Core practical: antiseptics, antibiotics and plant extractsSpec 5.18B
In short
In this core practical, bacteria are spread over sterile agar to make a lawn, and paper discs soaked in antiseptics, antibiotics or plant extracts are placed on top, with a water control disc. After incubating at 25 °C for about 48 hours, the clear zone around each disc is measured. A larger clear zone means a more effective substance.
Investigate the effects of antiseptics, antibiotics or plant extracts on microbial cultures.
The idea is to find out which substance is best at stopping bacteria growing. Bacteria are grown on agar jelly in a petri dish, and small paper discs soaked in different substances are placed on top.
- Using aseptic technique, spread a sample of bacteria evenly over the surface of a sterile agar plate to make a bacterial lawn.
- Using sterile forceps, place paper discs soaked in the different antiseptics, antibiotics or plant extracts on the agar, spaced apart.
- Place a control disc soaked in sterile water on the agar.
- Put the lid on and tape it in place. Label the base and incubate the dish upside down at 25 °C for about 48 hours.
- Measure the diameter of the clear zone (inhibition zone) around each disc and calculate its area.
| Type of variable | In this investigation |
|---|---|
| Independent | The type of antiseptic, antibiotic or plant extract (or its concentration) |
| Dependent | The size of the clear zone |
| Control | Type and volume of bacteria; type and volume of agar; size of disc and volume of liquid on it; temperature and time of incubation |
- Results: where the substance stops bacteria growing, a clear zone appears around the disc. The larger the clear zone, the more effective the substance. The water control disc should have no clear zone.
- Why 25 °C: a lower temperature makes it less likely that harmful pathogens that grow best at body temperature will grow.
- Why the dish is taped: to stop microorganisms escaping or entering; do not seal it fully, so that oxygen can enter.
- Safety: wear eye protection; do not open dishes after incubation; wash hands; disinfect the bench; sterilise used dishes in an autoclave; take care with flammable chemicals such as ethanol near a flame.
If you are asked why a control disc is used, say it shows that the water or the paper alone does not stop bacteria growing, so the clear zones are caused by the substances tested.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026