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Practical 6Spec ref: 4.3.1.8Higher Tier

Bacterial Culture

Linked to B3: Infection and Response

Aim

Investigate the effect of different antibiotics on bacterial growth using aseptic technique.

PDF Resources

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Practical Guide

Method & analysis

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Exam Questions

Past-paper style

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Mark Scheme

Model answers

Equipment

  • •Nutrient agar plates
  • •Bacterial culture
  • •Antibiotic discs
  • •Sterile spreader
  • •Sterile forceps
  • •Inoculation loop
  • •Bunsen burner
  • •Adhesive tape
  • •Ruler
  • •Incubator at 25°C

Variables

Independent variable
Type of antibiotic (different discs)
Dependent variable
Diameter of zone of inhibition (mm)
Controlled variables
Volume of bacteria spread, agar type, incubation temperature and time, same bacterial strain

Key Facts to Know

✓Zone of inhibition = clear area around antibiotic disc where bacteria cannot grow
✓Larger zone = more effective antibiotic against that bacterium
✓Zone diameter of 0 mm = bacteria are resistant
✓Aseptic technique prevents contamination with unwanted microorganisms
✓Incubate at 25°C NOT 37°C — reduces risk of culturing harmful pathogens

⚠️ Common Mistakes

  • ✗Incubating at 37°C (body temperature) — school rule is 25°C for safety
  • ✗Opening the plate after incubation — measure zones through the base
  • ✗Taping the lid all the way around — must allow gas exchange

💡 Exam Tips

  • ★Measure zone diameter (not radius) through the base without opening
  • ★Incubate at 25°C — examiners ask this every year with the reason: safer
  • ★For Higher: calculate zone area = π × (d/2)² for a more meaningful comparison

All Required Practicals

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