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. Ornithine is an amino acid that some bacteria can use because of an enzyme called ornithine decarboxylase. Ornithine decarboxylase broth is thus used to determine whether the microbe can use the amino acid ornithine for carbon and energy.
How is ornithine use determined? If ornithine can be used, the microbe will accumulate alkaline/basic metabolic products. The enzyme ornithine decarboxylase degrades ornithine to produce these alkaline/basic products. However, the enzyme does not do this unless the growth medium is acidified by other metabolic activities, so the test involves two distinct stages: first the microbe must acidify the medium; then the ornithine decarboxylase enzyme (if present) can metabolize the ornithine.
In a positive test, the microbe must first use the glucose present to cause the pH to drop. This is indicated by a change from purple to yellow during the first 24 hours of incubation. Once the medium has been acidified, the enzyme ornithine decarboxylase is activated. The culture is incubated an additional 24 hours at 35-37 C to allow the microbe to now use the ornithine. The final results are then observed. Change back to purple from yellow indicates a positive test for ornithine decarboxylase. Failure to turn yellow at 24 hours or to revert back to purple at 48 hours indicates a negative result.
What is the content of this medium? The medium is a nutrient broth to which 0.5% ornithine is added. An important component of the medium is a modest amount of glucose, necessary for the initial acidification of the medium. The pH indicator brom cresol purple is purple at neutral or alkaline/basic pH but turns yellow at pH <5.2.
How is the test performed? For information on how to determine the ability of a microbe to produce the enzyme ornithine decarboxylase, refer to the ornithine decarboxylase test.