Pathogens (Edexcel GCSE Combined Science: Biology): Revision Note

Types of Pathogen

• Communicable diseases are spread by pathogens

• A pathogen is any microorganism that causes disease in another organism (e.g. in plants or animals)

• Many microorganisms are pathogens including:

  • Bacteria

  • Fungi

  • Protists (protoctists)

  • Viruses

• Not all species within these groups (apart from the viruses) are pathogens, as many bacteria, fungi and protists are harmless and do not cause disease

• However, all viruses are pathogenic as they can only exist by living inside the living cells of other organisms (or by using these cells to create more viruses)

Pathogenic bacteria

• Pathogenic bacteria do not always infect the hosts of cells, they can remain within body cavities or spaces

• Toxins produced by the bacteria also damage cells

• They are small and can reproduce very quickly

• Bacterial infections include:

  • M. tuberculosis causes tuberculosis (TB) in humans

  • N. meningitidis causes bacterial meningitis in humans

  • Helicobacter Pylori causes stomach ulcers

  • V. Cholerae causes cholera in humans

Pathogenic fungi

• Fungal diseases are much more common in plants than animals

• Fungi can be single-celled or multicellular (with threads of hyphae)

• The spores they produce allow them to infect other organisms

• In plants, fungal diseases tend to be much more serious and can threaten entire crops

• Fungal diseases include:

  • Cattle ringworm and athletes foot are fungal diseases in animals

  • Black Sigatoka is a fungal disease in bananas

  • Chalara Ash Dieback is a fungal disease that affects ash trees

Pathogenic protists

• Protists are a diverse group of eukaryotic (and usually unicellular) organisms

• They are parasites which means they need a host in order to survive

• Only a small number of protists are pathogenic, but the diseases they cause are often serious

• Examples of diseases caused by protists are:

  • Plasmodium falciparum is a protist that causes severe forms of malaria in humans

  • P. infestans causes the infamous potato blight