A Petri dish (or Petri plate or cell culture dish) is a shallow glass or plastic cylindrical lidded dish that biologists use to culture cells or small moss plants. It was named after German bacteriologist Julius Richard Petri, who invented it when working as an assistant to Robert Koch. Glass Petri dishes can be reused by sterilization (for example, in an autoclave or by dry heating in a hot air oven at 160 °C for one hour). For experiments where cross-contamination from one experiment to the next can become a problem, plastic Petri dishes are often used as disposables.
Modern Petri dishes often have rings on the lids and bases, which allow them to be stacked so that they do not slide off one another. Multiple dishes can also be incorporated into one plastic container to create what is called a "multi-well plate". Petri dishes are usually used to culture bacteria.
Petri dishes are often used to make agar plates for microbiology studies. The dish is partially filled with warm liquid containing agar and a mixture of specific ingredients that may include nutrients, blood, salts, carbohydrates, dyes, indicators, amino acids and antibiotics. After the agar cools and solidifies, the dish is ready to receive a microbe-laden sample in a process known as inoculation or "plating." For virus or phage cultures, a two-step inoculation is needed: bacteria are grown first to provide hosts for the viral inoculum.
Petri plates are incubated upside down (agar on top) to lessen the risk of contamination from settling airborne particles and to prevent water condensation from accumulating and disturbing the cultured microbes.
Scientists have long been growing cells in natural and synthetic matrix environments to elicit phenotypes that are not expressed on conventionally rigid substrates. Unfortunately, growing cells either on or within soft matrices can be an expensive, labor intensive, and impractical undertaking.
Other uses 
Petri dishes are also used for eukaryotic cell culture in liquid medium or using solid agar. Empty Petri dishes may be used to observe plant germination or small animal behavior, or for other day-to-day laboratory practices such as drying fluids in an oven and carrying and storing samples. Their optical transparency and flat profile lend them to their common use as a temporary receptacle for viewing samples (especially liquid ones) under a low power microscope.
Notes and references 
- Mosby's Dental Dictionary (2nd ed.). Elsevier. 2008. Retrieved 2010-02-11.
- Ralf Reski (1998): Development, genetics and molecular biology of mosses. Botanica Acta 111, pp 1–15.
- Petri dish in the American Heritage Dictionary
- Gilbert, P.M. et al. Substrate elasticity regulates skeletal muscle stem cell self-renewal in culture. Science 329, 1078–1081 (2010).
- Chowdhury, F. et al. Soft substrates promote homogeneous self-renewal of embryonic stem cells via downregulating cell-matrix tractions. PLoS ONE 5, e15655 (2010).
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