Pathogens (Edexcel IGCSE Biology: Double 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
  • 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 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:
  • 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

HIV

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

HIV Table

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