Investigating microbial cultures (AQA GCSE Biology): Revision Notes
Investigating microbial cultures
What is this practical about?
This practical helps you learn how antiseptics affect bacterial growth. You measure this by looking at zones of inhibition - these are clear areas around antiseptic discs where bacteria cannot grow.
Antiseptics are chemicals that kill or stop the growth of microorganisms. The bigger the clear zone, the better the antiseptic works.
The core practical: testing antiseptics
Aim
The main goal is to investigate how different antiseptics affect bacterial growth using agar plates. This helps us understand which antiseptics work best at stopping bacteria.
What you need (apparatus)
Your teacher will prepare most of this equipment for you:
- Petri dish with bacteria spread on nutrient agar
- Filter paper discs (about 4mm across)
- Three different antiseptics to test
- Forceps for handling discs safely
- Adhesive tape to seal the dish
- Permanent marker for labelling
- Bunsen burner for sterilising equipment
- Heat-resistant mat for safety
The teacher will add a few drops of bacterial culture (often E. coli) to the agar and spread it evenly using a sterile glass spreader.
Method steps
Here's how to carry out the investigation safely:
Step 1: Turn the Petri dish upside down. Use a marker to divide the bottom into three equal segments.
Step 2: Mark a spot in the middle of each segment. Number each segment and write your name and date on the bottom.
Step 3: Add a different antiseptic to each philtre paper disc. Record which antiseptic you put on each segment.
Step 4: Carefully lift the Petri dish lid. Use sterile forceps to place each disc over the marked spots.
Step 5: Secure the lid with adhesive tape. This stops harmful bacteria getting out.
Step 6: Incubate the plate for 2 days at 25°C. This temperature is safe and allows bacterial growth.
Important safety points
Essential Safety Procedures:
Always wipe the bench with disinfectant solution before starting. Clean with paper towels to remove any harmful microorganisms.
Before using forceps, sterilise them by passing the ends through a Bunsen burner flame. This kills any bacteria on the equipment.
How to measure results
After incubation, you'll see clear zones around each disc where bacteria couldn't grow.
Measuring the zones:
- Measure the diameter of each clear zone in millimetres
- Take two measurements at 90° to each other
- Calculate the mean diameter from your two measurements
- Work out the area of each clear zone using the formula
Calculating the area
You need to find the area of circular clear zones. Use this formula:
Where = radius of the circle.
Remember: divide the diameter by 2 to find the radius.
Worked Example: Calculating Zone Area
If the diameter is 16mm, the radius is 8mm.
Step 1: Identify the radius
Step 2: Apply the formula
Understanding your results
The larger the clear zone, the more effective the antiseptic. This is because effective antiseptics stop bacteria growing over a bigger area.
Compare the mean areas of different antiseptics to see which works best. You can also compare your results with other students to make your conclusions more reliable.
Example data analysis
Worked Example: Comparing Antiseptic Effectiveness
If you got these results:
- Antiseptic A: mean diameter 15mm, area 177mm²
- Antiseptic B: mean diameter 25mm, area 491mm²
- Antiseptic C: mean diameter 20mm, area 314mm²
Conclusion: This would show that Antiseptic B is most effective because it has the largest clear zone.
Key Points to Remember:
- Antiseptics create zones of inhibition where bacteria cannot grow
- Larger clear zones mean more effective antiseptics
- Always use sterile technique to prevent contamination
- Calculate area using where is radius (diameter ÷ 2)
- Take multiple measurements and find the mean for accuracy
- Incubate at 25°C for safety - higher temperatures could grow harmful bacteria