SUMMARY Escherichia coli is the predominant nonpathogenic facultative flora of the human intestine. Some E. coli strains, however, have developed the ability to cause disease of the gastrointestinal, urinary, or central nervous system in even the most robust human hosts. Diarrheagenic strains of E. coli can be divided into at least six different categories with corresponding distinct pathogenic schemes. Taken together, these organisms probably represent the most common cause of pediatric diarrhea worldwide. Several distinct clinical syndromes accompany infection with diarrheagenic E. coli categories, including traveler’s diarrhea (enterotoxigenic E. coli), hemorrhagic colitis and hemolytic-uremic syndrome (enterohemorrhagic E. coli), persistent diarrhea (enteroaggregative E. coli), and watery diarrhea of infants (enteropathogenic E. coli). This review discusses the current level of understanding of the pathogenesis of the diarrheagenic E. coli strains and describes how their pathogenic schemes underlie the clinical manifestations, diagnostic approach, and epidemiologic investigation of these important pathogens.

The ability of enteropathogenic Escherichia coli (EPEC) to form attaching and effacing intestinal lesions is a major characteristic of EPEC pathogenesis. Using TnphoA mutagenesis we have identified a chromosomal gene (eae, for E. coli attaching and effacing) that is necessary for this activity. A DNA probe derived from this gene hybridizes to 100% ofE. coli of EPEC serogroups that demonstrate attaching and effacing activity on tissue culture cells as well as other pathogenic E. coli that produce attaching and effacing intestinal lesions, such as RDEC-1 (an EPEC of weanling rabbits) and enterohemorrhagic E. coli. The predicted amino acid sequence derived from the nucleotide sequence of eae shows significant homology to that of the invasin of Yersinia pseudotuberculosis.Enteropathogenic Escherichia coli (EPEC) are an important cause of infant diarrhea in the developing world (1-5). Diarrhea caused by EPEC can be severe, as evidenced by a 30% fatality rate in a recent nursery outbreak (5). Once a serious cause of "summer diarrhea" and nursery outbreaks in industrialized countries, diarrhea due to EPEC now occurs less frequently in these areas, although outbreaks in nurseries and day-care centers are reported occasionally (6, 7).Although EPEC were the first E. coli to be recognized as a diarrheal pathogen, the elucidation of EPEC virulence factors has lagged behind that of enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), and enterohemorrhagic E. coli (EHEC). Unlike that of ETEC and EHEC, the pathogenesis of EPEC does not appear to involve a toxin and no fimbrial colonization factors have been described. A major advance in the understanding of EPEC pathogenesis was the demonstration that EPEC possess a high molecular weight plasmid which is required for full virulence in volunteers (8) and is associated with the ability to adhere to HEp-2 epidermoid carcinoma cells in a pattern described as localized adherence (9, 10). This adherence phenotype is a characteristic of E. coli of the major EPEC serotypes (11) and is detectable with a DNA probe derived from one such plasmid called the EAF probe (EPEC adherence factor).Perhaps the most important feature of EPEC pathogenesis is the ability of EPEC to produce characteristic histopathological intestinal lesions in humans or experimental animal models. This lesion has been described by Moon et al. (12) as an "attaching and effacing" (A/E) lesion and is characterized by the intimate adherence of bacteria to the enterocyte, dissolution of the brush border at the site of bacterial attachment, and disruption of the cellular cytoskeleton. Within the enterocyte, high concentrations of filamentous actin are present at the site of bacterial attachment and the enterocyte membrane is frequently seen cupping the bacteria, often forming a pedestal-like structure. The production of this lesion can occur in the absence of the EAF plasmid, as evidenced by the observation that A/E lesions are produced by EAF plasmid-cured derivatives of EPEC isolates in experimental an...

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The ability to attach to epithelial cels, efface the microvillus surface, and disrupt the underlying cytoskeleton is characteristic of enteropathogenic Escherichia coli (EPEC). Recently, eae, a gene necessary for this phenomenon, was described (A. E.

SummaryAttaching and effacing (AE) bacteria are a diverse group of gastrointestinal pathogens, comprising members of four genera, that cause the intestinal epithelial microvilli to be replaced with raised clusters of filamentous actin that conform to the surface of attached bacteria. We have cloned a 35.4 kb 'pathogenicity island' from the prototype AE bacterium, enteropathogenic Escherichia coli, containing all previously described AE genes. Transfer of this pathogenicity island to avirulent E. coli converts the recipients into strains that secrete virulence proteins, induce host signaltransduction pathways, and cause AE lesions on cultured epithelial cells. These results demonstrate that this pathogenicity island contains all pathogen-specific genes necessary for inducing AE lesions, and that the defining feature of this class of pathogens can be acquired by an avirulent bacterium in a single genetic step.

Attenuated Vibrio cholerae vaccine strains specifically mutated in genes encoding cholera toxin (CT) are still capable of causing mild to moderate diarrhea. Culture supernatants of V. cholerae strains, both CT-positive and CT-negative, were examined in Ussing chambers, and a toxin was found that increases the permeability of the small intestinal mucosa by affecting the structure of the intercellular tight junction, or zonula occludens. The activity of this toxin is reversible, heat-labile, sensitive to protease digestion, and found in culture supernatant fractions containing molecules between 10 and 30 kDa in size. Production ofthis factor (named ZOT for zonula occludens toxin) correlates with diarrheagenicity of V. cholerae strains in volunteers and may represent another virulence factor of infectious diarrhea that must be eliminated to achieve a safe and effective live oral vaccine against cholera.Vibrio cholerae produces the copious diarrhea characteristic of cholera by means of a potent enterotoxin, cholera toxin (CT). The A subunit of CT, encoded by ctxA, stimulates adenylate cyclase in intestinal epithelial cells, which results in net secretion of fluid into the intestinal lumen (1). Initial recombinant V. cholerae vaccine strains, attenuated by removal of the ctxA gene, were greatly reduced in their ability to induce diarrhea in volunteers (2). However, despite the absence of CT, these strains were still capable of inducing an unacceptable amount of diarrhea. One such strain, V. cholerae CVD101, is a ctxA deletion mutant of V. cholerae strain 395 in which 94% of the sequence encoding the A1 peptide of CT has been removed (3). When evaluated in volunteer studies, CVD101 caused mild to moderate diarrhea (mean stool volume of 0.9 liter with a range of 0.3-to 2.1 liters) in 54% of subjects ingesting this organism (2). While greatly attenuated compared with the parent strain 395 [which induces a mean diarrheal stool volume of 5.5 liters with a range of 0.3-44 liters in >90% of volunteers (4)], the amount of diarrhea induced by CVD101 is still unacceptable for use of this strain as a vaccine.Given the magnitude of the diarrhea induced in the absence of CT, we hypothesized that V. cholerae produced a second toxin, which was still present in strains deleted of the ctxA sequence. To investigate this hypothesis, we examined V. cholerae strains with and without intact ctx genes by using rabbit intestinal tissue mounted in Ussing chambers, a classic technique for studying the process of transport across intestinal tissue (5, 6). The results indicate that V. cholerae produces a toxin that increases intestinal tissue conductance by altering the structure of intercellular tight junctions. Production of this toxin correlates with diarrheagenicity of V. cholerae strains in volunteers and may represent another virulence mechanism of infectious diarrhea. MATERIALS AND METHODSBacterial Strains and Growth Conditions. V. cholerae 395 is a classical Ogawa CT-positive strain that has been extensively studied in volunteer...

Escherichia coli that exhibit the aggregative pattern of adherence to HEp-2 cells (enteroadherent-aggregative E. coli [EA-AggEC]) have been epidemiologically incriminated as a cause of diarrhea. We undertook a preliminary microbiological and pathogenetic characterization of 42 isolates of this putative pathogen. The strains were negative by tests with DNA probes for enteropathogenic, enterotoxigenic, enteroinvasive, and enterohemorrhagic E. coli and, by serotype, did not fit these categories. Thirty-nine of 42 strains had a 55-65-megadalton plasmid; many shared DNA homology. With one representative strain, plasmid transfer was accompanied by transfer of smooth lipopolysaccharide, fimbriae expression, and the aggregative property. EA-AggEC caused characteristic lesions in rabbit and rat ileal loops. The intestinal lesions and (Shiga-like) limb paralysis and death in rabbits inoculated with live organisms suggest toxin involvement; assays for Shiga-like toxins were negative. These preliminary results support the contention that EA-AggEC may represent a distinct category of diarrheagenic E. coli.