Avoiding Contamination
Biology: AQA GCSE Higher, topic 1.1.29
Aseptic Technique
To prevent contamination (from skin, air, soil, or water), aseptic (free from contamination) techniques are used on all apparatus. These techniques kill and prevents the entry of unwanted bacteria. They involve:
Flames
- Inoculation loops (tools used to transfer bacteria) must be passed through a flame for sterilisation (removes bacteria).
Boiling
- Solutions and agar must be boiled for sterilisation.
Lids
- To stop air from getting into the Petri dish, the lid must be rapidly taken off and put back on when using the inoculating loop to add bacteria to the agar.
- This lid should be taped on and the dish should be stored upside down to prevent condensation forming on the lid and then dripping onto the agar.
Temperature
- In schools, the maximum temperature at which cultures should be incubated (stored) is 25 degrees Celsius.
- This extra precaution reduces the risk of harmful bacteria growing.
Test yourself on Avoiding Contamination
Which of the following is NOT a way of preventing contamination of bacterial cultures?
Why are bacterial cultures incubated at a maximum of 25°C in school laboratories?
Why must the lid of a Petri dish be removed and replaced quickly when adding bacteria with an inoculating loop?
What is most likely to happen if an inoculating loop is used without first being passed through a flame?
What is most likely to happen if agar is not boiled before being poured into a Petri dish?