Petri dish

A Petri dish is a shallow, round, transparent shell with overlapping edges, which is used generally in biology, medicine and chemistry.

Petri dishes were introduced in 1887 by German bacteriologist Julius Richard Petri and are named after him. They are usually used for the cultivation of microorganisms and cell culture. For this purpose, a flat layer of a gel-like culture medium in the Petri dish is produced: The medium is usually prepared on an agar base is sterilized in an autoclave by heating and yet poured hot and thus liquid in the trays, at room temperature it solidifies and forms a so-called agar plate. The nutrient medium supplied the growing microorganisms with water and nutrients, keep it but - in contrast to cultures in liquid media - at one point fixed. The spread of the microorganisms, such as by mixing in a liquid medium is prevented.

Major advantages are:

• The microorganisms remain, with few exceptions (eg, Proteus ) localized and caused by their multiplication colonies,
• The colonies show typical forms of growth and the microorganisms can be distinguished from each other because of the characteristic appearance of the colonies
• With corresponding Beimpfungstechniken (clones ) can consist of single cells resulting colonies identified and it genetically pure strains are isolated,
• At the growth front, the edge of the colonies, young microorganisms individuals are always (rather than a mixture of growing and dying individuals ) present.

Petri dishes are made in various sizes from laboratory glass and plastic ( transparent polystyrene). In biology and medical disposable plastic petri dishes are now almost exclusively used. Outer diameter commonly used are 50 and 92 to 93 mm (= 90 mm inner diameter ) at 15 mm in height. The lid of Petri dishes can be supported directly or through selective studs have a larger gap. These ensure optimal gas exchange and prevent condensation. Petri dishes without knobs are used for long incubation times, high temperatures or sensitive culture media.