alt=|thumb|A glass Petri dish, Mac Conkey agar and medium
A Petri dish (alternatively known as a Petri plate or cell-culture dish) is a shallow transparent lidded dish that biologists use to hold growth medium in which cells can be cultured, originally, cells of bacteria, fungi, and small mosses.
The container is named after its inventor, German bacteriologist Julius Richard Petri. It is the most common type of culture plate. The Petri dish is one of the most common items in biology laboratories and has entered popular culture. The term is sometimes written in lower case, especially in non-technical literature. which used an agar medium developed by Walther Hesse. Koch had published a precursor dish in a booklet in 1881 titled "" (On the Study of Pathogenic Organisms), which is now known as the "Bible of Bacteriology". He described a new bacterial culture method that used a glass slide with agar and a container (basically a Petri dish, a circular glass dish of 20 × 5 cm with matching lid) which he called ' ("moist chamber"). A bacterial culture was spread on the glass slide, then placed in the moist chamber with a small wet paper. Bacterial growth was easily visible.
Koch publicly demonstrated his plating method at the Seventh International Medical Congress in London in August 1881. There, Louis Pasteur exclaimed, "" ("What great progress, Sir!") It was using this method that Koch discovered important pathogens of tuberculosis (Mycobacterium tuberculosis), anthrax (Bacillus anthracis), and cholera (Vibrio cholerae). For his research on tuberculosis, he was awarded the Nobel Prize in Physiology or Medicine in 1905. His students also made important discoveries. Friedrich Loeffler discovered the bacteria of glanders (Burkholderia mallei) in 1882 and diphtheria (Corynebacterium diphtheriae) in 1884; and Georg Theodor August Gaffky, the bacterium of typhoid (Salmonella enterica) in 1884.
Petri made changes in how the circular dish was used. It is often asserted that Petri developed a new culture plate, but this is incorrect. Instead of using a separate glass slide or plate on which culture media were placed, Petri directly placed media into the glass dish, eliminating unnecessary steps such as transferring the culture media, using the wet paper, and reducing the chance of contamination.
Features and variants
Petri dishes are usually cylindrical, mostly with diameters ranging from , and a height to diameter ratio ranging from 1:10 to 1:4. Four sided versions are also available.
Petri dishes were traditionally reusable and made of glass; often of heat-resistant borosilicate glass for proper sterilization at 120–160 °C.
Since the 1960s, plastic dishes, usually disposable, are also common.
The dishes are often covered with a shallow transparent lid, resembling a slightly wider version of the dish itself. The lids of glass dishes are usually loose-fitting. Alternatively, some glass or plastic versions may have small holes around the rim, or ribs on the underside of the cover, to allow for air flow over the culture and prevent water condensation.
Some Petri dishes, especially plastic ones, feature rings and/or slots on their lids and bases so that they are less prone to sliding off one another when stacked or sticking to a smooth surface by suction.
Some versions may have grids printed on the bottom to help in measuring the density of cultures.
Petri dishes can be used to visualize the location of contamination on surfaces, such as kitchen counters and utensils, clothing, food preparation equipment, or animal and human skin. For this application, the Petri dishes may be filled so that the culture medium protrudes slightly above the edges of the dish to make it easier to take samples on hard objects. Shallow Petri dishes prepared in this way are called Replicate Organism Detection And Counting (RODAC) plates and are available commercially.
Petri dishes are also used for cell cultivation of isolated cells from eukaryotic organisms, such as in immunodiffusion studies, on solid agar or in a liquid medium.
Petri dishes may be used to observe the early stages of plant germination, and to grow plants asexually from isolated cells.
Petri dishes may be convenient enclosures to study the behavior of insects and other small animals.
Due to their large open surface, Petri dishes are effective containers to evaporate solvents and dry out precipitates, either at room temperature or in ovens and desiccators.
Petri dishes also make convenient temporary storage for samples, especially liquid, granular, or powdered ones, and small objects such as insects or seeds. Their transparency and flat profile allows the contents to be inspected with the naked eye, magnifying glass, or low-power microscope without removing the lid.
In popular culture
The Petri dish is one of a small number of laboratory equipment items whose name entered popular culture. It is often used metaphorically, e.g. for a contained community that is being studied as if they were microorganisms in a biology experiment, or an environment where original ideas and enterprises may flourish.
Unicode has a Petri dish emoji, "<span style="font-size:larger;">🧫</span>", which has the code point U+1F9EB (HTML entity "&#129515;" or "&#x1F9EB;", UTF-8 "0xF0 0x9F 0xA7 0xAB").