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Laboratory diagnosis of an infection is usually confirmed by isolating and culturing microorganisms in artificial media. Bacteria and fungi are cultured in either liquid (broth) or on solid (agar) artificial media. Koch pioneered the use of agar as a base for culture media. He developed the pour plate method and was the first to use solid culture media for culture of bacteria. At first, Koch cultured bacteria on the sterile surfaces of cut, boiled potatoes. This was unsatisfactory, because bacteria would not always grow well on potatoes. He then tried to solidify regular liquid media by adding gelatin. Separate bacterial colonies developed after the surface had been streaked with a bacterial sample. The sample could also be mixed with liquefied gelatin medium. When the gelatin medium hardened, individual bacteria produced separate colonies. Despite its advantages, gelatin was not an ideal solidifying agent because it was digested by many bacteria and melted when the temperature rose above 28°C. A better alternative was provided by Fannie EilshemiusHesse, the wife of Walther Hesse, one of Koch’s assistants. She suggested the use of agar as a solidifying agent—she had been using it successfully to make jellies for sometime. Agar was not attacked by most bacteria and did not melt until it reaches a temperature of 100°C. One of Koch’s assistants, Richard Petri, developed the Petri dish (plate), a container for solid culture media.
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