Overview
This lecture explains how to safely conduct a microbiology practical investigating the effectiveness of antibiotics and antiseptics on bacterial growth, using E. coli and agar plates.
Preparation and Safety
- Wash hands thoroughly and use a sterilized work mat before handling any materials.
- Only open agar plates near a Bunsen burner to maintain sterility.
- Label the bottom (not the lid) of agar plates with bacteria type, date, and your initials.
Inoculating the Agar Plate
- Work within the sterile area created by a blue flame Bunsen burner.
- Use a sterile pipette to transfer E. coli onto the plate, minimizing exposure time.
- Discard used pipettes and spreaders in disinfectant to kill any remaining bacteria.
Spreading Bacteria and Applying Treatments
- Use a sterile glass spreader to evenly distribute bacteria across the agar surface.
- Mark the plate into three labeled sections for treatments: antibiotic, tea tree oil, and commercial antiseptic.
- Place each treatment disk onto the designated section, keeping the lid open for minimal time.
Incubation and Observation
- Seal the plate with two pieces of tape (not all the way around) to allow oxygen in.
- Incubate plates upside down at 30°C to prevent condensation on the agar.
- After incubation, observe cloudy bacterial growth and clear zones (zones of inhibition) around treatment disks.
Data Analysis and Fair Test Considerations
- Measure the diameter of clear zones to compare treatment effectiveness.
- Share or repeat results for reliability.
- Consider if all variables (e.g., disk dryness, liquid viscosity, diffusion) were controlled for a fair test.
Real-World Application and Limitations
- Results may suggest effectiveness for treating bacterial illnesses, but practicality and safety (e.g., ingesting tea tree oil) must be considered.
Key Terms & Definitions
- Agar Plate — a petri dish containing a jelly-like growth medium for bacteria.
- Inoculate — to introduce microorganisms onto a growth medium.
- Sterile — free from bacteria or other living microorganisms.
- Zone of Inhibition — a clear area around a treatment disk where bacteria do not grow, indicating effectiveness.
- Diffusion — the movement of molecules from an area of high concentration to low concentration.
Action Items / Next Steps
- Disinfect your workstation after completing the practical.
- Measure and record the diameter of inhibition zones on your plate.
- Consider variables and suggest improvements for the experimental design.