Zones of inhibition – antiseptics and antibiotics on bacteria
BiologyMicrobiology & VirusesAges 15–16
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Sign in to playA paper-disc assay on an agar plate: perform or skip each step of aseptic technique (a skipped step lets contaminant colonies grow), place discs of antibiotics, antiseptics and a sterile-water control, incubate in time-lapse, then measure the zone diameters with an on-screen ruler. Calculate the area πr² step by step, repeat on three plates and compare the means.
Lesson: Required practical: effect of antiseptics and antibiotics on bacterial growth
What it shows
Paper discs soaked in antibiotics or antiseptics are placed on a lawn of bacteria growing on agar. Each substance diffuses outwards and stops growth wherever its concentration is high enough, leaving a clear zone of inhibition; a larger zone means a more effective agent at that concentration. The virtual plate also models aseptic technique: every step you skip lets contaminant colonies of different colours and shapes grow. Zone sizes follow a diffusion model with small random variation, so repeat plates and means matter. No bacteria are cultured, and the safety rules for the real practical are listed.
How to use
Choose the bacteria, incubation temperature and time, then an agent and concentration for discs A–D; disc E is the sterile-water control. Work through the steps, pressing a technique button or Skip, or press Quick run, every step correct. After incubation, use Zoom and drag the ruler to measure each zone diameter, enter it in the table and press Check measurements. Add up to three plates with New plate (repeat) and compare the mean areas.
Parameters you can change
- Bacteria cultured Bacillus subtilis (Gram-positive), E. coli K-12 (Gram-negative)
- Starting set of discs Antibiotics and antiseptics, Three antibiotics, Four antiseptics, Chlorhexidine at 4 concentrations
- Incubation temperature 25 °C (school maximum), 37 °C (not used in schools)
- Incubation time 24–72 h
- Random variation between plates 0–15 %
Questions to explore
- Why does the experiment need a paper disc soaked in sterile water?
- Does doubling the chlorhexidine concentration double the area of the clear zone?
- Why does penicillin give a large zone with Bacillus subtilis but almost none with E. coli?
