A student wants to use an autoclave before culturing bacteria on agar plates in a school laboratory. Explain why an autoclave is used as part of aseptic techniques, and describe what would happen to results if the agar plates and equipment were not sterilised before use.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student is preparing to culture a known, safe strain of bacteria. They have access to an autoclave, agar plates, inoculating loops and a growth medium containing carbohydrates, proteins, vitamins and minerals.
Model answer (5 marks)
An autoclave sterilises equipment and agar plates by exposing them to high temperatures (121 °C) and pressure (15 psi) for a set time, which kills all microorganisms and spores present.
This ensures that any bacteria, fungi or spores that might already be on the plates or on the inoculating loops are destroyed before the intended strain is introduced.
If the agar plates or equipment were not sterilised, environmental microorganisms would grow on the plates alongside the inoculated strain.
Such contamination would make it impossible to recognise which colonies belong to the intended strain, so the results would be unreliable and could not be interpreted.
Additionally, unsterilised equipment could introduce harmful pathogens, posing a health risk to the student and others in the laboratory.
This ensures that any bacteria, fungi or spores that might already be on the plates or on the inoculating loops are destroyed before the intended strain is introduced.
If the agar plates or equipment were not sterilised, environmental microorganisms would grow on the plates alongside the inoculated strain.
Such contamination would make it impossible to recognise which colonies belong to the intended strain, so the results would be unreliable and could not be interpreted.
Additionally, unsterilised equipment could introduce harmful pathogens, posing a health risk to the student and others in the laboratory.
Examiner tips
- Use the word "autoclave" and mention temperature/pressure; link to killing all microorganisms. Explain the consequence: contamination, inability to identify colonies, unreliable data, health risk. Keep answer to 5 points, no extra words.
Common mistakes
- Forgetting to mention pressure or temperature. Failing to state that contamination prevents colony identification. Not recognising the health risk of unsterilised equipment.
Mark scheme (5 marks)
- An autoclave sterilises equipment / agar plates by exposing them to high temperatures (and pressure)
- This kills any microorganisms / pathogens already present on the equipment or agar plates before culturing begins
- If equipment / agar plates are not sterilised, contaminating microorganisms from the environment would grow on the agar plates
- This contamination would make it impossible to identify which colonies belong to the intended bacterial strain / results would be unreliable
- Unsterilised equipment could also introduce pathogens, creating a health risk to the student
Key terms in this question
autoclave · aseptic techniques · agar plate · sterilise
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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