Enterotoxigenic E. coli (ETEC), an impoant cause of traveler's diarrhea, produces a toxin that activates intestinal adenylate or guanylate cyclase. Consequently, the intestinal mucosa overexpresses cAMP, resulting in a mild and self-limited secretory diarrhea. Enteroaggregative E. coli (EAEC) does not express a toxin, but is seen to cluster over the colonic mucosa in some individuals. Although occasionally found in patients with chronic diarrhea, no clear mechanism for mucosal pathophysiology has been determined for EAEC. Enterohemorrhagic E. coli (EHEC), classically associated with strain O157:H7 and present in undercooked hamburgers, expresses a Shiga-like toxin that causes bloody diarrhea and hemolytic-uremic syndrome. Enteroinvasive E. coli (EIEC) does not express a toxin, but rather is pathogenic through its capacity to invade the colonic mucosa and evoke an inflammatory response. The resulting dysentery-like inflammatory diarrhea generally necessitates vigilant hydration, but no antibiotics are indicated. Ref: Brooks G.F. (2013). Chapter 15. Enteric Gram-Negative Rods (Enterobacteriaceae). In G.F. Brooks (Ed), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.