directly that extracellular DNase is a major barrier to transformation of V. cholerae. Transformation of wild type and DNase-negative strains of V. cholerae was accomplished by electroporation. Efficiency of transformation by electroporation increased with field strength, decreased with plasmid size, and was relatively insensitive to changes in the electrolyte composition of the buffer as long as isotonic sucrose was present. Host-controlled modification/restriction systems also affected transformation efficiency in V. cholerae.

A novel locus of enterocyte effacement (LEE) pathogenicity island inserted at pheV in bovine Shiga toxin-producing Escherichia coli strain O103:H2.

We describe a locus of enterocyte effacement (LEE) which is part of a new pathogenicity island (PAI) detected in the bovine Shiga toxin-producing Escherichia coli strain RW1374 (O103:H2). This PAI is at least 80 kb in size and inserted in the vicinity of the pheV tRNA gene at 67 min of the E. coli chromosome. Furthermore, the PAI differs from the previously described LEEs by unique flanking regions at both sides, which harbor one copy each of an insertion element in an inverted orientation that is 96% identical to insertion site (IS)629. In addition, a 5-kb PAI-specific sequence downstream of the LEE core region and adjacent to the E. coli K12 region is duplicated upstream of the LEE core region as well. The duplicated sequences are more than 80% identical to each other and consist partially of prophage sequences.

Construction of genetically marked Vibrio cholerae O1 vaccine strains.

Attenuated Vibrio cholerae O1 vaccine strains lacking the gene encoding the A subunit of cholera toxin have proven efficacious in preventing experimental cholera. As these strains move from closed, contained testing environments to large-scale field trials, a readily assayable phenotypic trait to distinguish a vaccine strain from wild-type V. cholerae O1 is desirable. We have constructed three derivatives of the attenuated V. cholerae strain CVD 103 which carry a mercury resistance or urease marker in the hlyA gene. CVD 103-HgR was constructed using a protracted marker-exchange procedure; this strain was found to have somewhat lowered colonisation efficiency in infant mice in comparison to its parent strain, CVD 103. The insertion of the resistance marker was repeated using a suicide vector system; CVD 103-HgR2 was found to colonise infant mice as efficiently as CVD 103. Strain CVD 103-UR, in which sequences encoding urease were inserted using a suicide vector, also colonised infant mice as well as CVD 103. The genetically marked strains CVD 103-HgR, CVD 103-HgR2 and CVD 103-UR form the basis for a generation of defined oral vaccines that may give single-dose, long-lasting protection to populations at risk from cholera.

Comparative analysis of the hemolysin genes of Vibrio cholerae non-01, V. mimicus, and V. hollisae that are similar to the tdh gene of V. parahaemolyticus.

The genes encoding the hemolysins similar to the thermostable direct hemolysin (tdh gene) of Vibrio parahaemolyticus were cloned from chromosomes of V. mimicus and V. hollisae. These cloned hemolysin genes and previously cloned tdh genes of V. parahaemolyticus and V. cholerae non-01 were compared by physical mapping and by hybridization with oligodeoxyribonucleotide probes. The nucleotide sequences in the coding regions of all the cloned hemolysin genes were very homologous and had only minor variations but the sequences flanking the homolysin genes were dissimilar, indicating that the hemolysin genes have a common ancestor and suggesting that they may have been transferred between Vibrio species as a descrete genetic unit.

Characterization of the locus of enterocyte effacement (LEE) in different enteropathogenic Escherichia coli (EPEC) and Shiga-toxin producing Escherichia coli (STEC) serotypes.

All proteins involved in the attachment and effacement lesion produced by enteropathogenic Escherichia coli (EPEC) and Shiga-toxin producing E. coli (STEC) are encoded by the locus of enterocyte effacement (LEE). We studied the presence and insertion site of the LEE in different EPEC and STEC strains. In serotypes O119:H6/H-, O55:H6, O55:H7, O142:H6, O111ac:H9/H-, O111ab:H9/H- LEE is inserted downstream of selC as previously described for EPEC O127:H6 and STEC O157:H7. In serotypes O111ac:H8/H- and O26:H11/H- the LEE is inserted in pheU as previously described for STEC O26:H-. However in EPEC from serotype O111ab:H25 the LEE is not inserted in either site suggesting a third insertion site in the K12 chromosome. We also cloned fragments of 2.3 kb and 1.0 kb from the right and left hand sides of the LEE of a O111ac:H- strain and identified additional insertion sequences on these LEE fragments, suggesting that the LEE may be larger and may have undergone more recombination events in these serotypes.

Insertion site of the locus of enterocyte effacement in enteropathogenic and enterohemorrhagic Escherichia coli differs in relation to the clonal phylogeny of the strains.

The locus of enterocyte effacement pathogenicity island confers the attaching and effacing histopathology on epithelial cells infected with enteropathogenic and enterohemorrhagic Escherichia coli. We investigated the site of insertion of the locus of enterocyte effacement in E. coli strains in relation to their evolution based on conservation of housekeeping proteins in these strains. The results indicate that the insertion site of the locus of enterocyte effacement varies according to the evolutionary lineage, suggesting that it has inserted at multiple times and sites during the evolution of these pathogens.

Molecular and ultrastructural characterisation of EspA from different enteropathogenic Escherichia coli serotypes.

Enteropathogenic Escherichia coli (EPEC) encode a type III secretion system located on a pathogenicity island known as the locus for enterocyte effacement. Four proteins are known to be exported by this type III secretion system--EspA, EspB and EspD required for subversion of host cell signal transduction pathways and a translocated intimin receptor protein (Tir) required for intimin-mediated intimate attachment and attaching and effacing lesion formation. The espA gene is located within the locus for enterocyte effacement and the EspA polypeptide from the prototype EPEC strain E2348/69 (O127:H6) has recently been shown to be a component of a filamentous structure involved in bacteria-host cell interaction and locus for enterocyte effacement-encoded protein translocation involved in attaching and effacing lesion formation. In this study we have extended our investigation of EspA to strains belonging to other classical EPEC serotypes. DNA sequencing demonstrated that the espA gene from the different EPEC strains share at least 65% DNA identity. In addition, we detected morphologically and antigenically similar EspA filaments in all but one of the bacterial strains examined including recombinant, non-pathogenic E. coli expressing espA from a cloned locus for enterocyte effacement region (HB101(pCVD462)).

Genetics of cholera toxin.

Cholera is caused by the toxin secreted by Vibrio cholerae 01. Cholera toxin (CT) is a protein consisting of A and B subunits. The former contributes to intracellular toxicity whereas the B subunit is required for binding of CT to eukaryotic cell surface receptor. The structural genes encoding A and B subunits are designated as ctxA and ctxB respectively. These genes are located on the chromosome forming an operon in which ctxA precedes ctxB. The ctxAB have been cloned and sequenced. Classical strains contain two full copies of unlinked ctxAB. Most el tors have single copy. However, in some strains there are two copies which are arranged in tandem. The tandem duplication and amplification of ctxAB is controlled by a transposable element like DNA sequence called RS1. A number of genes have been identified which regulate the expression of ctx operon. V. cholerae seems to elaborate more than one toxin which are different from the one encoded by ctxAB genes.