15.1.3 Testing Antibiotics
Paper 6 Practical Skill: Zone of Inhibition Method
How Do We Test Antibiotic Effectiveness?
The Big Question
Which antibiotic is most effective at killing bacteria?
The Evidence
We measure the Zone of Inhibition - the clear circle around each antibiotic disc where bacteria have been killed.
💡 Larger zone = More effective antibiotic
Why This Works
- Antibiotic diffuses outward from disc into agar
- Bacteria die where antibiotic concentration is high enough
- This creates a clear, transparent circle
The Zone of Inhibition
Antibiotic Disc (paper soaked in antibiotic) Bacterial Lawn (cloudy, opaque layer) Zone of Inhibition (clear, transparent)