What is this medium used for?
Bacteria are identified based largely on what organic compounds they can break down. The range of compounds used depends on the collection of enzymes a species of bacteria can make. Glucose is a sugar that some bacteria can use because of an enzyme that begins the breakdown of this compound. Phenol red glucose (dextrose) broth is thus used to determine whether the microbe can use the sugar glucose for carbon and energy.
When many microbes ferment glucose they produce gases, mainly carbon dioxide and hydrogen. These gases bubble up through the medium and escape into the atmosphere. Tubes of broth media can be made with inverted tubes called Durham tubes which are filled with medium after sterilization. Durham tubes serve as gas traps for collecting bubbles generated by gas-producing microbes.
How is glucose fermentation determined?
If glucose can be used, the microbe will accumulate acidic byproducts. In a positive test, the pH indicator in the medium changes color from its normal red to yellow, indicating acid production.
How is gas production determined?
The Durham tube traps any gases microbes might produce during fermentation. These gases bubble up through the medium and produce obvious bubbles in the Durham tube.
What is the content of this medium?
The medium is a nutrient broth to which 0.5-1.0% glucose is added. The pH indicator phenol red is red at neutral pH but turns yellow at pH <6.8. It also changes to magenta or hot pink at pH >8.4.
How are the two tests performed?
For information on how to determine the ability of a microbe to use glucose, refer to the acid from glucose test. or for information on how to determine gas production during fermentation, refer to the gas from glucose test.