Durham tubes are used to detect gas-producing bacteria in biological samples. The gases produced by the microorganisms become trapped in the Durham tube, producing bubbles that are visually identified.
Durham tubes are simple lab apparatuses consisting of small test tubes placed inverted in larger ones. Prior to the inversion, the larger test tubes are filled with a seed culture of the microorganism being tested and a medium suitable for the growth of different kinds of bacteria. An example medium consists of a mixture of water and lactose, a simple sugar that many bacteria can metabolize. Varying the type of nutrient and its concentration promotes the growth of certain kinds of bacteria while suppressing the growth of others. Incubating bacteria consume the nutrient, producing gases as byproducts.
The type of bacteria contained in the sample can be estimated by observing the change in color of the culture after a fixed duration of time and determining the kind of gas trapped in the Durham tube by its odor or lack thereof. Using Durham tubes for microbiological identification takes considerable time; samples may be left to incubate for several weeks. Samples are usually sterilized through heating prior to examination, destroying the gas bubbles.