Purpose: To differentiate between bacteria that produce the enzyme urease and those that do not.
Principle: Urea may be hydrolyzed by some bacteria so that the ammonia that is liberated can be used as a nitrogen source. Phenol red indicator is added to the broth so that when the pH reaches 8.4 (due to ammonia production) the tube will turn from an orange color to a pink color.
The tubes at the right represent a negative and a positive urea tube.