AQA GCSE Biology · Cell BiologyTriple only
Microbiology required practical
Investigate the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring zones of inhibition.
Paper discs soaked in different antiseptics are placed on a lawn of bacteria. The bigger the clear zone around a disc, the better the antiseptic stops bacteria growing.
This one is only on the separate Biology GCSE, so it won’t come up in Combined Science.
Equipment
- Sterile nutrient agar plate (Petri dish)
- Culture of a safe bacterium, e.g. E. coli K-12
- Sterile pipette and spreader
- Paper discs and sterile forceps
- Different antiseptics, plus sterile water as a control
- Bunsen burner, sticky tape and a ruler
Variables
- Independent you change
- The type of antiseptic
- Dependent you measure
- The area of the clear zone (zone of inhibition) around each disc
- Control you keep the same
- Same bacteria and volume of culture
- Same size of paper disc
- Same volume of antiseptic on each disc
- Same incubation temperature and time
Method
- 1
Work near a lit Bunsen burner so the rising air carries microbes away from the plate.
- 2
Use a sterile pipette to add bacteria to the agar plate and spread them evenly with a sterile spreader.
- 3
Soak paper discs in each antiseptic (and one in sterile water as a control). Place them on the agar with sterile forceps, well apart.
- 4
Tape the lid on with a few strips of tape, not all the way round, and label the base.
- 5
Incubate the plate upside down at 25 °C for about 48 hours.
- 6
Measure the diameter of the clear zone around each disc and work out its area using πr².
Worked example
Clear zones measured after 48 hours at 25 °C.
| Disc | Zone diameter (mm) | Radius (mm) | Area (mm²) |
|---|---|---|---|
| Antiseptic A | 14 | 7.0 | 153.9 |
| Antiseptic B | 22 | 11.0 | 380.1 |
| Antiseptic C | 9 | 4.5 | 63.6 |
| Sterile water (control) | 0 | 0 | 0 |
Working
Area of a circle = πr², and r = diameter ÷ 2
Antiseptic B: r = 22 ÷ 2 = 11 mm
Area = π × 11² = 380.1 mm²
Conclusion
Antiseptic B has the largest clear zone, so it is the most effective at killing or stopping the growth of these bacteria. The water disc has no clear zone, which shows the effect comes from the antiseptics and not the paper disc.
Risks
- Growing harmful pathogensUse a safe strain, tape the lid on and never open the plate after incubation; incubate at no more than 25 °C.
- Bunsen flame and hot loopTie hair back and keep flammable antiseptics away from the flame.
- Contaminated equipmentAutoclave or disinfect everything afterwards and wash your hands.
Common mistakes
- Taping the lid all the way round. A few strips let oxygen in, which stops harmful anaerobic bacteria growing.
- Incubating at 37 °C. In schools it must be 25 °C or below, so human pathogens don’t grow well.
- Using diameter to compare when asked for area. Halve the diameter for the radius, then use πr².
Better results
- Repeat with three plates and calculate a mean area for each antiseptic.
- Measure each zone across two diameters at right angles and take the mean, in case it isn’t a perfect circle.
- Include a control disc soaked in sterile water so you know the disc itself has no effect.
Launching November 2026
7 exam questions on this practical.
Practise the method, the variables, the maths and what the results mean, then revise the Cell Biology flashcards. It opens to students soon: join the waitlist and we’ll email you once, when it does.