Durham tube
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Durham tubes are used in microbiology to detect production of gas by microorganisms. They are simply smaller test tubes inserted upside down in another test tube. This small tube is initially filled with the solution in which the microorganism is to be grown. If gas is produced after inoculation and incubation, a visible gas bubble will be trapped inside the small tube. Note that the initial air gap when the tube is inserted upside down goes away by itself during the standard sterilization procedure which is performed at 121°C for 15 minutes.