AQA 311: Pathogens

Click on continue reading to see a summary of the key points from this part of the specification.

  • A pathogen is an agent that causes disease.
  • Pathogens include bacteria [prokaryotic single-celled organisms such as Vibrio cholerae], fungi [eukaryotic multicellular organisms such as yeast] or viruses [such as HIV].
  • Disease is a malfunction of the body or mind that causes symptoms and has a negative impact on health
  • For a pathogen to cause disease it must (i) gain entry into the host (ii) colonise the tissues of the host (ii) resist the host defences (iv) cause damage to the host systems.
  • Infection can result from the penetration of an organism’s interface with the environment. Interfaces include:
  • The digestive system; [cholera, typhoid and dysentery] the gut is infected by contaminated food and water.
  • The gas exchange system; [tuberculosis, influenza, bronchitis] the lungs and airways are infected by breathing contaminated air.
  • The skin; [septicaemia] the subcutaneous tissues are contaminated after a wound or cut exposes the tissue to contaminated air.
  • To minimise the chances of infection the host has a number of local non-specific defences including (i) a mucous layer that traps particles (ii) stomach acid to kill invading particles (iii) production of enzymes that kill invading cells.
  • Once the pathogen has colonised the host tissues it may cause recognisable symptoms. At this stage it has caused disease.
  • A pathogen causes disease by (i) damaging the cells of the host or (ii) releasing toxins.
  • Damage to the host tissues include (i) overwhelming of normal functions by sheer numbers (ii) viruses invade host cells and interfere with DNA, RNA and protein synthesis (iii) break down the cell membranes of the host.
  • Toxins can have a wide variety of effects such as altering the permeability of the gut to ions such as chloride leading to chronic dehydration (cholera).
  • Once the pathogen has transferred from one individual to another it is known as transmission.
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