Required practical 2: antiseptics and antibiotics

Required practical 2: antiseptics and antibioticsSpec 4.1.1.6

1. Using aseptic technique, spread a sample of bacteria evenly over the surface of a sterile agar plate to make a bacterial 'lawn'.
2. Using sterile forceps, place sterile paper discs soaked in different antiseptics or antibiotics (or different concentrations of one) onto the agar. Include a control disc soaked in sterile water.
3. Secure the lid with adhesive tape, label the base, store the plate upside down and incubate at 25 °C for about two days.
4. Measure the diameter of the clear area (zone of inhibition) around each disc and calculate its area using πr².

Where the antiseptic or antibiotic has killed or stopped the growth of the bacteria, there is a clear zone around the disc. The larger the zone of inhibition, the more effective the substance is against that bacterium. The control disc should have no clear zone, which shows that the water itself does not kill the bacteria.

• Independent variable: the type or concentration of antiseptic or antibiotic.
• Dependent variable: the size (diameter or area) of the zone of inhibition.
• Control variables: type and amount of bacteria, type and depth of agar, size of disc, volume of solution on the disc, temperature, incubation time.

Safety: the cultures may contain harmful microorganisms. Use aseptic technique, wash hands, keep the plate taped shut, never open it after incubation, and disinfect the work area and dispose of plates safely.