F6 Microbes and Disease


F.6.1  List six methods by which pathogens are transmitted and gain entry to the body

  • Vector-based (carried by animals)
  • Inhalation (air-borne droplets)
  • Drinking contaminated water
  • Eating contaminated foods
  • Openings on surface of skin (i.e. injuries)
  • Sexual contact (unprotected)

F.6.2  Distinguish between intracellular and extracellular bacterial infection using Chlamydia and Streptococcus as examples

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F.6.3  Distinguish between endotoxins and exotoxins

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Exotoxin versus Endotoxin

exotoxins versus endotoxins

F.6.4  Evaluate methods of controlling microbial growth by irradiation, pasteurisation, antiseptics and disinfectants


  • Irradiation involves destroying pathogens using radiation (usually gamma radiation – UV radiation is less effective)
  • It is useful for sterilising food, medical instruments and equipment
  • Ionising (gamma) radiation produces free radicals which may affect the flavour of food, although the food itself does not become radioactive
  • However some people are reluctant to buy irradiated food, fearing possible effects


  • Pasteurisation involves heating a liquid to a specified temperature for a specific duration to kill pathogens
  • It does not effect the composition or nutritional value of the liquid itself
  • After pasteurisation, it is important to refrigerate the liquid to delay spoilage (it may be only bacteriostatic for non-pathogenic bacteria)
  • Pasteurisation is commonly used to maintain and improve the quality of milk and milk products


  • Disinfectants are chemicals that kill bacteria (but not endospores)
  • They are bacteriocidal in concentrated forms (e.g. bleaches, detergents) but may be only bacteriostatic when dilute (e.g. chlorination in pools)
  • Concentrated forms can be damaging to mucous membranes and skin


  • Mild chemicals that are less effective than disinfectants, but also less toxic to living tissue
  • They are used to kill microbes and prevent further growth on the surface of tissues and wounds, in order to prevent infection
  • However they cannot be taken internally or used on foods and will have an unpleasant taste

F.6.5  Outline the mechanism of the action of antibiotics, including inhibition of synthesis of cell walls, proteins and nucleic acids

Antibiotics interfere with some aspect of prokaryotic metabolism that is different to that of eukaryotes, thus targeting the bacteria without affecting the host

  • Antibiotics can taget enzymes involved in the synthesis of bacterial cell walls (e.g. penicillin)
  • Antibiotics can inhibit bacterial protein synthesis by interfering with 70S ribosomes (e.g. erythromycin, tetracycline)
  • Antibiotics can target enzymes involved in the bacterial synthesis of nucleic acids (e.g. rifampicin)

F.6.6  Outline the lytic life cycle of the influenza virus

The lytic cycle is a method of viral replication which involves the independent replication of viral DNA / RNA as part of virion assembly

  • This is in contrast to the lysogenic cycle, where the viral DNA is integrated into the host genome and is replicated with the cell's own DNA

The lytic life cycle of the influenza virus can be summarised in a number of key steps:

  • When the virus attaches to cell surface receptors it is internalised in an endosome, uncoated and released into the cytoplasm
  • Influenza is a retrovirus and the RNA of the viral genome is transported to the nucleus whereby it is replicated (reverse transcription is not involved)
  • The viral RNA acts as an mRNA template for the translation of viral core proteins, which assemble as a capsid around viral RNA in the cytoplasm
  • Newly constructed virus particles recognise regions of the cell membrane where viral proteins have attached and are released from these points
  • Unlike most lytic viruses, the influenza virus does not promote the lysis of the cell as part of its release

influenza life cycle

F.6.7  Define epidemiology

Epidemiology is the study of the occurrence, distribution and control of diseases

F.6.8  Discuss the origin and epidemiology of one example of a pandemic

A pandemic is an outbreak of a disease over a large geographical area (i.e. many countries)

In the late 20th century (1997) there was a pandemic outbreak of avian bird flu 

  • Infectious Agent: The outbreak was caused by the H5N1 virus 
  • Mode of Transmission:  The virus originated in domestic poultry but spread to humans through exposure to the birds and their faeces
  • Distribution:  The outbreak originated in Asia, and although it mainly remained there, there were cases noted in other parts of the world
  • Methods of Control:  Containment strategies were employed for the infected, infected birds were killed and global records of occurrences were kept
  • Problems of Control:  Migrating wild birds could spread virus, problems coordinating international communities, questions of equitable access to care

F.6.9  Describe the cause, transmission and effects of malaria, as an example of a disease caused by a protozoan

  • Malaria is caused by the protozoan Plasmodium and is transmitted to humans by the Anopheles mosquito (females only)
  • The life cycle of the Plasmodium parasite requires both a primary human host (adult stage) and intermediary mosquito host (juvenile stage)
  • When a mosquito ingests infected human blood, the parasite reproduces and matures in the mosquito's gut before migrating to the salivary glands
  • When a mosquito feeds on another human, the parasite is injected into the blood and travels to the liver for further development
  • After a period of time, the parasite migrates to erythrocytes (red blood cells), which then rupture as the parasite spreads
  • Symptoms of malaria include fever, nausea and chills – these symptoms may be cyclic due to the parasitic life cycle (period of dormancy in liver)
  • Additionally, the frequent rupturing of red blood cells can lead to the development of anaemia

Life Cycle of the Malarial Parasite (Plasmodium falciparum)


F.6.10  Discuss the prion hypothesis for the cause of spongiform encephalopathies

  • A prion (proteinaceous infectious particle) is a protein that has refolded abnormally into a structure that is capable of causing disease
  • It is also able to convert normally folded protein molecules into the abnormal form (mechanism of conversion not well understood)
  • Infectious prion proteins have a higher content of beta-pleated sheets, which increases structural stability making them more resistant to denaturation
  • This makes treatment of prion proteins extremely difficult (there are currently no known cures)
  • Prion proteins aggregate together to form amyloid fibrils capable of causing disease
  • Diseases caused by prion proteins are called spongiform encephalopathies, because they cause holes to form within the brain (neurodegeneration)
  • Examples of prion diseases include:
    • Mad cow disease (affects cows)
    • Scrapie (affects sheep)
    • Creutzfeld-Jacob disorder (CJD) and kuru (affects humans)
  • Mad cow disease has been used as an excuse for protectionism by some national governments, refusing beef imports to protect local interests

Folding and Replication of Prion Protein