A student is preparing a growth medium to culture bacteria in a school laboratory. Explain why it is important to sterilise the growth medium and the equipment used before culturing the bacteria, and describe how an autoclave is used in this process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student wants to investigate the effect of different nutrient concentrations on bacterial growth. They prepare several glass flasks containing liquid growth medium, which includes carbohydrates, proteins, vitamins and minerals. Before adding the bacteria, the student must follow aseptic techniques.
Model answer (5 marks)
Sterilising the growth medium and equipment removes or kills all unwanted microorganisms that could contaminate the culture. If contaminants are present the growth observed would be unreliable and it would be impossible to know which organism is responsible for the growth.
An autoclave sterilises by applying high‑temperature steam under pressure. The pressure raises the boiling point of water so that the medium and equipment reach temperatures of about 121 °C for 15–20 min, which kills all microorganisms, including resistant endospores.
After sterilisation, aseptic techniques (working close to a flame, using sterile inoculating loops, sealing flasks) are used to prevent re‑contamination when the culture is handled.
An autoclave sterilises by applying high‑temperature steam under pressure. The pressure raises the boiling point of water so that the medium and equipment reach temperatures of about 121 °C for 15–20 min, which kills all microorganisms, including resistant endospores.
After sterilisation, aseptic techniques (working close to a flame, using sterile inoculating loops, sealing flasks) are used to prevent re‑contamination when the culture is handled.
Examiner tips
- Mention removal of contaminants and reliability of results
- Explain autoclave temperature/pressure and time
- Include aseptic steps after sterilisation
Common mistakes
- Forgetting to state that spores are also killed
- Confusing sterilisation with disinfection
- Not linking sterilisation to reliability of results
Mark scheme (5 marks)
- Sterilising the growth medium and equipment removes/kills unwanted microorganisms (contaminants)
- If contaminants are present, results would be unreliable / it would be unclear which organism was growing
- An autoclave uses high temperature (steam under pressure) to sterilise equipment/growth medium
- The high temperature (and pressure) kills all microorganisms, including spores / endospores, present on the equipment or in the medium
- After sterilisation, aseptic techniques (e.g. working near a flame, using sterile inoculating loops, sealing flasks) must be used to prevent re-contamination when handling the culture
Key terms in this question
growth medium · autoclave · culture
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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