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, resulting in diarrhoea, dehydration and loss of ions from the blood

Sourav PanNovember 3, 2024

Answered

The cholera bacterium, Vibrio cholerae, produces a potent toxin known as cholera toxin (CT), which plays a critical role in the pathophysiology of cholera. Here’s how this toxin affects the body, leading to severe symptoms such as diarrhea, dehydration, and loss of ions from the blood:

Mechanism of Cholera Toxin Action

1. Toxin Production and Entry:
   • When Vibrio cholerae is ingested through contaminated water or food, it colonizes the small intestine. The bacteria then secrete cholera toxin into the intestinal lumen.
2. Activation of Cellular Signaling:
   • Cholera toxin consists of two subunits: A and B. The B subunit binds to specific receptors on the surface of intestinal epithelial cells, facilitating the entry of the A subunit into the cell.
   • Once inside the cell, the A subunit is activated and catalyzes the ADP-ribosylation of a G protein involved in regulating adenylate cyclase, an enzyme that converts ATP to cyclic AMP (cAMP).
3. Increased cAMP Levels:
   • The activation of adenylate cyclase leads to elevated levels of cAMP within intestinal epithelial cells. This increase in cAMP triggers a cascade of cellular responses.
4. Secretion of Chloride Ions:
   • The elevated cAMP levels stimulate the secretion of chloride ions (Cl⁻) through specific ion channels in the cell membrane, particularly the cystic fibrosis transmembrane conductance regulator (CFTR) channel.
   • The secretion of chloride ions into the intestinal lumen creates an osmotic gradient.
5. Osmotic Movement of Water:
   • As chloride ions are secreted into the gut, sodium ions (Na⁺) and water follow passively due to osmosis. This results in a massive outflow of water into the intestinal lumen.
6. Resulting Symptoms:
   • The combination of increased chloride secretion and water influx leads to profuse watery diarrhea, often described as “rice-water stools.” This rapid loss of fluid can result in severe dehydration.
   • Additionally, significant amounts of electrolytes (such as sodium and potassium) are lost along with the water, leading to electrolyte imbalances that can have serious physiological consequences.
7. Consequences for the Body:
   • The severe diarrhea can lead to rapid dehydration, hypovolemic shock, and potentially death if not treated promptly. The loss of ions from the blood can disrupt normal cellular functions and lead to complications such as muscle cramps and cardiac issues.