Aseptic techniques (AQA GCSE Biology): Revision Notes
Aseptic techniques
What are aseptic techniques?
Aseptic techniques are methods used to prevent contamination when working with bacteria. Scientists grow bacteria either in liquid nutrient broth or as colonies on agar gel plates. These techniques help ensure that only the bacteria you want to study are present in your experiment.
Contamination occurs when unwanted microorganisms interfere with your bacterial cultures, making experimental results unreliable.
Sterilisation methods
Sterilisation means destroying all microorganisms to prevent contamination. Different equipment needs different sterilisation methods:
Petri dishes:
- Made sterile using gamma radiation at the factory
- Come ready to use in sealed packaging
Culture media:
- Nutrient broth and agar gel are sterilised using an autoclave
- The autoclave uses very hot steam under high pressure to kill all microorganisms
Laboratory apparatus:
- Equipment like wire loops are sterilised using a Bunsen burner flame
- The flame burns away any unwanted microorganisms
How bacteria multiply
Bacteria reproduce through binary fission. This means one bacterial cell splits into two identical cells. Under the right conditions, bacteria can divide every 20 minutes. They need:
- Enough nutrients to grow
- A suitable temperature
This rapid multiplication is why sterile techniques are so important when studying bacterial growth. Without proper aseptic techniques, unwanted bacteria can quickly overwhelm your cultures.
Using a wire loop safely
A wire loop transfers bacterial samples between containers. Here's how to use it safely:
- Heat the loop in a Bunsen burner flame until it glows red
- Let it cool for a few seconds (don't blow on it)
- Collect your sample by touching the bacteria with the cooled loop
- Transfer the sample to your new container
- Sterilise the loop again in the flame before putting it away
This process prevents cross-contamination between different bacterial cultures.
Never blow on a hot wire loop to cool it down - this introduces bacteria from your breath and defeats the purpose of sterilisation.
Preparing Petri dishes
When growing bacteria on agar plates, follow these steps:
Inoculation process:
- Open the Petri dish upside down near a flame
- Gently sweep the sterilised wire loop across the agar surface
- Replace the lid immediately and seal with tape
- Label the dish with your name, date, and contents
Incubation temperature:
- School laboratories use 25°C for incubation
- This is cooler than human body temperature (37°C)
- The lower temperature reduces the risk of growing harmful bacteria that prefer body temperature
- The dish is left slightly unsealed to prevent dangerous anaerobic bacteria from growing
Calculating bacterial growth
You can work out how many bacteria will be present after a certain time:
Worked Example: Bacterial Growth Calculation
If bacteria divide every 30 minutes and you start with 1000 bacteria, after 4 hours you would have:
Step 1: Calculate number of divisions
Step 2: Apply the growth formula
Using powers on calculators:
- For , press: 2, then x^y (or y^x), then 8, then equals
- If your calculator doesn't have a power button, multiply:
Why aseptic techniques matter
Antiseptics are substances that kill microorganisms or stop their growth. Laboratory benches are wiped with antiseptic solutions before and after experiments to maintain sterile conditions.
Using proper aseptic techniques ensures your results are accurate and prevents the spread of potentially harmful bacteria in the laboratory.
Key Points to Remember:
- Sterilisation destroys all microorganisms using heat, radiation, or chemicals
- Always sterilise wire loops in a flame before and after use
- Incubate at 25°C in schools to avoid growing dangerous bacteria
- Bacteria multiply rapidly - they can double every 20 minutes under ideal conditions
- Seal and label all Petri dishes properly to prevent contamination