Urea is a waste product excreted in urine by animals. Some enteric bacteria produce the enzyme urease, which splits the urea molecule into carbon dioxide and ammonia. The urease test is useful in identifying the genera Proteus, Providentia, and Morganella, which liberate this enzyme.
Urease, which is produced by some microorganisms, is an enzyme that is especially helpful in the identification of Proteus vulgaris. Although other organisms may produce urease, their action on the substrate urea tends to be slower than that seen with Proteus species. Therefore, this test serves to rapidly distinguish members of this genus from other lactose-non-fermenting enteric microorganisms.
Urease is a hydrolytic enzyme that attacks the nitrogen and carbon bond in amide compounds such as urea and forms the alkaline end product ammonia. The presence of urease is detectable when the organisms are grown in a urea broth medium containing the pH indicator phenol red. As the substrate urea is split into its products, the presence of ammonia creates an alkaline environment that causes the phenol red to turn to a deep pink. This is a positive reaction for the presence of urease. Failure of the deep pink color to develop is evidence of a negative reaction.
Urea is unstable and is broken down at 15 psi, or pressure. It cannot be added to the medium for autoclaving and is therefore filter sterilized and added to the medium after autoclaving.
Examine the urea slants inoculated with Proteus vulgaris and compare them to the unknown. If your unknown is negative, continue to incubate the slant for 7 days to check for slow urease production.
