Communicable (infectious) diseases: Bacteria may produce poisons (toxins) that damage tissues and make us feel ill.

The idea

Many disease-causing bacteria harm the body not only by multiplying but by releasing toxins. These toxins are substances produced by the bacteria that interfere with host cells, damage tissues and trigger symptoms such as fever, vomiting or diarrhoea. For example, in food poisoning, bacteria such as Salmonella release toxins in the gut, irritating the lining and causing illness. The toxin may act near the site of infection or be carried in the blood to affect other organs. This explains why symptoms can appear even when the bacteria themselves remain in one place, and why treating the effects of toxins, as well as the infection, matters.

What earns the marks

• Some pathogenic bacteria release toxins as they grow and reproduce in the body.
• Toxins are poisons that damage host tissues and disrupt normal cell or organ function.
• Toxin damage produces symptoms such as fever, pain, vomiting or diarrhoea, making the person feel ill.
• Toxins may act locally at the site of infection or spread in the blood to affect other parts of the body.
• The harm caused by a bacterial infection can be due to toxins as well as to direct damage by the bacteria.

Common mistakes and how to put them right

• Confusing toxins with the bacteria themselves.

  Put it right: Toxins are poisonous substances produced by the bacteria.

• Thinking all bacteria produce toxins.

  Put it right: Only some pathogenic bacteria do, and harmless bacteria do not.

• Assuming toxins only affect the site of infection.

  Put it right: Toxins can be transported in the blood and affect distant tissues.

Exam tips

• State clearly that toxins are poisons produced by bacteria, not the bacteria themselves.
• Give a named example, such as food poisoning, to show how toxins cause symptoms.
• Link toxin action to tissue damage and to feeling ill, using both ideas in your answer.