Christensen’s Urea Agar was formulated to detect and differentiate urolytic and urea degrading microorganisms. Proteus species are rapid urease positives and can be differentiated from urease positive Enterobacteriaceae like Citrobacter, Enterobacter and Klebsiella. Also gram positive bacteria with urease activity can be detected. Christensen observed that the addition of gelatine peptone, dextrose and reduced content of buffer supports a luxuriant growth at an early stage. Urea is the substrate and can be degraded from certain organisms, which results in ammonia building. The ammonia makes the media alkaline and therefore the indicator phenol red change the colour to pink-red. Prolonged incubation may cause alkaline reaction in the medium.
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