Urease is a constitutively expressed enzyme that hydrolyzes urea to carbon dioxide and ammonia.
(NH2)2CO + H2O → CO2 + 2NH3
Urease test media contain 2% urea and phenol red as a pH indicator. An increase in pH due to the production of ammonia results in a color change from yellow (pH 6.8) to bright pink (pH 8.2). Urea broth (Stuart’s urea broth) is a highly buffered medium requiring large quantities of ammonia to raise the pH above 8.0 resulting in a color change. This medium provides all essential nutrients for Proteus, for which it is differential. Urea agar (Christensen’s urea agar) has a reduced buffer content and contains peptones and glucose. This medium supports the growth of many enterobacteria allowing for the observation of urease activity