Edexcel IGCSE Biology

Revision Notes

1.2.5 Pathogens

Types of Pathogen

  • A pathogen is any microorganism that causes disease in another organism (e.g. in plants or animals)
  • Many microorganisms are pathogens including:
    • Bacteria
    • Fungi
    • Protoctists
    • Viruses
  • Not all species within these groups (apart from the viruses) are pathogens, as many bacteria, fungi and protoctists 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
  • M. tuberculosis causes tuberculosis (TB) in humans
    • The bacteria infect the lungs, causing a chronic cough and bloody mucus
    • It is a disease often associated with poor hygiene and sanitation
    • M. bovine in cows can also transmit to humans to cause TB
  • N. meningitidis causes bacterial meningitis in humans
    • Very few bacteria can cross the barrier created by the meninges (the tissue that surrounds the brain and spinal cord) however N. meningitidis crosses this barrier to cause acute inflammation
    • Inflammation of the meninges causes symptoms such as fever, headache, neck stiffness and a characteristic rash

Pathogenic fungi

  • Fungal diseases are much more common in plants than animals
  • Cattle ringworm and athletes foot are fungal diseases that exist on the surface of the skin
  • Fungal diseases in plants tend to be much more serious and can threaten entire crops
  • Black Sigatoka is a fungal disease in bananas
    • It spreads through the leaves of the plant, reducing its ability to photosynthesise
    • The lack of photosynthesis causes parts of the leaf to die; producing black streaks
    • Eventually, the whole leaf dies

Pathogenic protoctists

  • Plasmodium falciparum is a protist that causes severe forms of malaria in humans
    • The parasite is spread by mosquitoes
    • Infected individuals experience fever, chills and fatigue
  • P. infestans causes the infamous potato blight
    • The pathogen is unusual as it has some fungal characteristics
    • It is transmitted via spores
    • The first signs of potato blight are small, dark brown marks on the leaves which quickly increase in size and number
    • The protist destroys potato and tomato crops leaving them completely inedible

Viruses

  • 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, IGCSE & GCSE Biology revision notes
Structure of a typical virus

  • Examples of viruses include:
    • The tobacco mosaic virus (TMV) causes discolouring of the leaves on tobacco plants by preventing the formation of chloroplasts
    • The HIV virus causes AIDS
    • The influenza virus causes the ‘flu

Tobacco mosaic virus

  • Tobacco mosaic virus (TMV) was the first virus to be isolated by scientists
  • It is a widespread plant pathogen that infects about 150 species of plants including tomato plants and cucumbers

TMV Table

Tobacco Mosaic Virus table, downloadable IGCSE & GCSE Biology revision notes

HIV

  • HIV (Human Immunodeficiency Virus) is a virus that can eventually lead to Acquired Immunodeficiency Syndrome (AIDS)

HIV Table

HIV table, downloadable IGCSE & GCSE Biology revision notes

Influenza virus

  • Three different influenza viruses infect humans to cause the flu
    • Influenza A, influenza B and influenza C infect the cells that line the airways
    • They cause a high temperature, body aches and fatigue
    • Influenza A is the virus that causes the most cases of flu globally
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