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. • 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 degrees C. • Students should be able to calculate cross-sectional areas of colonies or clear areas around colonies using pi r^2. • Students should be able to calculate the number of bacteria in a population after a certain time if given the mean division time.