Final answer:
The inactivation of Gi protein is caused by the cholera toxin produced by Vibrio cholerae, leading to severe diarrhea due to changes in ion channel regulation.
Step-by-step explanation:
The inactivation of Gi protein is an effect of the cholera toxin, which is produced by the water-borne bacterium Vibrio cholerae. This toxin is an example of an A-B toxin, where the A subunit is responsible for the enzymatic activity, and the B subunits are responsible for binding the toxin to a host cell. Once inside the host intestinal cells, the cholera toxin's A subunit modifies a G-protein involved in the regulation of ion channels. Specifically, it modifies and activates the G-protein that controls the opening of a chloride channel, resulting in increased cyclic AMP levels, causing a severe ion imbalance and leading to diarrhea.