pathogens

• Pathogen


  Any microorganism that causes disease in another organism (e.g. in plants or animals)
• Types of pathogens


  • Bacteria
  • Fungi
  • Protoctists
  • Viruses
• Not all species within bacteria, fungi and protoctists groups are pathogens, as many are harmless and do not cause disease
• 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
• Pathogenic bacteria


  • M. tuberculosis
  • N. meningitidis
• M. tuberculosis


  Causes tuberculosis (TB) in humans, infects the lungs, causing a chronic cough and bloody mucus, 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, can cross the barrier created by the meninges (the tissue that surrounds the brain and spinal cord) to cause acute inflammation, inflammation of the meninges causes symptoms such as fever, headache, neck stiffness and a characteristic rash
• Pathogenic fungi


  Fungal diseases that can infect organisms
• Fungal diseases are much more common in plants than animals
• Fungal diseases


  • Cattle ringworm
  • Athletes foot
• 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
• Fungal disease in plants

  • Black Sigatoka in bananas
• How Black Sigatoka affects bananas

  1. Spreads through the leaves

  2. Reduces ability to photosynthesise

  3. Causes parts of leaf to die, producing black streaks

  4. Whole leaf eventually dies
• Pathogenic protoctists


  Protist diseases that can infect organisms
• Pathogenic protoctists


  • Plasmodium falciparum (causes malaria)
  • P. infestans (causes potato blight)
• How Plasmodium falciparum causes malaria

  1. Parasite is spread by mosquitoes

  2. Infected individuals experience fever, chills and fatigue
• P. infestans


  • Has some fungal characteristics
  • Transmitted via spores
• How P. infestans affects potatoes and tomatoes

  1. First signs are small, dark brown marks on leaves

  2. Marks quickly increase in size and number

  3. Protist destroys crops leaving them completely inedible
• Viruses are not usually included in the classification of living organisms as they are not considered to be alive
• Viruses do not carry out the 8 life processes for themselves
• Reproduction


  The only life process viruses seem to display, but they must take over a host cell's metabolic pathways to carry it out
• Biological characteristics of viruses

  • 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
• HIV

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


  1. Enter host cell

  2. Take over host's genetic machinery

  3. Make more virus particles

  4. Host cell dies

  5. Virus particles released to infect more cells
• Body's immune system


  Destroys the virus and the person recovers
• Virus cannot be destroyed by the immune system quickly enough


  May cause permanent damage or death
• Virus attacks cells of the immune system

  This is the case with HIV (the Human Immunodeficiency Virus), which causes the illness called AIDS (Acquired Immune Deficiency Syndrome)
• Viruses that infect plant cells

  • Tobacco mosaic virus
• Tobacco mosaic virus


  Interferes with the ability of the tobacco plant to make chloroplasts, causing mottled patches to develop on the leaves
• Prokaryotes lack membrane-bound organelles such as mitochondria and endoplasmic reticulum.