Culturing microorganismsSpec 4.1.1.6
Bacteria multiply by simple cell division called binary fission, as often as once every 20 minutes if they have enough nutrients and a suitable temperature.
Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate. Uncontaminated cultures of microorganisms are required for investigating the action of disinfectants and antibiotics, so that the only microorganisms present are the ones being tested.
Preparing an uncontaminated culture using aseptic technique
- Sterilise the Petri dish and the agar culture medium before use (for example by heating), to kill any unwanted microorganisms.
- Sterilise the inoculating loop by passing it through a flame.
- Use the sterile loop to transfer the microorganisms to the agar. Keep the lid of the Petri dish open for as short a time as possible.
- Secure the lid of the Petri dish with adhesive tape and store it upside down.
- Incubate the plate at 25 °C.
| Step | Reason |
|---|---|
| Sterilise Petri dishes and culture media | Kills unwanted microorganisms that could contaminate the culture |
| Pass the inoculating loop through a flame | Kills microorganisms already on the loop, so that only the intended microorganisms are transferred |
| Secure the lid with adhesive tape | Stops microorganisms from the air getting in, and stops the cultured microorganisms escaping |
| Store the dish upside down | Condensation drips onto the lid rather than onto the agar surface |
| Incubate at 25 °C in school laboratories | Reduces the chance of growing pathogens that are harmful to humans, which grow best at higher temperatures such as body temperature |