Petri dish
A Petri dish is a flat, round, transparent dish with an overlapping lid, which is usually used in biology , medicine or chemistry .
Petri dishes were invented in 1887 by the German bacteriologist Julius Richard Petri and are named after him. They are mostly used for the cultivation of microorganisms and for cell culture . For this purpose, a flat layer of a gel-like nutrient medium is created in the Petri dish: The medium is usually made on an agar basis, sterilized by heating in an autoclave and then poured into the dishes while still warm and therefore liquid, at room temperature it solidifies and forms what is known as a so-called Agar plate . The nutrient medium supplies the growing microorganisms with water and nutrients, but - in contrast to cultures in liquid media - holds them in one place. The spread of the microorganisms by mixing like in a liquid medium is thus prevented.
The main advantages are:
- With a few exceptions ( e.g. Proteus ), the microorganisms remain fixed and colonies arise through their reproduction.
- the colonies show typical growth forms and the microorganisms can be distinguished from one another based on the characteristic appearance of the colonies,
- With appropriate inoculation techniques, colonies ( clones ) formed from individual cells can be recognized and genetically pure strains isolated from them,
- on the growth front, the edge of the colonies, there are always young microorganism individuals (instead of a mixture of growing and dying individuals).
Petri dishes are made of laboratory glass and plastic (clear polystyrene ) in various sizes . In biology and medicine nowadays almost exclusively single-use plastic petri dishes are used. Frequently used outer diameters are 50 and 92 to 93 mm (= 90 mm inner diameter) at a height of 15 mm. The lid of Petri dishes can lie directly on it or have a larger gap due to punctiform knobs. These ensure an optimal gas exchange and prevent condensation. Petri dishes without knobs are used for long incubation times, high temperatures or sensitive culture media.
literature
- RJ Petri: A small modification of the Koch plate method. In: Centralblatt für Bacteriologie und Parasitenkunde . Vol. 1, 1887, pp. 279-280. Online at archive.org
