Proposal for a New Inclusive Designation for Extraintestinal Pathogenic Isolates ofEscherichia coli:ExPEC

Although Escherichia coli is probably the best-known bacterial species and one of the most common isolates in clinical microbiology laboratories, misconceptions abound with regard to the various types of E. coli and the infections that they cause. E. coli strains of biological significance to humans can be broadly classified (on the basis of both genetic and clinical criteria) into 3 major groups: commensal strains, intestinal pathogenic (enteric or diarrheagenic) strains, and extraintestinal pathogenic strains. Commensal strains of E. coli constitute the bulk of facultative fecal flora in most healthy humans, other mammals, and birds. These strains appear to be adapted for peaceful coexistence with the host and do not appear to cause disease within the intestinal tract. Furthermore, in humans they usually do not cause disease outside the intestinal tract, except when precipitating factors are present, such as an indwelling foreign body or impairment of host defenses. Most human commensal strains originate from E. coli phylogenetic group A, as defined by multilocus enzyme electrophoresis, as do the familiar laboratory strains of E. coli (e.g., K-12 and its derivatives) 1, 2. Commensal strains of E. coli typically lack the specialized virulence traits that are present in intestinal and extraintestinal pathogenic strains.