Pathogens (Edexcel IGCSE Biology: Double Science)

• A pathogen is any microorganism that causes disease in another organism
• Many microorganisms are pathogens, including:
  • bacteria
  • fungi
  • protoctists
  • 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

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 protoctist that causes severe forms of malaria in humans
  • The parasite is spread by mosquitoes
  • Infected individuals experience fever, chills and fatigue

• 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 eight 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
• The symptoms of tobacco mosaic virus are the distinctive mosaic pattern of discolouration of the leaves as the virus infects the chloroplasts
  • The plant will not grow as much due to the lack of photosynthesis - this reduces the yield of crops
• The virus is spread by:
  • Plants in direct contact with an infected plant
  • The virus can stay in the soil for about 50 years

• There is no treatment for tobacco mosaic virus and the best method of control is good field hygiene to prevent the spread of the virus
  • Farmers are also using tobacco mosaic virus-resistant strains of crop plants

HIV

• HIV (Human Immunodeficiency Virus) is a virus that can eventually lead to Acquired Immunodeficiency Syndrome (AIDS)
• Symptoms:
  • HIV starts with a flu-like illness
  • If untreated it can travel to the lymph nodes and attack cells of the immune system
  • It can stay hidden in the immune system for many years until the immune system is so badly damaged it can no longer deal with other infections or cancers (this is late-stage HIV infection, known as AIDS)
• The virus is spread by:
  • Direct sexual contact
  • The exchange of bodily fluids such as blood, for example, can be transmitted when drug users share needles
  • From mother to child during birth or in breast milk
• There is no cure for HIV, although the use of antiretroviral drugs used early in the infection can now effectively control the disease to slow or halt the progress to 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