Ability of enteroaggregative Escherichia coli strains to adhere in vitro to human intestinal mucosa

Adhesion of enteroaggregative Escherichia coli to pediatric intestinal mucosa in vitro

Organ cultures of small- and large-intestinal mucosa from children were used to examine the interactions of enteroaggregative Escherichia coli (EAEC) with human intestine. Mucosae from patients aged between 3 and 190 months were cultured with five EAEC strains isolated from infants with diarrhea in the United Kingdom and with two well-described prototype EAEC strains, 17-2 and 221. The prototype strains adhered to jejunal, ileal, and colonic mucosae. The wild-type strains also adhered to this tissue but showed a variable pattern of adhesion: two adhered to all intestinal levels, one adhered to jejunum and ileum, one adhered to ileum only, and one adhered to ileum and colon. Adherence was in an aggregative or stacked-brick pattern, resembling that seen on HEp-2 cells. Electron microscopy of infected small intestinal mucosa revealed bacteria in association with a thick mucus layer above an intact enterocyte brush border, which contained extruded cell fragments. This mucus layer was not present on controls. EAEC adherence to colonic mucosa was associated with cytotoxic effects including microvillous vesiculation (but without evidence of an attaching/effacing lesion), enlarged crypt openings, the presence of intercrypt crevices, and increased epithelial cell extrusion. These results demonstrate that in vitro organ culture of intestinal mucosa from children can be used to investigate EAEC pathogenesis in childhood directly. EAEC strains appear able to colonize many regions of the gastrointestinal tract, without overt changes to small intestinal mucosa but with cytotoxic effects on colonic mucosa.

T84 cells in culture as a model for enteroaggregative Escherichia coli pathogenesis

Enteroaggregative Escherichia coli (EAEC) is an important cause of persistent diarrhea in many developing parts of the world, yet the pathogenetic mechanisms of EAEC diarrhea are unknown. Experiments with animal models suggest that EAEC strains damage the intestinal mucosa, and a putative cytotoxin has been described. To characterize the mucosal effects of EAEC, we studied strain 042, which we have shown to cause diarrhea in adult volunteers. Strain 042 was incubated in an in vitro organ culture model with biopsy-derived normal intestinal mucosa from pediatric patients. Strain 042 adhered strongly to samples of jejunal, ileal, and colonic mucosa. In addition, scanning electron microscopic examination of in vitro-infected intestinal biopsies revealed cytotoxic effects marked by exfoliation of mucosal epithelial cells. To develop an in vitro model to study these effects, we incubated 042 with polarized monolayers of the human intestinal epithelial cell lines Caco-2 and T84. Strain 042 adhered strongly to T84 cells but not to Caco-2 cells. T84 cells infected with 042 displayed marked toxic effects, most prominently in areas where bacteria were adhering. The apical membrane of damaged cells exhibited vesiculation and shedding of microvilli. The cytoplasm of affected cells displayed subnuclear vacuolization, and in some cases, nuclei of affected cells became separated from the surrounding cytoplasm. Severely affected cells ruptured, releasing their nuclei. Vacuolated remnant cells were seen throughout the monolayer. Strain 042 was not internalized by T84 cells. We concluded that EAEC strain 042 alters intestinal cell morphology, ultimately leading to cell death. Although the factor(s) required for this effect remains to be elucidated, T84 cells may serve as a valuable model in EAEC pathogenesis studies.

The emergency department approach to diarrhea

Enteric disease represents a significant medical problem on a worldwide basis. The evaluation of patients with diarrhea in the emergency department should follow a stepwise methodology to identify potentially serious disorders. The evaluation of the stool for fecal leukocytes is an important differentiation point in the evaluation of the moderately to seriously ill diarrhea patient. Oral rehydration alone can treat the vast majority of diarrhea patients. Oral rehydration solution takes advantage of the sodium glucose coupled active absorption mechanism, which is largely unaffected by enteric toxins. Antimicrobial or antidiarrheal agents are rarely indicated in the treatment of diarrhea.

Characterization of a novel hemagglutinin of diarrhea-associated Escherichia coli that has characteristics of diffusely adhering E. coli and enteroaggregative E. coli

Escherichia coli 73-1 (serotype O73:H33) and 5-2 (serotype O89:H-) isolated from patients with diarrhea adhered to tissue culture cells (HeLa and HEp-2) as well as coverslips (plastic and glass) in a diffuse pattern. Adherence of strain 73-1 was mediated by a 110-kbp plasmid designated pEDA1 and correlated with D-mannose-resistant hemagglutinin (MRHA) detected with bovine, sheep, or human erythrocytes. The MRHA region was duplicated on pEDA1 and mediated the production of the 57-kDa outer membrane protein whose N-terminal amino acid sequence was hydrophobic. In accordance with MRHA and adherence, the 57-kDa outer membrane protein was observed best at 37 degrees C and to a lesser extent at 25 degrees C. In human intestine, adherence to mucus and colonic epithelium was obvious. No detectable pili were observed. The enteroaggregative E. coli heat-stable enterotoxin 1 (EAST1) gene, whose nucleotide sequence was 99.1% homologous to that of enteroaggregative E. coli, was present adjacent to the MRHA region on pEDA1. Strain 5-2 also exhibited MRHA activities and adherence and had sequences corresponding to those of the MRHA region and EAST1 gene. The data suggest that strain 73-1 (and strain 5-2), which has characteristics of both diffusely adhering E. coli and enteroaggregative E. coli, possesses a novel hemagglutinin associated with diffuse adherence.

Enteroaggregative Escherichia coli associated with an outbreak of diarrhoea in a neonatal nursery ward

Over a 9-day period in February 1995, 16 newborn babies (age range 2-11 days) and 3 infants (24, 47 and 180 days of age) in a neonatal nursery ward developed diarrhoea accompanied by pyrexia and weight loss. Known enteropathogens were not detected in their stools but Escherichia coli displaying aggregative adherence to HEp-2 cells (enteroaggregative E. coli) were found in 12 (63%) ill infants and in none of 5 well neonates (P = 0.02). The illness lasted 3-9 days (mean 5.2) in 16 babies, whereas in 3 neonates it showed a protracted course of 18-20 days. The source of infection and the mode of transmission remained unclear. The outbreak isolates manifested properties common in this new group of diarrhoeagenic E. coli: mannose-resistant haemagglutination, haemolysis on blood agar, and clump formation in liquid culture medium. They belonged to the O4 E. coli serogroup and expressed multiple antibiotic resistance.

Characteristics of Escherichia coli strains belonging to enteropathogenic E. coli serogroups isolated in Italy from children with diarrhea

Fifty-five Escherichia coli strains belonging to enteropathogenic E. coli (EPEC) serogroups were examined for phenotypic and genetic factors associated with virulence. The strains were isolated in Italy from children with diarrhea and identified as EPEC by clinical laboratories using commercially available antisera. O:H serotyping showed that 35 strains (27 of O26, O111, and O128 serogroups) belonged to 11 serotypes considered to be classical EPEC O:H serotypes. The other 20 isolates were classified as 15 nonclassical EPEC O:H serotypes. All the potential EPEC virulence factors associated with bacterial adhesion (localized adherence, fluorescentactin staining test positivity, presence of the attaching and effacing [eaeA] gene), the production of verotoxin, and the positivity with the enterohemorrhagic E. coli probe were significantly more frequent among isolates belonging to classical than nonclassical serotypes. Strains displaying an aggregative adhesion and hybridizing with the enteroaggregative DNA probe were found in serogroups O86, O111, and O126. Verotoxin-producing isolates belonged to serogroups O26, O111, and O128. Only one of the isolates hybridized with the EPEC adherence factor (EAF) probe, but 33 strains gave positive results with the eae probe, confirming that the former is more suitable in epidemiological studies in European countries. These results indicate that up to 75% of strains identified as EPEC by commercial antisera may possess potential virulence properties and/or belong to classical EPEC O:H serotypes and suggest that O grouping is still a useful diagnostic tool for presumptive identification of diarrheagenic E. coli in clinical laboratories.

Controlled study of cytolethal distending toxin-producing Escherichia coli infections in Bangladeshi children

The role of cytolethal distending toxin (CDT)-producing Escherichia coli, a newly described category of E. coli, in the causation of diarrhea was studied by screening E. coli isolates from 546 children < 5 years of age with diarrhea and 215 matched controls without diarrhea by using a specific DNA probe. Although CDT-positive E. coli strains were isolated from more children with diarrhea than from healthy controls (3.1 versus 0.93%), this difference did not reach statistical significance (P = 0.082). All CDT-positive strains also possessed the virulence factors of enteropathogenic E. coli or enteroaggregative E. coli isolates.

Enteroaggregative strains of Escherichia coli belonging to serotypes O126:H27 and O44:H18 express antigenically similar 18 kDa outer membrane-associated proteins

Outer membrane-associated proteins of 18 kDa were expressed by enteroaggregative Escherichia coli (EAggEC) belonging to serotypes O126:H27 and O44:H18, which hybridized with a probe derived from a plasmid necessary for enteroaggregative adhesion. The 18 kDa proteins expressed by strains of E. coli, belonging to these serotypes, were surface exposed and antigenically similar but not structurally identical.

Colonization factors of diarrheagenic E. coli and their intestinal receptors

While Escherichia coli is common as a commensal organism in the distal ileum and colon, the presence of colonization factors (CF) on pathogenic strains of E. coli facilitates attachment of the organism to intestinal receptor molecules in a species- and tissue-specific fashion. After the initial adherence, colonization occurs, and the involvement of additional virulence determinants leads to illness. Enterotoxigenic E. coli (ETEC) is the most extensively studied of the five categories of E. coli that cause diarrheal disease, and has the greatest impact on health worldwide. ETEC can be isolated from domestic animals and humans. The biochemistry, genetics, epidemiology, antigenic characteristics, and cell and receptor binding properties of ETEC have been extensively described. Another major category, enteropathogenic E. coli (EPEC), has virulence mechanisms, primarily effacement and cytoskeletal rearrangement of intestinal brush borders, that are distinct from ETEC. An EPEC CF receptor has been purified and characterized as a sialidated transmembrane glycoprotein complex directly attached to actin, thereby associating CF-binding with host-cell response. Three additional categories of E. coli diarrheal disease, their colonization factors and their host cell receptors, are discussed. It appears that biofilms exist in the intestine in a manner similar to oral bacterial biofilms, and that E. coli is part of these biofilms as both commensals and pathogens.

Antibodies raised against the outer membrane protein interrupt adherence of enteroaggregative Escherichia coli

Enteroaggregative Escherichia coli (EAggEC) is a distinct category of diarrheal pathogen implicated as the cause of persistent diarrhea. The pathogen exhibits a characteristic "stacked-brick" pattern of aggregation when incubated with HEp-2 cells. The outer membrane protein (OMP) profile of a prototype EAggEC strain (F03) reflected the presence of one major 30-kDa protein. The OMP is expressed in the presence of the 60-MDa plasmid that the strain harbors. Antibodies were raised against the OMP by injecting the protein into a rabbit. The manifestation of an adherence phenotype on HEp-2 cells was observed for F03 and other strains that express OMP in the presence and absence of anti-OMP serum. Clumps of bacteria forming an aggregative pattern were observed in the HEp-2 cell assay in the absence of OMP antibodies, whereas a few bacteria attached to the cells in the presence of OMP antibodies. Mannose-resistant hemagglutination of human erythrocytes observed in the presence of EAggEC strains was inhibited in the presence of anti-OMP serum. Sequence analysis of a peptide generated by protease digestion of OMP exhibited 90% homology to a peptide of flagellin protein encoded by the hag gene of Serratia marcescens. Immunolabeling of the outer membrane by colloidal gold confirmed the protein to be an OMP. Our results suggest that the OMP of EAggEC have common antigenic properties. Antibodies raised against the protein can prevent adherence in vitro and could potentially interrupt the natural disease.

Aggregative adherence of Klebsiella pneumoniae to human intestine-407 cells

Aggregative adhesion of Klebsiella pneumoniae LM3 to Intestine-407 (Int-407) cells was studied. Adhesive capacities were affected by the bacterial growth phase (with a maximum of adherence obtained during the exponential phase), temperature, multiplicity of infection, and length of incubation with Int-407 cells. Adhesion occurred through a cytochalasin D-sensitive process and was greatly reduced after treatment of Int-407 with cycloheximide, indicating that aggregative adhesion requires active participation of Int-407 cells. Transmission electron microscopy revealed that adherent bacteria were surrounded by a capsule-like material, apparently involved in both bacterium-Int-407 cell and bacterium-bacterium adherence. Examination with a scanning electron microscope showed interactions of intestinal cell microvilli with bacteria and formation in 3 h of a fibrous network within and around the bacterial clusters. We speculate that aggregative adhesion of K. pneumoniae mediated by a capsule-like extracellular material might explain the persistence of these strains inside the host gastrointestinal tract.

Expression of antigenically distinct fimbriae with hemagglutination and HeLa cell adherence properties by an enteroaggregative Escherichia coli strain belonging to the enteropathogenic serogroup

An enteroaggregative Escherichia coli (EAggEC) strain (DS92), isolated from a case of infantile diarrhea, was shown to express mannose-resistant hemagglutination and HeLa cell adhering properties when grown at 37 degrees C but not at 28 degrees C. Cellular adherence properties of DS92, which belonged to enteropathogenic serogroup 0125, were shown to correlate well with the expression of fimbriae that were encoded by a 112 kb plasmid. The fimbriae of the EAggEC strain DS92 were composed of 20 kDa subunit proteins and were serologically distinct from fimbrial or non-fimbrial cell surface antigen(s) of other diarrheagenic E. coli strains including the reference EAggEC strain 17-2. Interestingly, the 20-kDa fimbrial protein was found to be antigenically related to 18- and 14.5-kDa cell surface proteins of two other locally isolated EAggEC strains belonging to the enteropathogenic serogroup 086.

Identification and characterization of a gene cluster mediating enteroaggregative Escherichia coli aggregative adherence fimbria I biogenesis

The aggregative pattern of adherence (AA) exhibited by enteroaggregative Escherichia coli upon HEp-2 cells is a plasmid-associated property which correlates with aggregative adherence fimbria I (AAF/I) expression and human erythrocyte hemagglutination. By using cloning and mutagenesis strategies, two noncontiguous plasmid segments (designated regions 1 and 2) required for AA expression have previously been identified in enteroaggregative E. coli 17-2. TnphoA mutagenesis was performed on clones containing region 1, and 16 TnphoA mutants which were negative for the AA phenotype were analyzed. The TnphoA insertion site for each mutant was determined by junctional DNA sequencing. All 16 mutations occurred within a 4.6-kb span in region 1. Nucleotide sequence analysis of the region revealed four contiguous open reading frames, designated aggDCBA, in the same span. AA-negative TnphoA insertions into all open reading frames except aggB were obtained. On the basis of mutational analysis and protein homology data, it is inferred that aggA, aggC, and aggD are involved in biogenesis of AAF/I, encoding a major fimbrial subunit, outer membrane usher, and periplasmic fimbrial chaperone, respectively. By immunogold electron microscopy, polyclonal antiserum raised against the aggA gene product decorated AAF/I fimbriae, affirming that AggA encodes an AAF/I subunit.

Sequence heterogeneity of the eae gene and detection of verotoxin-producing Escherichia coli using serotype-specific primers

The distribution of the Escherichia coli attaching and effacing (eae) gene in strains of verotoxin-producing E. coli (VTEC) isolated from cattle and humans was studied. The majority of strains isolated from humans with bloody diarrhoea or HUS and cattle with severe diarrhoea were eae positive (82 and 83% respectively). In contrast, 59% of VTEC isolated from asymptomatic cattle were eae negative and of the remaining 41% that were eae positive, the majority were serotype O157. H7. The nucleotide sequence of the 3' end of the eae gene of enteropathogenic E. coli (EPEC) of serotype O55. H7 was found to be almost identical to that of serotype O157. H7. Specific primers are described which detect the eae sequences of VTEC serotypes O157. H7, O157. H-, and EPEC serotypes O55. H7 and O55. H-. The nucleotide sequence of the 3' end of the eae gene of serotype O111. H8 differed significantly from that of O157. H7. Primers were developed to specifically identify the eae sequences of VTEC serotypes O111. H- and O111. H8. We conclude that whereas the majority of VTEC associated with disease in cattle and humans possess the eae gene, the gene itself may not be necessary to produce haemorrhagic colitis and HUS. Sequence heterogeneity in the 3' end of eae alleles of VTEC permits specific identification of subsets of these organisms.

The significance of enteroaggregative Escherichia coli in the etiology of hospitalized diarrhoea in Calcutta, India and the demonstration of a new honey-combed pattern of aggregative adherence

Previous studies have identified enteroadherent Escherichia coli that exhibit localized adherence, diffuse adherence and atypical diffuse adherence as diarrhoeagenic agents associated with infantile diarrhoea in Calcutta, India. In this study, a DNA probe specific for enteroaggregative adherence was used to determine the etiological significance of enteroaggregative E. coli in the causation of diarrhoea. From a total of 330 strains of E. coli recovered from 159 cases of acute secretory diarrhoea and 174 cases of invasive diarrhoea, 20 strains hybridized with the probe, whereas of the 25 E. coli strains recovered from 25 healthy controls only 1 strain hybridized with the probe. Of the 21 probe positive strains, 19 adhered to HeLa cells in the typical stacked-brick pattern while 2 strains recovered from 2 cases of secretory diarrhoea adhered to the glass surface in a hitherto undescribed formation which we have termed, based on the appearance, as the honey-comb pattern. The enteroaggregative E. coli strains identified in this study did not produce any conventional enterotoxins and were significantly associated with patients with secretory diarrhoea (10.7%) than with invasive diarrhoea (1.7%). The results of this study indicate that enteroaggregative E. coli play a causal role in acute secretory diarrhoea in this part of the world which lends credence to the involvement of a potent toxin in the pathogenesis of EAggEC mediated infections.

Adhesion and its role in the virulence of enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) organisms are an important cause of diarrheal disease in young children. The virulence of EPEC is a multifactorial process and involves a number of distinct stages. Initial adherence to intestinal mucosa is mediated by fimbriae which bring about a distinct form of adhesion, localized adhesion. Intimate adhesion of the bacterium to the eukaryotic membrane occurs, resulting in the activation of signal transduction pathways. Microvilli are disrupted and effaced from the apical membrane which then cups around the organism to form pedestal structures, the attaching and effacing lesion. Diarrhea may be produced by alteration of the permeability of the apical membrane and also through a malabsorption mechanism. The pathways involved in the production of the attaching and effacing lesion are described. EPEC organisms were originally thought to belong to a number of distinct serogroups; it is now apparent that many isolates belonging to these serogroups are not pathogenic or belong to other pathogenic groups of E. coli. In addition, isolates falling outside of these serogroups are considered to be true EPEC. The definition of EPEC based on serotyping is inaccurate and should be replaced by methods that specifically detect the virulence properties of EPEC.

Hemagglutinating properties of enteroaggregative Escherichia coli

Many intestinal bacterial pathogens possess hemagglutinating properties, which are indicative of their adhesive properties to the intestinal mucosal surface. To understand the bacteria-mucosa interaction, 41 strains of enteroaggregative Escherichia coli (EAggEC), a recently described category of diarrheagenic E. coli, isolated mostly from children with diarrhea in Bangladesh, India, Thailand, Central America, and South America were screened for mannose-sensitive hemagglutination and mannose-resistant hemagglutination of erythrocytes from humans, rats, mice, sheep, cattle, and rabbits. Some strains demonstrated mannose-sensitive hemagglutination of erythrocytes. Most isolates showed mannose-resistant hemagglutination of erythrocytes from all species except rabbits. The hemagglutination patterns could be classified into 18 groups. Studies with three selected isolates suggested that hemagglutinins are cell bound and are protein in nature. On the basis of the pattern of inhibition of hemagglutination by various chemicals, 39 isolates were classified into 19 groups. Hemagglutinations of many isolates were inhibited by sialic acid-containing compounds, suggesting that these compounds may be the receptors for these organisms on erythrocytes and possibly on the intestinal mucosa. These data indicate that strains of EAggEC are a heterogeneous group of organisms with different types of hemagglutinins or adhesins for the intestinal mucosal surface. Also, the adhesion characteristics of EAggEC strains may be too complex to be assessed by simple hemagglutination tests.

Mechanism of transovarian transmission of Salmonella enteritidis in laying hens

To understand the mechanism of transovarian transmission of Salmonella enteritidis in laying hens, experiments were conducted to examine the isolation of S. enteritidis from the preovulatory follicles of experimentally infected hens. Salmonella enteritidis was isolated from the preovulatory follicles in 16 birds (from follicle membrane alone in 10 birds, from the follicle yolk alone in 4 birds, and from both membrane and yolk in 2 birds). In addition, 83 S. enteritidis isolates of the major phage types prevalent in United States were tested for attachment to hen ovarian granulosa cells and HEp-2 cells. Salmonella enteritidis demonstrated three different patterns of attachment to granulosa cells, namely, local, diffuse, and aggregative; whereas, only local attachment pattern was observed on HEp-2 cells. The total number of S. enteritidis isolates that demonstrated any pattern of attachment was significantly greater on the granulosa cells than on HEp-2 cells (P < .05). Salmonella enteritidis isolates of phage Types 8 and 28 demonstrated similar patterns of attachment on granulosa cells derived from the mature and developing follicles of the hen ovary. This suggest that S. enteritidis can colonize the preovulatory follicles at different stages of development. Preincubation of bacteria with the tetrapeptide arg-gly-asp-ser, the amino acid sequence known to mediate the interaction of adhesive proteins with cells, abrogated the local attachment of bacteria to granulosa cells. These results suggest that S. enteritidis can colonize the preovulatory follicles by interacting with the ovarian granulosa cells and that adhesive proteins may be involved in this process.

Morphologic evaluation of the pathogenesis of bacterial enteric infections

Current advances in the understanding of the pathogenicity of the agents of diarrheal infections, Vibrio cholerae, diarrheagenic E. coli, Shigella, Salmonella, and enteropathogenic Yersinia, have, to a great extent, become possible due to morphological studies of host-pathogen interactions in natural and experimental infections. Despite a multigenic nature and a diversity of pathogenic features in the bacterial species and even in serogroups of the same species, it is now possible to delineate four major patterns of interaction of enteric pathogens with their cellular targets, the enterocytes, and with the immune apparatus of the gut. These patterns, epicellular cytotonic, epicellular restructuring cytotonic, invasive intraepithelial cytotonic and cytotoxic, and invasive transcellular cytotonic and cytotoxic bacteremic, underlie early pathogenesis and clinical manifestations in the respective diarrheal diseases. In this review, the results of the morphological analyses of these patterns over the last 3 decades as well as some methodological problems encountered in the interpretation of morphological observations are discussed.

Diarrhoeal disease: current concepts and future challenges. Enteroadherent Escherichia coli: a heterogeneous group of E. coli implicated as diarrhoeal pathogens

Despite a great expansion in our knowledge of the causative agents of infectious diarrhoea over the past 20 years, a significant proportion of diarrhoeal cases remains undiagnosed. Enteroadherent Escherichia coli are a relatively recently identified group of enteric bacteria which have been implicated as diarrhoeal pathogens. These organisms, defined by their ability to adhere to human epithelial-derived tissue culture cells, have been closely studied over the past 10 years and appear to be quite heterogeneous. This review summarizes our current understanding of enteroadherent E. coli and the recognized subgroups. At least 3 distinct tissue culture cell adherence patterns have been recognized: localized adherence, characteristic of enteropathogenic E. coli, diffuse adherence, and aggregative adherence. Studies examining the epidemiological and pathogenic significance of the latter 2 groups, so-called diffusely adhering E. coli and enteroaggregative E. coli, are discussed in detail.

Diffuse adherence of enteropathogenic Escherichia coli strains--processing of AIDA-I

The adherence of pathogenic Escherichia coli to the mucosa of the small intestine is an important step in the development of diarrhoea. To study the molecular basis of the diffuse adherence (DA) pattern of E. coli strains expressing the classical serotypes of enteropathogenic E. coli (EPEC), strain 2787 (O126:H27) was investigated. By expression cloning, a plasmid-derived 6.0 kb DNA fragment was identified which conferred the DA phenotype on recipient K-12 strains. This fragment encoded the 100 kDa adhesin involved in diffuse adherence (AIDA-I) which by mild heat shock treatment was isolated from the surface of the wild-type and recombinant DA-positive strains. Analysis of the entire DNA fragment revealed two open reading frames coding for proteins of 45 kDa and 132 kDa, respectively. The 132 kDa protein has been identified as the AIDA-I precursor protein which after cleavage of the signal sequence undergoes additional C-terminal processing for maturation to AIDA-I. Though the function of the cytoplasmic 45 kDa protein is not known, preliminary evidence indicates that authentic expression of the protein is a prerequisite for the correct processing of the 132 kDa precursor to AIDA-I. The AIDA-I precursor exhibits significant homology to the virG (icsA) protein of Shigella flexneri which apparently plays a major role in the events leading to the intercellular spread of invasive Shigella organisms.

Colonization by enteroaggregative Escherichia coli in travelers with and without diarrhea

Enteroaggregative Escherichia coli (EAggEC) has been found to be associated with pediatric diarrhea in developing countries. In order to determine the role of EAggEC as an agent of traveler's diarrhea, we used a sensitive and specific DNA probe for EAggEC to screen bacterial colony blots from 278 volunteers before and after travel. Colonization with EAggEC was infrequent (2.5%) prior to travel but rose to 27 to 33% after travel in volunteers who took either placebo or trimethoprim-sulfamethoxazole. Travelers who took trimethoprimsulfamethoxazole were colonized with organisms that were uniformly resistant to that antimicrobial agent; when volunteers received ciprofloxacin, colonization with EAggEC was prevented (2.0%). Although colonization rates were high in the placebo and trimethoprim-sulfamethoxazole groups, only a minority of travelers who were colonized with EAggEC experienced diarrhea. On the basis of our data, we suggest that colonization with EAggEC alone is not sufficient to cause traveler's diarrhea.

Enteroaggregative Escherichia coli strains secrete a heat-labile toxin antigenically related to E. coli hemolysin

A protein toxin of approximately 120,000 Da secreted by nonhemolytic enteroaggregative Escherichia coli strains cross-reacted in Western blots (immunoblots) with antibodies raised against the C-terminal region of E. coli hemolysin. Treatment of HEp-2 cells with enteroaggregative E. coli or culture supernatants caused elevation of intracellular calcium and stimulated calcium-dependent protein phosphorylation.