Pathogens (WJEC GCSE Biology: Combined Science)

• 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 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)

Bacterial pathogen

• 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
• A bacterial cell consists of the following key features:
  • A cell wall
  • Cell membrane
  • Cytoplasm
  • No distinct nucleus

Structure of a bacterium diagram

A simple bacterium structure

Fungal pathogen

• Fungal diseases are much more common in plants than animals, including humans
• 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 athlete's 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

Protist pathogen

• Protists are a diverse group of eukaryotic (and usually unicellular) organisms
• They are parasites which means they need a host 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

Viral pathogen

• Viruses are not part of any classification system as they are not considered living things
  • They do not carry out the seven life processes for themselves, instead they take over a host cell’s metabolic pathways so they can make multiple copies of themselves
• Examples of diseases caused by viruses are:
  • Human immunodeficiency virus (HIV)
  • Coronavirus (COVID-19)
  • Measles
• Virus structure is as follows:
  • Genetic material/genes (RNA or DNA)
  • A protein coat
  • Some viruses contain an outer envelope as shown in the image below
• As a result, they can only replicate inside living cells

Structure of a virus diagram

Structure of a typical virus

Communicable diseases

• Pathogens cause communicable diseases which means they transfer from a diseased host to a healthy organism during infection, in other words, the disease is infectious

• Pathogens can be passed on from host to host in different ways, including:
  • Direct contact - the pathogen is passed directly from one host to another by transfer of body fluids such as blood or semen (eg HIV, gonorrhoea, hepatitis B & C)
  • Indirect contact - the pathogen leaves the host and is carried in some way to another, uninfected individual

Methods of transmission table