How to Culture Microorganisms Safely

Microorganisms are cultured safely by using aseptic technique: sterilise the equipment and growth medium, minimise exposure to the air, secure the Petri dish and incubate it at a safe temperature.

This is biology-only content for AQA GCSE Biology, so it is not required for Combined Science. It applies to both Foundation and Higher tiers and appears in Cell biology on Paper 1.

What is aseptic technique?

Aseptic technique is a set of procedures used to prevent unwanted microorganisms contaminating a culture. It also reduces the risk of cultured microorganisms escaping into the environment.

Safe method

1. Sterilise the Petri dish and agar growth medium before use. This kills unwanted microorganisms already present.
2. Sterilise the inoculating loop by passing it through a flame. Allow it to cool before collecting the microorganisms so that it does not kill them.
3. Open the Petri dish lid as little as possible when transferring microorganisms. This reduces contamination by microorganisms in the air.
4. Use the sterile loop to spread the microorganisms across the agar surface.
5. Replace the lid quickly and secure it with adhesive tape. Do not remove the lid after incubation.
6. Store the Petri dish upside down. This prevents condensation from dripping onto the agar and spreading colonies.
7. In school laboratories, incubate the culture at about $25{}^{\circ }\mathrm{C}$. This reduces the chance of growing harmful human pathogens, which often grow well near body temperature.

Example exam explanation

Why must the inoculating loop be passed through a flame?

The flame sterilises the loop by killing microorganisms already on it. This prevents them from contaminating the culture.

Common exam mistake

Do not simply write that equipment is sterilised “to keep it clean”. Explain that sterilisation kills unwanted microorganisms, preventing contamination and making the results more valid.