An inoculum from a pure culture is transferred aseptically to a sterile tube of ornithine decarboxylase broth. The inoculated tube is incubated at 35-37 C for 24 hours and the preliminary results are determined. The microbe must first use the glucose present to cause the pH to drop. This is indicated by a change from purple to yellow. 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 obtained by observing the tube at 48 hours. 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.