Culturing microorganisms (biology only)

Bacteria multiply by simple cell division (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.

MS 1a, 1b, 1d, 2a, 2h

Calculate the number of bacteria in a population after a certain time if given the mean division time.

MS 5c

Calculate cross-sectional areas of colonies or clear areas around colonies using πr².

Students should be able to describe how to prepare an uncontaminated culture using aseptic technique.

They should be able to explain why:

• Petri dishes and culture media must be sterilised before use
• inoculating loops used to transfer microorganisms to the media must be sterilised by passing them through a flame
• the lid of the Petri dish should be secured with adhesive tape and stored upside down
• in school laboratories, cultures should generally be incubated at 25°C.