n 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.