The symptoms of infection and resulting complications are dependent upon the strain of E. coli and the site of infection. These bacteria produce toxins that have a wide range of effects. Symptoms caused by some E. coli infections range from mild to severe, bloody diarrhea, acute abdominal pain, vomiting, and fever. Gastrointestinal complications that can cause E. coli infections include irritable bowel syndrome (IBS) ischemic colitis, appendicitis, perforation of the large bowel, and in some instances gangrene in the colon. Other known E. coli-causing infections may include chronic renal failure, pancreatitis, and diabetes mellitus. Some neurological symptoms such as drowsiness, seizure and coma may occur. In infants, E. coli infections are present in cases of infantile gastroenteritis and neonatal meningitis.

Strains of E. coli that produce diarrhea were initially distinguished by their O (somatic) antigens found on the bacterial surface. Although there is an overlap in characteristics between strains, they may be classified into four main groups; enterohemorrahagic (0157),enteropathogenic (055,0111), enterotoxigenic (06,078), and enteroinvasive (0124,0164).