E. coli is the most common facultative bacterium in the human colon, and the majority of these strains are unable to cause disease in the normal host. However, the presence of foreign bodies, compromised host defenses, or large challenge inocula may enable these strains to cause disease. These isolates contribute to colonization resistance which decreases susceptibility to infection by enteric pathogens. Their virulence traits are for the most part distinct from strains of E. coli that cause disease outside of the bowel. This is not surprising considering site dependent differences in host environments and the defense mechanisms.
Acquisition results in disease or transient colonization.
Each group causes gastroenteritis via a distinct pathophysiologic mechanism, however, these strains rarely, if ever, invade outside of the bowel. Enteric infection due to ETEC, EPEC, EIEC, EAEC, and DAEC occurs primarily in developing countries and is uncommon in the United States. From both genetic, pathogenic and clinical perspectives, extraintestinal isolates of E. coli are distinct from strains that cause enteric disease. This designation, however, is a poor choice since the spectrum of infections caused by this group of E. coli is far from being limited to the urinary tract. These strains make up part of the normal human fecal flora.