Core practical: antiseptics, antibiotics and plant extractsSpec 5.18B
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.