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. Lactose is a sugar that some bacteria can use because of an enzyme that begins the breakdown of this compound. Phenol red lactose broth is thus used to determine whether the microbe can use the sugar lactose for carbon and energy.
How is lactose fermentation determined? If lactose 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.
What is the content of this medium? The medium is a nutrient broth to which 0.5-1.0% lactose 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 is the test performed? For information on how to determine the ability of a microbe to use lactose, refer to the lactose fermentation test.