Urease broth is a differential medium that tests the ability of an organism to produce an exoenzyme, called urease, that hydrolyzes urea to ammonia and carbon dioxide. The broth contains two pH buffers, urea, a very small amount of nutrients for the bacteria, and the pH indicator phenol red. Phenol red turns yellow in an acidic environment and fuchsia in an alkaline environment. If the urea in the broth is degraded and ammonia is produced, an alkaline environment is created, and the media turns pink.
Many enterics can hydrolyze urea; however, only a few can degrade urea rapidly. These are known as “rapid urease-positive” organisms. Members of the genus Proteus are included among these organisms.
Urea broth is formulated to test for rapid urease-positive organisms. The restrictive amount of nutrients coupled with the use of pH buffers prevent all but rapid urease-positive organisms from producing enough ammonia to turn the phenol red pink.
In our lab, we only use the urease broth, not the agar.
Proteus mirabilis is rapid urease positive as evidenced by the pink color of the media.
Escherichia coli on the right is negative.
Urease broth can be used to differentiate members of the genus Proteus (as well as those of Morganella and Providencia, but we don’t use those in our lab) from other enterics.