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Pathogenic Escherichia coli

Christine A Wanke, MD
Section Editor
Stephen B Calderwood, MD
Deputy Editor
Allyson Bloom, MD


Escherichia coli are normal inhabitants of the human gastrointestinal tract and are among the bacterial species most frequently isolated from stool cultures. When E. coli strains acquire certain genetic material, they can become pathogenic. E. coli are among the most frequent bacterial causes of diarrhea and are classified by clinical syndrome they produce (table 1) [1].

The characteristics of diarrheal illness caused by enterotoxigenic E. coli (ETEC), enteropathogenic E. coli (EPEC), enterohemorrhagic E. coli (EHEC, also called Shiga toxin-producing E. coli or STEC), enteroinvasive E. coli (EIEC), and enteroaggregative E. coli (EAEC or EAggEc) will be reviewed here. EHEC is also discussed in greater detail separately. (See "Microbiology, pathogenesis, epidemiology, and prevention of enterohemorrhagic Escherichia coli (EHEC)" and "Clinical manifestations, diagnosis and treatment of enterohemorrhagic Escherichia coli (EHEC) infection".)


E. coli can be cultured readily from the stool under aerobic conditions. On selective media, such as MacConkey agar, E. coli usually appear as pink colonies, indicating that the organism ferments lactose (picture 1). Additional biochemical identification should also be performed, since up to 10 percent of E. coli do not ferment lactose or ferment lactose relatively slowly. The most useful biochemical identification test for E. coli is the indole test, which is positive in up to 99 percent of E. coli strains.

Pathogenic E. coli are not distinguishable from other strains or from each other by the appearance on culture plates or by the results of the usual biochemical tests. To determine whether the isolated strain is one of the pathogenic strains or merely a constituent of the normal flora, additional identification techniques must be employed, which are generally limited to research laboratory settings.

EHEC O157 is the only pathogenic strain that can be identified readily in the clinical laboratory [2]. Further testing on pathogenic E. coli isolates must be performed in a research or reference laboratory. This testing can include determining the serotype of the strain, performing biological assays (bioassays) that demonstrate strain virulence, and using genetic tools, such as DNA probing or polymerase chain reaction (PCR) amplification, to identify genetic material that encodes for specific virulence factors.


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Literature review current through: Sep 2016. | This topic last updated: Sep 8, 2015.
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