AQA GCSE FOUNDATION Biology: Investigating Microbiology - Antibiotic Effectiveness

Investigating Antibiotic Effectiveness: A Practical Guide

This tutorial will guide you through a practical investigation of antibiotic effectiveness, a key topic in AQA GCSE Foundation Biology. We will explore the principles of culturing bacteria safely, measuring inhibition zones, and interpreting the results to determine antibiotic efficacy.

1. Culturing Bacteria Safely

• Sterilization is Key: Microorganisms are everywhere, so it's crucial to maintain a sterile environment to prevent contamination. Use sterile equipment and techniques throughout the experiment.
• Agar Plates: Agar plates provide a nutrient-rich surface for bacterial growth. Use sterile agar plates and work in a sterile environment to prevent contamination.
• Bacterial Cultures: Obtain a sterile bacterial culture from a reliable source. Handle the culture carefully to avoid contamination.
• Aseptic Technique: Use aseptic technique to transfer bacteria to agar plates. This involves working near a Bunsen burner flame to create an upward airflow that minimizes air contamination. Flame sterilize equipment before and after use to kill any microorganisms present.

2. Applying Antibiotic Discs

• Antibiotic Discs: Obtain pre-prepared antibiotic discs. These discs contain different antibiotics at known concentrations.
• Agar Plates: Once the bacterial culture has been inoculated on the agar plate, allow it to incubate until a uniform lawn of bacterial growth is visible.
• Disc Placement: Using sterile forceps, carefully place the antibiotic discs onto the surface of the agar plate. Ensure they are evenly spaced and not too close to the edge of the plate.
• Incubation: Incubate the plates at an appropriate temperature for a specific time period (usually 24-48 hours).

3. Measuring Inhibition Zones

• Inhibition Zones: After incubation, observe the agar plates. Areas around the antibiotic discs where bacterial growth is inhibited will appear clear. These are called zones of inhibition.
• Measurement: Measure the diameter of each zone of inhibition using a ruler. Alternatively, you can measure the radius and calculate the area using the formula: Area = ? * radius^2.
• Data Recording: Record the diameter or area of the inhibition zone for each antibiotic disc.

4. Interpreting Results

• Antibiotic Effectiveness: The size of the inhibition zone reflects the effectiveness of the antibiotic against the bacteria. A larger zone indicates greater effectiveness.
• Comparison: Compare the inhibition zones of different antibiotics. Which antibiotic was most effective? Which was least effective?
• Factors Influencing Results: Consider factors that may have influenced the results, such as:
  • Bacterial Species: Different bacterial species may have varying sensitivities to antibiotics.
  • Antibiotic Concentration: The concentration of the antibiotic on the disc can influence the size of the inhibition zone.
  • Incubation Conditions: Temperature and time of incubation can impact bacterial growth and antibiotic effectiveness.

5. Disposal Protocols

• Safe Disposal: Dispose of all materials used in the experiment properly. Follow your school or laboratory's protocols for disposal of contaminated materials.
• Sterilization: Sterilize all used equipment before and after use.
• Biohazard Waste: Dispose of agar plates and bacterial cultures as biohazard waste.

By following these steps, you can conduct a safe and informative experiment on antibiotic effectiveness. This investigation will provide you with practical experience and deepen your understanding of microbiology and the role of antibiotics in treating bacterial infections.