Enteric Rods: Ornithine decarboxylation test
Ornithine decarboxylase broth is used to aid in the differentiation of
Gram-negative rods. When ornithine is decarboxylated by bacteria, alkaline
by-products cause the pH indicator to turn purple. Other organisms ferment
glucose in the medium, producing acid, which turns the pH indicator yellow. A
decarboxylase base broth is always inoculated for this test because some
bacteria neither ferment glucose nor decarboxylate ornithine. For these
metabolically inactive organisms, the color of the test medium will remain the
same as that of the base medium.
Note that the broth in the tubes is overlaid with mineral oil to create the
anaerobic conditions needed for decarboxylase reactions to occur.
