Explain that the cholera bacterium produces a toxin that causes secretion of chloride ions into the small intestine, causing osmotic movement of water into the gut, causing diarrhoea, dehydration and loss of ions from the blood

Cholera is a disease caused by the bacterium Vibrio cholerae, which produces a potent toxin known as cholera toxin. This toxin plays a crucial role in the pathogenesis of the disease by disrupting normal physiological processes in the intestines, leading to severe diarrhea and dehydration. Here’s an explanation of how cholera toxin operates:

Mechanism of Cholera Toxin Action

1. Binding to Intestinal Cells:
   • Cholera toxin consists of one enzymatically active A subunit and five B subunits. The B subunits bind to specific receptors (GM1 gangliosides) on the surface of intestinal epithelial cells (enterocytes) in the small intestine, facilitating the entry of the toxin into these cells.
2. Activation of Adenylate Cyclase:
   • Once inside the cell, the A subunit is released and activates adenylate cyclase, an enzyme that converts ATP into cyclic adenosine monophosphate (cAMP). This increase in cAMP levels is critical for the subsequent effects of the toxin.
3. Secretion of Chloride Ions:
   • Elevated cAMP levels lead to the activation of protein kinase A (PKA), which stimulates the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel on the apical membrane of enterocytes. This results in an efflux of chloride ions (Cl⁻) from the intestinal cells into the lumen of the intestine.
4. Osmotic Movement of Water:
   • The secretion of chloride ions creates an osmotic gradient that draws water into the intestinal lumen. As water follows the movement of chloride ions osmotically, this leads to a significant increase in fluid accumulation in the intestines.
5. Diarrhea and Dehydration:
   • The result is profuse watery diarrhea, often described as “rice-water stools,” which can lead to rapid dehydration and loss of electrolytes, including sodium and potassium ions from the blood. This severe fluid loss can result in hypovolemic shock and, if untreated, can be fatal.