Required Practical: Growth (AQA GCSE Biology): Revision Note

Microbiology required practical

Aim:

• To investigate the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring zones of inhibition

Procedure:

• Use aseptic technique to set up the investigation as follows:

  1. Wash hands with soap and disinfect work area

  2. Label a Petri dish containing agar that has been inoculated with bacteria to indicate the treatments to be tested, e.g. by dividing your dish into sections A, B, C and D

     • Safe strains of E. coli or Micrococcus luteus bacterial cultures will be used in this practical

  3. Soak a filter paper disc in your first antibiotic / antiseptic solution

  4. Partially lift the lid of the Petri dish and use forceps to transfer the soaked paper disc to the appropriate labelled section of your Petri dish

     • The chemicals present in the discs will diffuse into the agar

  5. Repeat steps 3-4 with the other antibiotics / antiseptic solutions / control solution

     • One of the paper discs should be soaked in sterile water to function as an experimental control

     • This demonstrates that any differences in bacterial growth observed can be attributed to the presence of the antiseptic or antibiotic used and not some other factor (such as the presence of paper discs)

  6. Secure the lid of the dish with short strips of tape

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• Incubate the prepared Petri dishes for a set period of time, e.g. three days

  • In school this will be carried out at 25 °C to reduce the chances of growing harmful pathogens

• Measure the zone of inhibition around the growing colonies of bacteria

• Calculate the area of each zone to compare the effect of different antiseptics / antibiotics

Analysis

• It is possible to judge which antiseptic or antibiotic is the most effective by eye, but a quantitative comparison can be carried out by calculating the area of each clear zone around the paper discs

  • Clear zones of inhibition are not always perfectly circular, so the diameter of each zone should be measured twice (at 90° angles to each other) and a mean diameter calculated for each clear zone

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