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. Citrate is an organic acid that some bacteria can use for carbon and energy. Simmons citrate agar is thus used to determine whether the microbe can use the compound citrate for carbon and energy.
How is citrate fermentation determined? If citrate can be used, the microbe will accumulate alkaline/basic byproducts. In a positive test, the pH indicator in the medium changes color from its normal green to deep blue, indicating alkaline/basic products.
What is the content of this medium? Simmons citrate agar is a mineral salts broth with 0.2% sodium citrate added. All media must have a nitrogen source, and ammonium dihydrogen phosphate serves that purpose. The pH indicator brom thymol blue is green at neutral pH, yellow at acidic pH <6.0 and turns blue at alkaline (basic) pH >7.6.
How is the test performed? For information on how to determine the ability of a microbe to use citrate, refer to the Citrate Utilization test.