# 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.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 5.1 Microbial growth and culture · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. Sterilising the growth medium and equipment removes/kills unwanted microorganisms (contaminants)
2. If contaminants are present, results would be unreliable / it would be unclear which organism was growing
3. An autoclave uses high temperature (steam under pressure) to sterilise equipment/growth medium
4. The high temperature (and pressure) kills all microorganisms, including spores / endospores, present on the equipment or in the medium
5. 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

- [growth medium](https://www.gradenine.co.uk/glossary/growth-medium)
- [autoclave](https://www.gradenine.co.uk/glossary/autoclave)
- [culture](https://www.gradenine.co.uk/glossary/culture)

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-preparing-a-growth-e543d275) · Published by Druglandscape Ltd.