The enterotoxins of both Vibrio cholerae O1 and toxigenic strains of Escherichia coli produce serious diarrhea by what mechanism?

• A. Stimulation of adenylate cyclase, which gives rise to excessive fluid secretion by the cells of the small intestine
• B. Penetration of the bowel mucosa
• C. Stimulation of colicin production
• D. Elaboration of a dermonecrotizing toxin

Answer: (A)

Secretory diarrhea from these toxins comes from turning on adenylate cyclase in the intestinal lining, which raises intracellular cAMP. The toxins modify a G-protein (Gs) so it stays active, driving continuous cAMP production. Higher cAMP activates protein kinase A, leading to opening of the CFTR chloride channel and a large loss of chloride and water into the gut lumen. That rapid, watery secretion is what causes the characteristic diarrheal output. This is not due to invading the mucosa or destroying tissue; it’s a secretory effect driven by altered signaling in enterocytes. (Note: the heat-stable toxin from some E. coli strains increases cGMP, a different pathway.)