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Escherichia coli are Gram-negative bacteria that are part of the normal intestinal flora of humans and animals. Most strains do not cause illness in healthy persons, but some have virulence attributes that enable them to cause disease. Pathogenic E. coli are categorized into pathotypes based on virulence genes that reflect distinctive aspects of their pathogenesis. Several pathotypes are associated with diarrhea, including enteropathogenic E. coli (EPEC), enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), Shiga toxin-producing E. coli (STEC),1 and enteroaggregative E. coli (EAEC). Other pathotypes cause urinary tract infections, bloodstream infections, and meningitis. The relative importance of diarrheagenic pathotypes in the burden of disease globally and regionally has been difficult to assess because routine diagnostic methods for their detection are not readily available. Prevalence estimates for the different pathotypes largely come from studies that have used a variety of laboratory-developed tests.1 As syndromic-focused assays that target a broad range of diarrheal pathogens are adopted, our estimates for the global and regional burden of disease attributable to these pathotypes should improve.

E. coli are also characterized by serotype, which is defined by a combination of up to four surface antigens (O, K, H, and F). The two commonly used surface antigens for serotyping diarrheagenic E. coli are the O antigen (repeating oligosaccharides of the lipopolysaccharide molecule) and the H antigen (flagellin of the flagellum). Historically, O:H serotyping has been useful for subtyping strains and predicting a strain’s pathotype, because a relatively strong correlation exists between specific serotypes and combinations of virulence genes. Predicting pathotype is more reliably done by determining virulence gene content. Serotyping, however, still provides valuable context for understanding a strain’s pathogenesis and relating historical with current findings. Because virulence genes are commonly on mobile genetic elements (e.g., bacteriophages, plasmids, and pathogenicity islands), E. coli often have virulence factors associated with more than one pathotype. The continuous movement of virulence genes within E. coli leads to the emergence of new strains capable of causing disease with clinical features reflective of new combinations of genes. An example is the O104:H4 strain that caused a large outbreak of bloody diarrhea in Germany in 2011; it produced Shiga toxin and, unlike other STEC, had adherence properties typical of EAEC.2


STEC are also called verotoxigenic E. coli (VTEC), and the term enterohemorrhagic E. coli is sometimes used to specify STEC strains capable of causing human illness, especially bloody diarrhea and hemolytic uremic syndrome (HUS).


EPEC was the first pathotype widely recognized as a cause of diarrheal illness. In the 1940s, bacteria later classified as EPEC were linked to “infantile summer diarrhea.” Investigations of outbreaks in newborn nurseries in industrialized countries, many with high fatality rates, implicated EPEC.3,4 Although outbreaks in nurseries are now almost never reported from industrialized countries, EPEC continue to be an important cause of diarrheal illness ...

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