Treating, curing and preventing disease - OCR Gateway How do antiseptics affect the growth of bacteria?

Investigation - the effects of anti-microbial solutions on bacterial growth

Scientists can test out the effectiveness of antibiotics and antiseptics on bacterial growth. Bacteria will grow easily on an agar plate. By adding filter paper soaked in a variety of anti-microbial solutions to the pre-prepared agar plate scientists can find out how good the solutions are at killing bacteria. Antibiotics or antiseptics can be tested experimentally on pre-prepared agar plates. A clear area (zone of inhibition) in the plate indicates that the bacteria have been killed by the solution or have not been able to reproduce.

Method

1. Soak filter paper discs in a variety of solutions. For example, use different concentrations of the same antibiotic solution, or a variety of different antibiotic or antiseptic solutions.
   • Reason - the effectiveness of the solutions at killing the bacteria can be tested experimentally.
2. Place the discs onto the surface of the pre-prepared agar plate, in contact with the bacteria. A control disc must be also included which has no anti-microbial substance on it. Leave for at least 24 hours at room temperature.
   • Reason - the antibiotic or antiseptic will diffuse through the agar and come into contact with the bacteria.
3. Measure the diameter of the clear area around the soaked filter paper discs using a ruler.
   • Reason – size of zone of inhibition indicates the effect of the substance tested on the growth of the specific bacterium. The larger the clear zone, the more bacteria were killed so the higher the effectiveness of the substance.