10.2 - Cholera

Cholera and its toxin

• Cholera is a disease caused by a bacterium.
• It's transmitted through water contaminated with the bacterium - for example, drinking water that has been polluted by sewage.
• Its main effects are diarrhoea, dehydration, and the loss of ions from the blood.
• These effects are all produced by a toxin that the bacterium releases.

How the toxin causes its effects

1. The toxin makes the lining of the small intestine secrete chloride ions into the gut.
2. This raises the concentration of dissolved ions in the small intestine, which lowers the water potential there.
3. The gut now has a lower water potential than the blood, so water moves out of the blood and into the small intestine by osmosis.
4. The extra water in the gut produces watery diarrhoea.
5. So much water is lost from the blood that the person becomes dehydrated.
6. Chloride ions and other ions are also lost in the diarrhoea, so the balance of ions in the blood is disturbed.