Heat-stable enterotoxins (STs) are secretory peptides produced by some bacterial strains, such as enterotoxigenic Escherichia coli[2] which are in general toxic to animals.

Different STs recognize distinct receptors on the surface of animal cells and thereby affect different intracellular signaling pathways.

For example, STa enterotoxins bind and activate membrane-bound guanylate cyclase, which leads to the intracellular accumulation of cyclic GMP and downstream effects on several signaling pathways.

[7] The mature STa protein from Escherichia coli, which is the cause of acute diarrhoea in infants and travellers in developing countries, is a 19-residue peptide containing three disulphide bridges that are functionally important.

The disulfide bonds are crucial for the toxic activity of the protein, and are required for maintenance of the tertiary structure, and subsequent interaction with the particulate form of guanylate cyclase, increasing cyclic GMP levels within the host intestinal epithelial cells.