• 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

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