Urea Agar and Urease Test Broth are used for the differentiation of organisms, especially the Enterobacteriaceae, on the basis of urease production.
SUMMARY AND EXPLANATION
Urea Agar was devised by Christensen for use as a solid medium for the differentiation of enteric bacilli. It differentiates between rapid urease-positive Proteeae organisms (Proteus spp., Morganella morganii subsp. morganii, Providencia rettgeri, and some Providencia stuartii) and other urease-positive organisms: Citrobacter, Enterobacter and Klebsiella and bacteria other than Enterobacteriaceae; i.e., some Bordetella and Brucella spp.2
Urease Test Broth was developed by Rustigian and Stuart. It may be used for the identification of bacteria on the basis of urea utilization and it is particularly recommended for the differentiation of members of the genus Proteus from those of Salmonella and Shigella in the diagnosis of enteric infections. The medium is positive for Proteus, Morganella morganii subsp. morganii, Providencia rettgeri, and a few Providencia stuartii strains with the reclassification of the members of the Proteeae.
The urea medium of Rustigian and Stuart is particularly suited for the differentiation of Proteus species from other gram-negative enteric bacilli capable of utilizing urea; the latter are unable to do so in Urease Test Broth because of limited nutrients and the high buffering capacity of the medium. To provide a medium with greater utility, Urea Agar was devised by Christensen with peptone and dextrose included and reduced buffer content to promote more rapid growth of many of the Enterobacteriaceae and permit a reduction in incubation time. The complete Urea Agar contains 15.0 g/L of agar in addition to the ingredients in the base medium.
When organisms utilize urea, ammonia is formed during incubation which makes the reaction of these media alkaline, producing a red-pink color. Consequently, urease production may be detected by the change in the phenol red indicator.