Petri dish - wikipedia Petri dish This is an old revision of this page, as edited by Corker1 ( talk contribs ) at 21 : 33, 19 May 2017 ( → ‎ Other uses : add references to photographs of immunodiffusion in Petri dishes ). The present address ( URL ) is a permanent link to this revision, which may differ significantly from the current revision. Revision as of 21 : 33, 19 May 2017 by Corker1 ( talk contribs ) ( → ‎ Other uses : add references to photographs of immunodiffusion in Petri dishes ) ( diff ) ← Previous revision Latest revision ( diff ) Newer revision → ( diff ) The bottom half of a glass Petri dish A Petri dish ( sometimes spelled `` Petrie dish '' and alternatively known as a Petri plate or cell - culture dish ), named after the German bacteriologist Julius Richard Petri, is a shallow cylindrical glass or plastic lidded dish that biologists use to culture cells -- such as bacteria -- or small mosses. Modern Petri dishes usually feature rings and / or slots on their lids and bases so that when stacked, they are less prone to sliding off one another. Multiple dishes can also be incorporated into one plastic container to create a `` multi-well plate ''. While glass Petri dishes may be reused after sterilization ( via an autoclave or one hour 's dry - heating in a hot - air oven at 160 ° C, for example ), plastic Petri dishes are often disposed of after experiments where cultures might contaminate each other. Contents 1 Microbiology 2 Other uses 3 Notes and references 4 External links Microbiology Further information : Agar plate A Petri dish with bacterial colonies on an agar - based growth medium 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 or antibiotics. Once the agar cools and solidifies, the dish is ready to be inoculated ( `` plated '' ) with a microbe - laden sample. Virus or phage cultures require a two - stage inoculation : after the agar preparation, bacteria are grown in the dish to provide hosts for the viral inoculum. Petri plates are incubated upside - down to lessen the risk of contamination from airborne particles settling on them and to prevent the accumulation of any water condensation that may otherwise disturb or compromise a culture. While Petri dishes are widespread in microbiological research, smaller dishes tend to be used for large - scale studies in which growing cells in Petri dishes can be relatively expensive and labor - intensive. A special type of petri dishes are Replicate Organism Detection And Counting often also referred to as rodac plates or contact plates. Those are petri dishes in which the medium protrudes ( raised agar level ) the edges of the dish to make it easier to take samples on hard objects. The plates are also divided in squares to make counting of CFUs easier. They are often used to monitor how clean certain areas ( eg in kitchens ) are. Other uses See also : Microbial art Petri dishes are also used for eukaryotic cell culture in a liquid medium or on solid agar. They are used in immunodiffusion studies when partially filled with agar or agarose gel. Empty Petri dishes may be used to observe plant germination, the behavior of very small animals or for other day - to - day laboratory practices such as drying fluids in an oven and carrying or storing samples. Their transparency and flat profile also mean they are commonly used as temporary receptacles for viewing samples, especially liquids, under a low - power microscope. Notes and references Jump up ^ Petri dish in the American Heritage Dictionary. Jump up ^ Petri, R.J. ( 1887 ) `` Eine kleine Modification des Koch'schen Plattenverfahrens '' ( A small modification of Koch 's plate method ), Centralblatt für Bakteriologie und Parasitenkunde, 1 : 279 -- 280. Jump up ^ Mosby 's Dental Dictionary ( 2nd ed. ). Elsevier. 2008. Retrieved 2010 - 02 - 11. Jump up ^ Reski, Ralf ( 1998 ). `` Development, genetics and molecular biology of mosses '' ( PDF ). Botanica Acta. 111 : 1 -- 15. doi : 10.1111 / j. 1438 - 8677.1998. tb00670. x. Jump up ^ Gilbert, P.M. ( 2010 ). `` Substrate elasticity regulates skeletal muscle stem cell self - renewal in culture ''. Science. 329 ( 5995 ) : 1078 -- 1081. PMC 2929271. PMID 20647425. doi : 10.1126 / science. 1191035. Jump up ^ Chowdhury, F. ( 2010 ). `` Soft substrates promote homogeneous self - renewal of embryonic stem cells via downregulating cell - matrix tractions ''. PLoS ONE. 5 ( 12 ) : e15655. PMC 3001487. PMID 21179449. doi : 10.1371 / journal. pone. 0015655. Jump up ^ Lemmen, Sebastian W. ; Häfner, Helga ; Zolldann, Dirk ; Amedick, Günter ; Lutticken, Rüdolf ( 2001 ). `` Comparison of two sampling methods for the detection of Gram - positive and Gram - negative bacteria in the environment : Moistened swabs versus Rodac plates ''. International Journal of Hygiene and Environmental Health. 203 ( 3 ) : 245. PMID 11279821. doi : 10.1078 / S1438 - 4639 ( 04 ) 70035 - 8. Jump up ^ ( 1 ) `` Immunodiffusion ''. ScienceDirect. Elsevier B.V. Archived from the original on 2017 - 05 - 19. Retrieved 2017 - 05 - 19. ( 2 ) `` Photograph of double immunodiffusion in a Petri dish '' ( photograph ). Retrieved 2017 - 05 - 15. ( 3 ) `` Photgraph of radial immunodiffusion in a Petri dish '' ( photograph ). Edvotek, Inc. 2017. Retrieved 2017 - 05 - 15. External links Wikimedia Commons has media related to Petri dishes. 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Question: why are bacterial agar plates turn upside-down when they are placed in the incubator to grow
Answer: to lessen the risk of contamination from airborne particles settling on them and to prevent the accumulation of any water condensation that may otherwise disturb or compromise a culture