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. What was later called Petri dish was originally developed by German physician Robert Koch in his private laboratory in 1881, as a precursor method. Petri, as assistant to Koch, at Berlin University made the final modifications in 1887 as used today. Penicillin, the first antibiotic, was discovered in 1929 in London when Alexander Fleming noticed that penicillium mold contaminating a bacterial culture in a Petri dish had killed the bacteria around it.
History The Petri dish was developed by German physician Julius Richard Petri (for whom the dish is named) while working as an assistant to Robert Koch at Berlin University. Petri did not invent the culture dish himself; rather, it was a modified version of Koch's invention which used an agar medium developed by Walther Hesse. Koch had published a precursor dish in a booklet in 1881 titled "Zur Untersuchung von Pathogenen Organismen" (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 feuchte Kammer ("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, "C'est un grand progrès, Monsieur!" ("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. He published the improved method in 1887 as "Eine kleine Modification des Koch'schen Plattenverfahrens" ("A minor modification of Koch's plating technique"). Although it could have been named "Koch dish", the final method was given the eponymous name Petri dish.
Features and variants
Petri dishes are usually cylindrical, mostly with diameters ranging from 30 to 200 millimetres (1.2 to 7.9 in), 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. Plastic dishes may have close-fitting covers that delay the drying of the contents. 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. Small dishes may have a protruding base that can be secured on a microscope stage for direct examination. Some versions may have grids printed on the bottom to help in measuring the density of cultures. A microplate is a single container with an array of flat-bottomed cavities, each being essentially a small Petri dish. It makes it possible to inoculate and grow dozens or hundreds of independent cultures of dozens of samples at the same time. Besides being much cheaper and convenient than separate dishes, the microplate is also more amenable to automated handling and inspection. Some plates are separated into different media known as biplates, triplates, and quadplates.
Uses
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