A student is investigating bacterial growth in a liquid growth medium. The student inoculates the medium with bacteria and leaves the culture for 48 hours. Explain why it is important to use an autoclave on the growth medium and equipment before the investigation begins, and describe what the student should observe in the growth medium after 48 hours if the culture is successful.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A growth medium is a liquid nutrient-rich medium containing carbohydrates, proteins, vitamins and minerals used to cultivate microorganisms.
Model answer (5 marks)
Use an autoclave to sterilise the growth medium and all equipment before inoculation. Sterilisation destroys spores and cells of unwanted microorganisms, preventing contamination of the culture. If the medium is not sterilised, other microbes could grow and interfere with the experiment, compromising its validity and reliability.
After 48 h of incubation a successful culture will make the liquid cloudy or turbid. This cloudiness is caused by the large number of bacterial cells that have multiplied in the medium.
After 48 h of incubation a successful culture will make the liquid cloudy or turbid. This cloudiness is caused by the large number of bacterial cells that have multiplied in the medium.
Examiner tips
- Mention both medium and equipment sterilisation
- Explain contamination risk
- State the visual change (cloudy/turbid)
- Link cloudiness to bacterial growth
Common mistakes
- Omitting the reason for sterilising equipment
- Confusing sterilisation with disinfection
- Failing to describe the cloudiness as evidence of growth
Mark scheme (5 marks)
- The autoclave sterilises the growth medium and equipment
- This prevents contamination of the culture by unwanted microorganisms
- Without sterilisation, unwanted microorganisms could grow and affect the validity / reliability of the results
- The growth medium would become cloudy / turbid after 48 hours
- This cloudiness indicates bacterial growth / an increase in the number of bacteria in the medium
Key terms in this question
autoclave · growth medium · culture
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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