A laboratory technician investigates how opening a Petri-dish lid affects contamination of sterile nutrient agar. Ten plates are used for each treatment. No microorganisms are deliberately added, and all the plates are incubated under the same conditions.
| How the lid is opened | Time the lid is open (s) | Mean number of colonies per plate |
|---|---|---|
| Lifted by 1 cm | 5 | 0.4 |
| Fully removed | 5 | 2.8 |
| Lifted by 1 cm | 60 | 4.6 |
| Fully removed | 60 | 18.2 |
The technician concludes that only the width of the opening affects contamination.
Evaluate the technician's conclusion. Suggest how the lid should be handled while a bacterial culture is added and after inoculation.
87 exam-style questions on Edexcel GCSE Biology 5.3 Immune system and medicines, covering 5.3.1 The specific immune system, 5.3.2 Response to immunisation, 5.3.3 Advantages and disadvantages of immunisation, 5.3.4 Antibiotics and bacterial infections, 5.3.5 Aseptic techniques in culturing, 5.3.6 Cross-sectional areas of cultures, 5.3.7 Stages of developing new medicines, and 5.3.8 Production of monoclonal antibodies. Each one has a worked solution and a mark scheme showing where the marks go.