# A scientist wishes to investigate whether a new antiseptic is effective against a bacterial pathogen. The scientist plans to culture the bacteria on agar plates, apply the antiseptic, and then incubate the plates. Explain why the scientist must use aseptic techniques throughout this procedure, and describe two specific aseptic techniques the scientist should use when preparing the agar plates.

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

> Antiseptics are chemicals that kill bacterial and fungal pathogens. When culturing bacteria on agar plates in a laboratory, contamination from unwanted microorganisms can make results unreliable.

## Mark scheme (5 marks)

1. Aseptic techniques are needed to prevent contamination of the culture by unwanted microorganisms
2. Contamination would make results unreliable / it would be unclear whether the antiseptic was affecting the target pathogen or a contaminant
3. The agar plates / growth medium should be sterilised (e.g. using an autoclave) before use to kill any existing microorganisms
4. The inoculating loop / instrument used to transfer bacteria should be sterilised, e.g. by passing it through a flame until it glows red hot
5. The agar plate should be opened as briefly as possible / the lid should be kept on except when inoculating, to prevent airborne contaminants entering

## Key terms

- [aseptic techniques](https://www.gradenine.co.uk/glossary/aseptic-techniques)
- [culture](https://www.gradenine.co.uk/glossary/culture)
- [agar plate](https://www.gradenine.co.uk/glossary/agar-plate)

## 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-scientist-wishes-to-investigate-whether-b4109e17) · Published by Druglandscape Ltd.