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6.3.2Unit 6: Practical Skills in Biology II

Core Practical 14: The Effects of Different Antibiotics — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 6 parts Lesson 14 of 48 Unit quiz

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Part 1 of 6Background: How Antibiotics Act

6.3.2 Core Practical 14: The Effects of Different Antibiotics
Types
  • Bactericidal antibiotics kill bacteria, whereas bacteriostatic antibiotics stop their growth and reproduction.
Cell wall
  • Penicillin prevents the cross-links that strengthen bacterial cell walls, so water enters by osmosis and the cell bursts.
Protein synthesis
  • Some antibiotics bind to bacterial ribosomes and stop enzymes from being made.
Why humans are safe
  • Human cells lack cell walls and have different ribosomes and enzymes, so antibiotics do not harm them, and viruses are unaffected.

Part 2 of 6Method

1
Disinfect the bench and set up a Bunsen burner nearby so that the updraught draws airborne microorganisms away from the work area.
2
Spread a sterile pipette sample of bacterial culture evenly over an agar plate using a sterile spreader.
3
Soak equal-sized paper discs in different antibiotics, or in different concentrations of one antibiotic, and place them apart with sterile forceps.
4
Include a disc soaked in distilled water as a negative control, so that any clear zone can be attributed to the antibiotic.
5
Tape the lid lightly, invert the plate and incubate at about 25 °C for 24 to 48 hours.

Part 3 of 6Aseptic Reasoning

Aseptic Reasoning
Light taping
  • Using only a little tape lets oxygen reach the bacteria, and avoids creating anaerobic conditions that may favour pathogens.
Inverted plate
  • Condensation collects in the lid rather than dripping on the agar, which could spread bacteria or contaminate the plate.
Incubation temperature
  • Staying near room temperature prevents harmful pathogens growing, although research laboratories may use higher temperatures.
Sterilising
  • Metal loops are flamed, while forceps are flamed or placed in disinfectant.

Part 4 of 6Results and Evaluation

Bacterial lawn
  • The bacteria form a continuous layer over the agar except where they cannot grow.
Clear zone
  • A clear area around a disc shows that the antibiotic prevented bacterial growth.
Interpretation
  • A larger clear zone means that the antibiotic is more effective, and no zone suggests resistance.
Quantifying
  • Measure the diameter and calculate the area of each zone using πr², since area is proportional to the effect.

Part 5 of 6Worked Example

Question

  • Antibiotic A produced a clear zone of diameter 14 mm and antibiotic B a zone of diameter 22 mm.
  • Compare their effects using the area of each zone.

  1. Radius of A = 7 mm, so area = π × 7² = 153.9 mm².
  2. Radius of B = 11 mm, so area = π × 11² = 380.1 mm².
  3. Compare380.1 ÷ 153.9 = 2.47.

Answer B has a clear zone about 2.5 times the area of A, so it is more effective against these bacteria.

Part 6 of 6Exam Checklist & Tips

6.3.2 – Key Points for the Exam

  • Bacteriostatic Describing an antibiotic that prevents bacteria from growing or dividing.
  • Bactericidal Describing an antibiotic that kills bacteria.
  • Clear zone The area around an antibiotic disc where bacteria have not grown.
  • Negative control A treatment, such as a water disc, that should have no effect, used for comparison.

Common Mistakes

  • Forgetting a control disc, which means the effect cannot be linked to the antibiotic.
  • Sealing the dish fully with tape, which could encourage the growth of anaerobic pathogens.
  • Comparing zone diameters rather than areas when asked to quantify the inhibition.
  • Saying that the antibiotic works on the whole plate, when it only acts around the disc.

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