Pathogens (Edexcel GCSE Biology)

• 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

• Viruses are not usually included in the classification of living organisms as they are not considered to be alive
  • This is due to the fact that viruses do not carry out the 8 life processes for themselves

• In fact, the only life process they seem to display is reproduction but even to carry out this process they must take over a host cell’s metabolic pathways in order to make multiple copies of themselves
• Viruses, which have a wide variety of shapes and sizes, all share the following biological characteristics:
  • They are small particles (always smaller than bacteria)
  • They are parasitic and can only reproduce inside living cells
  • They infect every type of living organism
  • They have no cellular structure but have a protein coat and contain one type of nucleic acid, either DNA or RNA

Structure of a typical virus

• Examples of viruses include:
  • Tobacco mosaic virus (TMV) causes discolouring of the leaves on tobacco plants by preventing the formation of chloroplasts
  • HIV virus causes AIDS
  • Influenza virus causes the ‘flu’
  • Ebola virus causes haemorrhagic fever

• When a virus infects a host cell, it can then reproduce using two different pathways, known as:
  • The lytic pathway
  • The lysogenic pathway

The lytic pathway

1. The virus infects the host cell and injects its DNA into the cytoplasm
2. Next, the virus uses proteins and enzymes within the host cell to produce new virus particles
3. Finally, the cell bursts, releasing the virus particles into the host organism to infect more cells

The lysogenic pathway

1. The virus injects its DNA into the host cell and the DNA becomes incorporated into the host DNA
2. As the host cell replicates, the viral DNA replicates also, but no new virus particles are made during this time (the virus is dormant)
3. Changes in the environment (e.g. a chemical trigger) cause the viral DNA to move to the lytic pathway to make new virus particles

A virus can follow two different pathways after infecting the host cell