Culturing Microorganisms (Edexcel International A Level Biology): Revision Note

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

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