Urease Test: Principle, Procedure, Interpretation & Urease-Positive Organisms

The urease test determines whether an organism produces urease, an enzyme that hydrolyzes urea into ammonia and carbon dioxide. The ammonia raises the pH of the medium, which is detected by a color shift in the phenol red indicator: light orange at neutral pH, bright pink (magenta) at alkaline pH.

Why It Matters

Proteus mirabilis is one of the most prolific urease producers in clinical microbiology, and that single enzyme has a direct downstream consequence: in a urinary tract infection, urease-driven ammonia production alkalinizes the urine enough to precipitate magnesium ammonium phosphate crystals — struvite stones. This is the classic mechanism behind staghorn calculi and recurrent UTIs, and it's the reason urease activity isn't just an identification tool — it's a disease mechanism you can trace directly from one enzyme to a kidney stone on imaging.

Urease also shows up in a completely different clinical setting: a rapid urease test (sometimes called the CLO test) performed on a crushed gastric biopsy sample is one of the fastest ways to get presumptive evidence of Helicobacter pylori during an endoscopy — same principle, same color change, completely different patient story (peptic ulcer disease workup instead of a UTI).

Principle

Urea + H₂O → (urease) → 2 NH₃ + CO₂

The ammonia combines with CO₂ and water to form ammonium carbonate, which alkalinizes the medium. Phenol red shifts from light orange to bright pink (magenta) as the pH rises.

Media: Christensen's urea agar (slant) or Stuart's urea broth — both contain phenol red as the indicator, but they don't always agree with each other (see below).

Procedure

Christensen's Urea Agar

1. Streak the entire slant surface with a heavy inoculum from an 18–24 hour pure culture. Do not stab the butt — it's left unstabbed deliberately, to serve as a color control against the slant.
2. Incubate with the cap loosened at 35°C.
3. Read at 6 hours and again at 24 hours — don't rely on a single reading.

Stuart's Urea Broth

1. Inoculate with a heavy inoculum from an 18–24 hour pure culture and shake gently to suspend.
2. Incubate with the cap loosened at 35°C.
3. Read at 8, 12, and 24 hours.

Figure: Urease test using urea broth. Left: Positive, Right: Negative

Where students actually get confused

• Rapid vs. delayed producers need different reading windows. Proteus, Morganella morganii, and some Providencia stuartii are rapid urease producers — strong positive within 1–6 hours. Klebsiella and Enterobacter are delayed producers — weak positive at 6 hours that intensifies over 24 hours. If you only check once at 24 hours, you'll still catch both, but if you check early and stop, you'll miss the delayed group.
• Broth and agar can give genuinely different answers for the same organism. This is the trap that actually matters: Stuart's broth has a higher buffering capacity than Christensen's agar, which means delayed-positive organisms like Enterobacter may produce a false negative in broth even though the same organism reads correctly as delayed-positive on agar. If a result looks wrong, check which medium you used before assuming a technical error.
• The unstabbed butt is a control, not a shortcut. On Christensen's agar, comparing the slant color to the unstabbed butt is how you confirm the color shift is real and not just lighting or an old tube.
• A false positive can come from peptone, not urease. In overnight tests run on peptone-containing medium, peptone hydrolysis alone can alkalinize the medium and mimic a positive — this is unrelated to urease activity. Be cautious interpreting overnight positives on peptone-rich media.
• Urea reagent is light-sensitive. It undergoes autohydrolysis if not stored properly — keep at 2–8°C in the dark, and never reheat prepared medium, since urea decomposes under heat.

Result Interpretation

Figure: Urea agar test (a) uninoculated, (b) Proteus mirabilis (rapidly urease positive), (c) Klebsiella pneumoniae (delayed urease positive), (d)Escherichia coli (urease negative). (Benita A. Brink, Adams State College, Alamosa, CO)

The PUNCH Mnemonic

Proteus · Ureaplasma · Nocardia · Cryptococcus neoformans / Corynebacterium · Helicobacter pylori

Note: Brucella is also a strong urease producer used diagnostically but doesn't have a letter in this mnemonic — worth remembering as a named exception alongside PUNCH.

Diagnostic Applications

• Proteus identification: distinguishes Proteus from other non-lactose-fermenting Enterobacteriaceae.
• H. pylori (gastric biopsy): a crushed tissue sample placed directly into urease broth gives presumptive evidence of infection — the basis of the rapid/CLO test used during endoscopy. The related urea breath test uses the same enzymatic principle non-invasively.
• Cryptococcus vs. Candida: Cryptococcus neoformans is rapidly urease-positive; Candida albicans is not — useful for presumptive yeast differentiation.

Quality Control

Limitations

1. Some organisms split urea rapidly (Brucella, H. pylori), others slowly — interpretation depends on knowing which group you're testing.
2. Peptone hydrolysis in overnight tests can mimic a true urease-positive reaction.
3. Urea is light-sensitive and undergoes autohydrolysis — store at 2–8°C in the dark.
4. The test is less sensitive on unbuffered medium.

References and further reading

1. Benita Brink. 2010. Urease test protocol. American Society for Microbiology
2. Clinical Microbiology Procedures Handbook, Fourth Edition. (2016). American Society of Microbiology. https://doi.org/10.1128/9781555818814