Edexcel International A Level Biology

Revision Notes

6.1 Culturing Microorganisms

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Culturing Microorganisms

  • Most microorganisms are only visible using a microscope 
  • For microbe investigations it is therefore necessary to culture microorganisms
    • This grows enough microorganisms to make measurements during investigations
    • E.g. bacteria reproduce by cloning themselves, so when they are grown on agar gel they form a colony of identical individuals that is visible to the naked eye
  • Microorganisms must be provided with everything they need to grow such as
    • Nutrients
    • Oxygen
      • Note that anaerobic microorganisms would require the absence of oxygen
    • Optimum pH
    • Favourable temperature
  • Microorganisms should be cultured with great care
    • There is always the risk that a mutation could lead to the formation of pathogenic strains
    • Pathogenic bacteria from the environment could contaminate the bacterial culture being investigated
  • Remember to
    • Follow health and safety precautions
    • Ensure that all equipment are sterilised before culturing the bacteria
      • Sterilising involves killing microorganisms, e.g. by heating to a high temperature or the use of antimicrobial chemicals
    • Keep the culture in the laboratory 
    • Seal cultures in a plastic bag and sterilise at high temperature and pressure before disposal

Culturing steps

  • Obtain a supply of the type of microorganism to be cultured
  • Provide them with the correct type of nutrients to facilitate growth
    • A nutrient growth medium (plural media) containing carbon, nitrogen, and minerals is typically used
    • The medium could be in the form of a liquid culture or a solid nutrient agar, a type of gel extracted from seaweed
  • Ensure that the nutrient medium is kept under sterile conditions until use
  • By adjusting the type of nutrients in the medium, conditions will be created for the optimal growth of a certain type of microorganism; this is known as a selective medium
    • Selective media can be used to identify mutant strains of microorganisms or those that are resistant to antibiotics
    • They are also useful for identifying genetically modified microorganisms
  • Microorganisms are introduced to a growth medium using inoculation with a sterilised inoculation loop 
  • Inoculation can be used to transfer microorganisms between media, e.g.
    • From agar gel into a liquid culture flask
    • From a liquid culture flask onto agar gel
  • The new medium should be sealed or covered to avoid contamination from microorganisms in the air; if growing aerobic microorganisms any seal or cover should not be airtight 
    • Flasks can be sealed with a sterile cotton wool stopper 
    • Petri dishes can be covered with a lid
  • Label the medium clearly and incubate at around 20 °C to prevent the growth of pathogenic microorganisms
    • Microorganisms that are pathogenic to humans will grow best at around 37 °C 
    • In a hospital or research laboratory a higher temperature might be used to obtain faster results

Culturing microorganisms 1
Culturing microorganisms 2
Culturing microorganisms 3

A metal inoculating loop can be used to transfer microorganisms from a liquid broth medium onto an agar gel medium. A Bunsen burned enables sterilisation of the loop between uses.

Growing a single type of microorganism

  • In order to grow a single type of microorganism, or a pure culture, the specific microorganism must be isolated
  • This can be done by using knowledge about the needs of the microorganism to be cultured or those of microorganisms that may contaminate the culture
  • Examples include
    • Growing the culture under either aerobic or anaerobic conditions to reduce the variety of microorganisms in the culture
    • Using a selective medium that is tailored to the specific requirements of the desired microorganism
    • Indicator media can provide a colour change to distinguish desired colonies from the rest
  • Being able to isolate pathogenic microorganisms is useful in the diagnosis and treatment of diseases

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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.