4.6.16 How to investigate the effect of different antibiotics on bacteria

The effectiveness of antibiotics can be measured using a disc diffusion technique.


  1. A bacterial lawn is grown on an agar plate (either by spreading the bacteria over the plate, or by using a pour plate).


  1. A disc of blotting paper is soaked in antibiotic of known concentration and placed in the centre of the


  1. A clear circle of dead bacteria will form around the disc


  1. The diameter / radius of the circle of dead bacteria is proportional to the effectiveness of the antibiotic
  2. This can be compared to other antibiotics, as long as the same concentration of antibiotic is used. In addition, one can also compare the effectiveness of an antibiotic with a disinfectant or sanitiser (e.g. Phenol coefficient)