Causative Agent

Most of the diarrhea-causing E. coli fall into four groups: enterotoxigenic (ETEC), enteroinvasive (EIEC), enteropathogenic (EPEC), and enterohemorrhagic E. coli (EHEC). In contrast to other strains of E. coli, strains in these groups possess virulence factors that allow them to be intestinal pathogens.

Enterotoxigenic E. coli (ETEC) is a common cause of traveler's diarrhea and diarrhea in infants. Some members of this group are responsible for significant mortality in young livestock due to diarrhea. ETEC strains usually possess adhesins that allow them to adhere to and colonize the intestinal epithelium, where they secrete one or more toxins. One such toxin is nearly identical to cholera toxin in action and antigenicity.

Enteroinvasive E. coli (EIEC) infections result in a disease closely resembling that caused by Shigella sp.

Enteropathogenic E. coli (EPEC) strains cause diarrheal outbreaks in hospital nurseries and in bottle-fed infants in developing countries and also can cause chronic diarrhea in infants. They possess plasmid-dependent adhesins and cause loss of microvilli and a thickening of the cell surface at the site where the organisms attach. Enteropathogenic E. coli strains are easily distinguished from other E. coli strains by using fluorescent antibodies to identify their somatic and capsular antigens.

The fourth group of diarrhea-producing strains, enterohemorrhagic E. coli (EHEC), was discovered in 1982. Almost all of these organisms belong to a single serological type, O157:H7. They often produce severe illness including bloody diarrhea. This strain has been shown to produce a potent group of toxins that cause the death of intestinal epithelium by interfering with protein synthesis. The toxins are closely related to the Shiga toxins of Shigella dysenteriae, mentioned earlier. Unlike shigellas, however, enterohemorrhagic E. coli generally do not penetrate intestinal epithelial cells. Their toxin production depends on lysogenic conversion by a distinct bacteriophage. Many infected individuals develop hemolytic uremic syndrome, marked by lysis of red blood cells and kidney failure. Fatalities are common in infants

24.5 Bacterial Diseases of the Lower Alimentary System 615

and the elderly. Outbreaks of O157 E. coli disease have occurred commonly in Canada and Great Britain, as well as in a number of states in the United States. A large 1993 epidemic in Washington State was traced to inadequately cooked hamburgers served by a fast-food restaurant chain. Mass production of hamburger has created conditions for large outbreaks, and several recalls of processed meat have been necessary. Food and drink contaminated with cattle feces have been the source of a number of epidemics. ■ enterohemorrhagic E. co/i epidemic, pp. 256,496

Characteristics of diarrhea-causing E. coli are summarized in table 24.7.

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