The Non-Enteric Rods: Ornithine decarboxylation test
Arginine and ornithine decarboxylase broths are used to aid in the
differentiation of Gram-negative rods. When ornithine or arginine are
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 these tests because some bacteria neither ferment glucose nor decarboxylate
arginine/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.
