Illustration of the cholera toxin in the gut

Cholera is an infectious gastroenteritis caused by enterotoxin-producing strains of the bacterium Vibrio cholerae. Vibrio cholerae is a Gram-negative bacterium that produces cholera toxin. The cholera toxin is a protein complex made up of six protein subunits: a single copy of the A subunit (blue and red in the image), and five copies of the B subunit (shown in green). The five B subunits form a five-membered ring which binds to the surface of the intestinal epithelium. The A subunit has two important segments. The A1 portion of the chain (CTA1, blue) is a globular enzyme payload that ADP-ribosylates G proteins, while the A2 chain (CTA2, red) forms an extended alpha helix which seats snugly in the central pore of the B subunit ring. The action of the A subunit leads to secretion of chloride and water into the lumen of the intestine resulting in rapid dehydration. In its most severe forms, cholera is one of the most rapidly fatal illnesses known.