(P) Effect of Antibiotics on Bacterial Growth

• 1. Place paper discs soaked in different types (or different concentrations) of antibiotics on an agar plate that has an even covering of bacteria. Leave some space between the discs

  
• 2. The antibiotic should diffuse (soak) into the agar jelly. Antibiotic-resistant bacteria will continue to grow on the agar around the paper discs, but non-resistant strains will die. A clear area will be left where the bacteria have died - this is called an inhibition zone

  
• 3. Make sure you use a control. This is a paper disc that has not been soaked in an antibiotic. Instead, soak it in sterile water. You can then be sure that any difference between the growth of the bacteria around the control disc and around one of the antibiotic discs is due to the effect of the antibiotic alone

  
• 4. Leave the plate for 48 hours at 25°C

  
• 5. The more effective the antibiotic is against the bacteria, the larger the inhibition zone will be

  
• You can compare the effectiveness of different antibiotics (or antiseptics) on bacteria by looking at the relative sizes of the inhibition zones
• The larger the inhibition zone around a disc, the more effective the antibiotic is against the bacteria
• To calculate the area of an inhibition zone, you need to use this equation:

  Area = πr² units are likely to be in cm² or mm²
• The equaion Area = πr² can also be used to calculate the area of a bacterial colony