A Durham fermentation tube is a small test tube that is inverted and placed in a larger test tube filled with a carbohydrate broth. Common sugars used to make the broth include glucose, sucrose and lactose, explains a Portland Community College website.
The carbohydrate fermentation test helps identify different types of bacteria based on which sugars they are able to ferment. If a bacterium can ferment the sugar, it results in acid production, gas production or both. If fermentation results in acid production, the broth changes colors. Gas production is seen as a gas bubble in the Durham fermentation tube. If the bacteria cannot ferment the sugar, there is no change in the broth.