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cholera toxin is secreted by the gram-negative bacterium Vibrio cholerae. It is a gastrointestinal toxin that increases adenylate cyclase activity of mucosa of the small intestine, thereby increasing cyclic AMP within these cells. Cyclic AMP stimulates active transport of ions by these epithelial cells, resulting in a large efflux of Na+ and water into the gut. The toxin enters cells by interacting with a GM1 ganglioside on the cell surface. After entry, the A1 subunit catalyzes the transfer of an ADP-ribose unit from NAD+ to an arginine side chain of the adenylate cyclase regulatory G protein, blocking its GTPase activity. Thus the adenylate cyclase deactivation mechanism is destroyed, and cyclic AMP is continually produced. The major sign of cholera poisoning is severe diarrhea; several liters of body water may be lost within a few hours, leading to shock and death if fluid is not replaced. Treatment involves physiological support (including i.v. fluids) and antibiotics.