Petri dishes are shallow glass or plastic plates used to culture cells and microorganisms such as bacteria and yeast. Petri dishes may be reused once sterilised.
Sterilisation of petri dishes is important to ensure that only the desired cells grow on the dish. This is usually performed by autoclave or heating to 160 degrees for an hour. In the bottom of the petri dish is a disk of agar which contains the nutrients needed to culture the desired cells on. Nutrients are mixed into the agar jelly to either promote growth of a particular kind of bacteria, or to create conditions that certain bacteria cannot survive in; these are called ‘selective cultures’ and are important in diagnostic medical microbiology.