Required practical 2: antiseptics and antibioticsSpec 4.1.1.6
In short
Required practical 2 tests the effect of antiseptics or antibiotics on bacterial growth. Paper discs soaked in each substance are placed on a bacterial lawn on an agar plate, with a sterile water control, and incubated at 25 °C. The larger the clear zone of inhibition around a disc, the more effective the substance is against that bacterium.
Investigate the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring zones of inhibition.
- Using aseptic technique, spread a sample of bacteria evenly over the surface of a sterile agar plate to make a bacterial 'lawn'.
- 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.
- Secure the lid with adhesive tape, label the base, store the plate upside down and incubate at 25 °C for about two days.
- 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.
Quick check
How is the genetic material of a bacterial cell arranged?
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As a single DNA loop that is not enclosed in a nucleus. There may also be plasmids.
Which sub-cellular structure is the site of aerobic respiration?
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Mitochondria.
What is the formula for magnification?
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Magnification = size of image ÷ size of real object.
Why does an electron microscope show more detail than a light microscope?
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It has much higher magnification and resolving power.
Triple only: why are inoculating loops passed through a flame?
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To sterilise them and kill unwanted microorganisms.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026