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.

Diarrheagenic Escherichia coli

Few microorganisms are as versatile as Escherichia coli. An important member of the normal intestinal microflora of humans and other mammals, E. coli has also been widely exploited as a cloning host in recombinant DNA technology. But E. coli is more than just a laboratory workhorse or harmless intestinal inhabitant; it can also be a highly versatile, and frequently deadly, pathogen. Several different E. coli strains cause diverse intestinal and extraintestinal diseases by means of virulence factors that affect a wide range of cellular processes.

Pathogenic Escherichia coli

Various gram-negative animal and plant pathogens use a novel, sec-independent protein secretion system as a basic virulence mechanism. It is becoming increasingly clear that these so-called type III secretion systems inject (translocate) proteins into the cytosol of eukaryotic cells, where the translocated proteins facilitate bacterial pathogenesis by specifically interfering with host cell signal transduction and other cellular processes. Accordingly, some type III secretion systems are activated by bacterial contact with host cell surfaces. Individual type III secretion systems direct the secretion and translocation of a variety of unrelated proteins, which account for species-specific pathogenesis phenotypes. In contrast to the secreted virulence factors, most of the 15 to 20 membrane-associated proteins which constitute the type III secretion apparatus are conserved among different pathogens. Most of the inner membrane components of the type III secretion apparatus show additional homologies to flagellar biosynthetic proteins, while a conserved outer membrane factor is similar to secretins from type II and other secretion pathways. Structurally conserved chaperones which specifically bind to individual secreted proteins play an important role in type III protein secretion, apparently by preventing premature interactions of the secreted factors with other proteins. The genes encoding type III secretion systems are clustered, and various pieces of evidence suggest that these systems have been acquired by horizontal genetic transfer during evolution. Expression of type III secretion systems is coordinately regulated in response to host environmental stimuli by networks of transcription factors. This review comprises a comparison of the structure, function, regulation, and impact on host cells of the type III secretion systems in the animal pathogens Yersinia spp., Pseudomonas aeruginosa, Shigella flexneri, Salmonella typhimurium, enteropathogenic Escherichia coli, and Chlamydia spp. and the plant pathogens Pseudomonas syringae, Erwinia spp., Ralstonia solanacearum, Xanthomonas campestris, and Rhizobium spp.

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Type III Protein Secretion Systems in Bacterial Pathogens of Animals and Plants

Since their initial recognition 20 years ago, Shiga toxin-producing Escherichia coli (STEC) strains have emerged as an important cause of serious human gastrointestinal disease, which may result in life-threatening complications such as hemolytic-uremic syndrome. Food-borne outbreaks of STEC disease appear to be increasing and, when mass-produced and mass-distributed foods are concerned, can involve large numbers of people. Development of therapeutic and preventative strategies to combat STEC disease requires a thorough understanding of the mechanisms by which STEC organisms colonize the human intestinal tract and cause local and systemic pathological changes. While our knowledge remains incomplete, recent studies have improved our understanding of these processes, particularly the complex interaction between Shiga toxins and host cells, which is central to the pathogenesis of STEC disease. In addition, several putative accessory virulence factors have been identified and partly characterized. The capacity to limit the scale and severity of STEC disease is also dependent upon rapid and sensitive diagnostic procedures for analysis of human samples and suspect vehicles. The increased application of advanced molecular technologies in clinical laboratories has significantly improved our capacity to diagnose STEC infection early in the course of disease and to detect low levels of environmental contamination. This, in turn, has created a potential window of opportunity for future therapeutic intervention.

Pathogenesis and Diagnosis of Shiga Toxin-Producing Escherichia coli Infections

Although Escherichia coli can be an innocuous resident of the gastrointestinal tract, it also has the pathogenic capacity to cause significant diarrheal and extraintestinal diseases. Pathogenic variants of E. coli (pathovars or pathotypes) cause much morbidity and mortality worldwide. Consequently, pathogenic E. coli is widely studied in humans, animals, food, and the environment. While there are many common features that these pathotypes employ to colonize the intestinal mucosa and cause disease, the course, onset, and complications vary significantly. Outbreaks are common in developed and developing countries, and they sometimes have fatal consequences. Many of these pathotypes are a major public health concern as they have low infectious doses and are transmitted through ubiquitous mediums, including food and water. The seriousness of pathogenic E. coli is exemplified by dedicated national and international surveillance programs that monitor and track outbreaks; unfortunately, this surveillance is often lacking in developing countries. While not all pathotypes carry the same public health profile, they all carry an enormous potential to cause disease and continue to present challenges to human health. This comprehensive review highlights recent advances in our understanding of the intestinal pathotypes of E. coli.

Recent Advances in Understanding Enteric Pathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC) adhere to the intestinal mucosa and produce an attaching and effacing (AE) lesion in the brush border microvillous membrane; the AE lesion is characterized by localized destruction of microvilli and intimate attachment of bacteria to the apical enterocyte membrane. A similar lesion is seen when bacteria adhere in vitro to a variety of human tissue culture cell lines. In both cases, dense concentrations of microfilaments are present in the apical cytoplasm beneath attached bacteria. Using a fluorescein-labeled phallotoxin, we have shown that these microfilaments are composed of actin. Cells infected with EPEC and EHEC strains known from electron microscopic studies to produce the AE lesion all exhibited intense spots of fluorescence which corresponded in size and position with each adherent bacterium; cells infected with adherent E. coli strains known not to produce the AE lesion did not produce this striking pattern of fluorescence and were indistinguishable from uninfected control cells. These results indicate that such site-specific concentrations of cytoskeletal actin are characteristic of the AE membrane lesion and can form the basis of a simple, highly sensitive diagnostic test for EPEC and EHEC. Images

Actin accumulation at sites of bacterial adhesion to tissue culture cells: basis of a new diagnostic test for enteropathogenic and enterohemorrhagic Escherichia coli.

A collection of 44 enteroaggregative Escherichia coli (EAggEC) strains isolated from infants with diarrhea in India and the United Kingdom were examined for their ability to adhere in vitro to human intestinal mucosa and by electron microscopy for production of putative adherence factors. None of the strains adhered to human duodenal mucosa, and six strains tested did not adhere to ileal mucosa; all 44 strains, however, adhered to human colonic mucosa in localized aggregates. Electron microscopy of infected colonic mucosa indicated fimbrially mediated adhesion of the EAggEC strains. Four morphologically distinct kinds of fimbriae, including a new morphological type of E. coli fimbriae consisting of bundles of fine filaments, were identified among the EAggEC strains; this new type of fimbria was observed in 43 of the 44 EAggEC strains. Forty-three of the 44 EAggEC strains were positive with a DNA probe developed to identify EAggEC, and most of the strains belonged to serotypes unrelated to the other major classes of diarrheic E. coli. These results suggest that EAggEC may be a large-bowel pathogen and colonize the colon by a fimbrially mediated adhesion mechanism.

https://iai.asm.org/content/iai/60/5/2083.full.pdf

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

EPEC colonise the small bowel and cause diarrhoea by a mechanism that is unrelated to production of recognised enterotoxins or mucosal invasion. Intestinal ion transport is thought to be regulated by intracellular second messenger molecules which function through activation of their appropriate protein kinases. Since EPEC do not induce elevated levels of cyclic nucleotide second messengers, we considered the possibility that EPEC cause diarrhoea by a mechanism which involves an intracellular calcium second messenger. Cultured HEp-2 cells were used as a model of EPEC infection and levels of intracellular calcium determined fluorimetrically using the calcium indicator dye, Quin-2. HEp-2 cells infected with EPEC for 3 hours showed a 5-6 fold increase in calcium levels (280-320 nanomolar) compared to uninfected cells (55 nanomolar). Alteration in the phosphorylation state of proteins following infection of 32P treated HEp-2 cells with EPEC was analysed by autoradiography following electrophoresis. A protein band of Mr 21-23 kDa showed increased phosphorylation within 2 hours of infection; the same band was phosphorylated a) on treatment of cells with TPA, a potent secretagogue and activator of protein kinase C (PKC), and b) on addition of purified PKC to HEp-2 cell lysates. These results suggest that EPEC pathogenesis involves contact of bacteria with target cells, a calcium intracellular signal and phosphorylation of a critical host cell protein.

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T J Baldwin

Papers

Actin accumulation at sites of bacterial adhesion to tissue culture cells: basis of a new diagnostic test for enteropathogenic and enterohemorrhagic Escherichia coli.

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

Elevated calcium levels and protein kinase c activation in response to infection by enteropathcgenic esciierichia coli (epec)

Comparative early histologic healing quality of magnetic versus stapled small bowel anastomosis.