Pathogens (WJEC GCSE Biology)

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 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