Enteropathogenic Escherichia coli

Organism. Escherichia coli strains currently recognized to cause human diarrhea can be distinguished on the basis pof pathogenic mechanisms and separated into five categories: enterotoxigenic, enteroinvasive, enterohemorrhagic (EHEC), enteroaggregative, and enteropathogenic (EPEC). Additionally, strains exhibiting diffuse adherence to tissue culture cells have been proposed to be diarrheagenic but, unlike members of the other categories, have not yet been consistently shown in epidemiologic (36, 38) or challenge (91) studies to be pathogenic. Despite the fact that EPEC strains were the E. coli organisms recognized as causing diarrhea, understanding of the pathogenic mechanisms of EPEC disease initially lagged behind that of some of the other categories of diarrheagenic E. coli. EPEC strains have long been defined by exclusion as those E. coli strains of serotypes epidemiologically linked to diarrheal disease that do not belong to any of the other categories. However, as our understanding of EPEC pathogenesis has improved, it has become apparent that serotyping is an outmoded method of identifying EPEC. It has recently been appreciated that EPEC strains represent a group of clonally derived organisms that may be more related to each other than to other E. coli organisms with the same 0-antigen type (7, 72, 90). In addition, organisms which do not belong to traditional EPEC serotypes but possess putative virulence traits characteristic ofEPEC have been cultured from children with diarrhea and may be pathogenic (52). Conversely, mere membership within an enteropathogenic serotype does not guarantee virulence. It has now become more appropriate to define EPEC on the basis of unique genetic determinants that encode characteristic pathogenic properties. In light of increased knowledge regarding these genetic determinants, it is no longer necessary to distinguish among classes ofEPEC (69). Similarly, since the diffuse adhering and enteroaggregative E. coli strains lack the major putative virulence determinants described for classic enteropathogenic strains and differ in their epidemiologic features, they should not be grouped in the same category as EPEC. The purpose of this review is to summarize recent advances in our understanding of the genetic foundation of EPEC pathogenesis that allow us to describe EPEC strains, on the basis of their characteristic virulence determinants, as a unique and fascinating category of diarrheal pathogens. Early history. Credit for the first epidemiologic description of the diarrheagenic potential of E. coli is often given to John Bray (9), although prior reports exist (for more extensive reviews of early EPEC history, see references 56 and 79).