A student investigated bacteria on a computer keyboard and on a recently cleaned ceramic tile.
The student rubbed sterile swabs over equal areas of each surface and transferred each sample onto sterile nutrient agar. Another set of plates was prepared using sterile swabs that had not touched either surface. All the plates were treated in the same way.
After incubation, the student counted the well-separated colonies.
| Source of sample | Number of colonies on plate 1 | Number of colonies on plate 2 | Number of colonies on plate 3 |
|---|---|---|---|
| Computer keyboard | 52 | 47 | 50 |
| Cleaned ceramic tile | 7 | 9 | 8 |
| Swab that did not touch a surface | 6 | 8 | 7 |
The student concluded that the computer keyboard contained bacteria but the cleaned ceramic tile did not.
Evaluate whether the results support the student's conclusion.
282 exam-style questions on AQA GCSE Biology 1.1 Cell structure, covering 1.1.1 Eukaryotes and prokaryotes, 1.1.2 Animal and plant cells, 1.1.3 Cell specialisation, 1.1.4 Cell differentiation, 1.1.5 Microscopy, and 1.1.6 Culturing microorganisms. Each one has a worked solution and a mark scheme showing where the marks go.