# A student is investigating the effect of different antiseptics on the growth of bacteria. The student places filter paper discs soaked in different antiseptics onto an agar plate that has been inoculated with bacteria, then incubates the plate at 25 °C for 48 hours. After incubation, the student observes clear zones around some of the discs where no bacterial colonies have grown. Explain why aseptic techniques must be used throughout this investigation and describe TWO specific aseptic techniques the student should use when preparing the inoculated agar plate.

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

> Antiseptics are chemicals that kill bacterial and fungal pathogens. When testing their effectiveness, it is important that any results are due only to the antiseptic being tested and not to contamination from other sources.

## Mark scheme (5 marks)

1. Aseptic techniques are needed to prevent contamination of the agar plate by unwanted microorganisms from the environment
2. Contamination would produce misleading results / make it impossible to determine whether zones of inhibition are due to the antiseptic or unwanted microorganisms
3. Specific aseptic technique 1: The inoculating loop / spreader should be sterilised by flaming / passed through a Bunsen burner flame before use (and allowed to cool) to kill any microorganisms on it
4. Specific aseptic technique 2: The lid of the agar plate should be kept closed as much as possible / only briefly lifted at an angle when inoculating to prevent airborne microorganisms falling onto the agar
5. The neck of any container holding the bacterial culture / broth should be flamed before and after use OR work should be carried out near a Bunsen burner flame to create an upward convection current that reduces airborne contamination

## Key terms

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

## 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-the-effect-b1ef0bff) · Published by Druglandscape Ltd.