# 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.

> Pearson Edexcel A-Level Biology A (9BI0) — 5.1 Microbial growth and culture · Explain · 5 marks

> A growth medium is a liquid nutrient-rich medium containing carbohydrates, proteins, vitamins and minerals used to cultivate microorganisms.

## Mark scheme (5 marks)

1. The autoclave sterilises the growth medium and equipment
2. This prevents contamination of the culture by unwanted microorganisms
3. Without sterilisation, unwanted microorganisms could grow and affect the validity / reliability of the results
4. The growth medium would become cloudy / turbid after 48 hours
5. This cloudiness indicates bacterial growth / an increase in the number of bacteria in the medium

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](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-investigating-bacterial-growth-38335865) · Published by Druglandscape Ltd.