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. Increases the acidity of the solution, thereby changing colour of indicator from yellow to purple. If the organism fails to produce ornithine-decarboxylase, the medium is acid (yellow).