Urease Test (Christensen's Method)
A bench guide to Christensen's urea agar slant test, used to detect urease activity to rapidly identify Proteus species and differentiate other Enterobacteriaceae.
Urea slants A-B
Compare the slant and butt color reactions. Hydrolysis of urea creates basic ammonia, turning the phenol red indicator pink/magenta.
The entire slant and butt turn a vibrant magenta-pink color, demonstrating rapid urea hydrolysis and alkaline pH shift.
Agar slant and butt remain their original light peach/orange color, indicating no urea hydrolysis occurred.
The entire slant and butt turn a vibrant magenta-pink color, demonstrating rapid urea hydrolysis and alkaline pH shift.
Agar slant and butt remain their original light peach/orange color, indicating no urea hydrolysis occurred.
What to look for
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Magenta / Pink color | Ammonia accumulation alkalizes the medium, shifting phenol red pH | Positive result |
| Light orange / peach color | No urea hydrolysis; pH remains neutral/acidic | Negative result |
| Rapid reaction | Rapidly urease-positive organisms (like Proteus) turn the medium pink within 24 hours | Key diagnostic clue |
Urease Interpretive Matrix
| Pattern | What to call it | How to think about it |
|---|---|---|
| Vibrant magenta-pink throughout | Positive (Rapid) | Organism produces active urease (e.g., Proteus vulgaris). |
| Pink slant only or delayed pink | Weakly Positive | Slow urease production (e.g., Klebsiella pneumoniae). |
| No change (remains light orange) | Negative | No urease produced (e.g., Escherichia coli). |
- The urease test detects the enzyme urease, which breaks down urea into ammonia (NH3) and carbon dioxide (CO2).
- Phenol red is the pH indicator: magenta/pink at pH 8.1 (positive), orange at pH 6.8 (negative).
- It is highly specific for separating Proteus (rapidly positive) from other lactose non-fermenting Gram-negative rods.
Pink = Positive (rapid Proteus). Orange = Negative (E. coli). Ammonia makes it basic (pink).