Microbiology · Enterobacterales
Proteus mirabilis !
- Bacterium
- Gram-negative
- Exam priority: medium
A bacterium that causes urinary infections, especially with catheters, and builds large stones by making urine alkaline.
Updated 2026-11-01
At a glance
- Gram
- Gram-negative
- Shape
- Rod
- Oxygen
- Facultative anaerobe
- Family
- Morganellaceae
- Stain
- Gram: red rods
Morphology
- Highly motile Gram-negative rod
- Spreads in waves across the whole agar plate (swarming)
- Non-lactose fermenter; makes H2S; strongly urease-positive
- Indole-negative (P. vulgaris is indole-positive)
Transmission
- Endogenous infection from gut flora
- Climbs the urethra, especially in people with long-term urinary catheters
Reservoir: Human gut, soil and water
Isolation precautions
What the care team adds to standard precautions.
- Standard
Virulence factors
- Urease: splits urea into ammonia, raises urine pH and precipitates struvite (magnesium ammonium phosphate) stones
- Crystalline biofilm that blocks catheters
- Flagella and swarming motility: ascent up the urinary tract
- Fimbriae, hemolysin and proteases
Diseases it causes
- Complicated urinary tract infection, mainly catheter-associated Urinary tract infection (acute cystitis) →
- Struvite stones, sometimes staghorn calculi filling the renal pelvis Kidney stones (urolithiasis) →
Diagnosis
- Alkaline urine pH with struvite ("coffin lid") crystals
- Urine culture: colonies that swarm over the plate, with a typical smell
- Imaging to look for staghorn stones
Treatment
By antibiogram: usually susceptible to ampicillin, cephalosporins, trimethoprim-sulfamethoxazole and fluoroquinolones. It is intrinsically resistant to nitrofurantoin, tetracyclines and polymyxins. Infected stones are removed, because the bacteria persist inside them.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Avoiding unneeded catheters and changing or removing them on time
- Treatment of recurrent urinary infections
Test yourself
Think of the answer, then tap the question to see it.
Which Proteus mirabilis enzyme explains struvite stones?
Urease, which turns urea into ammonia and makes urine alkaline.
What does Proteus look like on an agar plate?
It spreads in waves over the whole plate because of swarming motility.
Which cystitis drug does not work against Proteus because of intrinsic resistance?
Nitrofurantoin.
Related
Related conditions
← Back to the group: Enterobacterales
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Proteeae Infections (MSD Manual Professional Edition, 2024); Kidney stones (MedlinePlus, 2026).
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Representative model, for educational purposes only.