Common Pathogens & Communicable Diseases (OCR A Level Biology): Revision Note

Common Pathogens & Communicable Diseases

• A disease is an illness or disorder of the body or mind that leads to poor health

• Each disease is associated with a set of signs and symptoms

• Communicable/infectious diseases are caused by pathogens and are transmissible (can be spread between individuals within a population)

• Both plants and animals can be affected by pathogens

Infectious & non-infectious diseases table

• Many microorganisms are pathogens including:

  • Bacteria

  • Viruses

  • Fungi

  • Protoctists

• To control disease, it is very important to know what pathogen is causing it

Bacteria

• Bacteria are a diverse range of prokaryotic organisms

• Some bacteria are non-pathogenic (they do not cause any disease or damage) while others are pathogenic

• 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

• Ring rot diseases in potato plants are caused by bacterial pathogens

  • The bacteria infect the vascular tissue and prevent the transport of water, causing the plant to wilt and die

  • The infection spreads into the potato tubers where the vascular tissue is arranged in a ring, producing the characteristic black ring of rot

Viruses

• Viruses do not have a cellular structure

  • This means they can't respire, produce ATP, replicate genetic material or synthesise protein

• They infect host cells and hijack their machinery to replicate their own genetic material and proteins

• The first virus ever discovered was the Tobacco Mosaic Virus (TMV)

• TMV infects several plant species

  • It causes a distinct yellowing of the leaves which produces a mosaic pattern

• 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

    • It has a capsid that surrounds 8 single-stranded molecules of RNA

• The human immunodeficiency virus (HIV) infects specific cells of the immune system

  • It is an enveloped retrovirus

  • The viral enzyme reverse transcriptase produces single-stranded DNA from its viral RNA

  • DNA polymerase synthesises double-stranded DNA from this single-stranded DNA

  • The double-stranded DNA is inserted into the host DNA and can remain inactive for many years

  • Once activated the DNA provirus is used to synthesise new viruses

Protoctista

• Protists are unicellular eukaryotes

• 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

Fungi

• Fungi have a similar structure to plants

  • Their eukaryotic cells have cell walls and large central vacuoles

  • However, instead of being made of separate cells, their bodies consist of filaments known as hyphae

  • These hyphae form a network and spread throughout a host/soil

• Fungal diseases are much more common in plants than animals

• Athletes foot is a fungal disease 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

Common pathogens and related communicable diseases in humans table

Common pathogens and related communicable diseases in plants table