Urease hydrolyzes urea into ammonia, carbon dioxide and water.
Urea is NH2-CO-NH2 (diamide of carbonic acid.
The release of ammonia increases the pH of the media. Phenolphthalein is colorless at a pH 8.1, but becomes pink-red if the pH is > 8.1.
Bacteria with urease vary in its activity:
(1) some species hydrolyze urea rapidly, with 2 hours
(2) other species hydrolyze urea slowly, over several days
The most commonly used test media are:
(1) Stuart's urea broth
(2) Christensen's urea agar
Both contain a large amount of urea.
The inoculated media is incubated at 35°C.
Causes of a false positive:
(1) mixed isolate
Causes of a false negative:
(1) reading the tube at 24 hours (will fail to detect species with weak urease activity)