Proteus mirabilis
Gram-negative bacilli
Identification & Growth
Identification
- Oxidase-negative, strongly urease-positive, H₂S-positive on TSI (K/A H₂S+)
- Indole-negative (differentiates from P. vulgaris)
Growth Conditions
- Blood agar: swarming motility spreading as a veil across the plate
- MacConkey agar: lactose-negative, no swarming; 35 °C, 24 h
Clinical Significance
- UTI, especially in catheterized patients; struvite stones from urine alkalinization
Intrinsic Resistance
- Colistin/polymyxin B (EUCAST Expected Resistant Phenotypes v1.2)
- Nitrofurantoin and tetracyclines (including tigecycline)
Bench Alerts
- Do not report nitrofurantoin on a urine culture with Proteus
- Imipenem breakpoints do not apply to this genus
- Swarming can mask another organism — use MacConkey agar to separate colonies
Clinical Notes
Clinical presentations
- A common cause of urinary tract infection, especially in patients with indwelling urinary catheters or urologic abnormalities.
- Its urease production favors struvite stone formation and chronic pyelonephritis associated with nephrolithiasis.
- Can cause bacteremia secondary to complicated urinary infection in institutionalized elderly patients.
Specimens and collection
- Midstream or straight catheter urine culture should be processed promptly, as urease-mediated urine alkalinization can affect interpretation.
- Surgically removed urinary stones should be sent for culture and compositional analysis when an infection stone is suspected.
Epidemiology and at-risk populations
- Patients with long-term urinary catheterization and structural urinary tract abnormalities have increased risk of recurrent infection.
- More prevalent in complicated and healthcare-associated urinary infections than in uncomplicated community-acquired UTI.
Resistance and therapeutic implications
- It is intrinsically resistant to nitrofurantoin and tigecycline, which should be reflected in the report regardless of the in vitro result.
- Unlike Proteus vulgaris and other Proteeae, it is generally susceptible to broad-spectrum beta-lactams in the absence of acquired mechanisms.
- ESBL- or carbapenemase-producing strains, though less frequent than in Klebsiella/E. coli, require laboratory confirmation when suspected.
Bench and reporting notes
- Gram-negative rod with characteristic swarming motility on blood agar, forming a concentric growth pattern.
- Strongly positive urease test within hours aids presumptive differentiation from other Enterobacterales.
- The report should state intrinsic nitrofurantoin resistance, avoiding its use even when the in vitro result appears susceptible.
Sources
- PMC — Classification, Identification, and Clinical Significance of Proteus, Providencia, and Morganella
- EUCAST — Expected Phenotypes
Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.
FAQ: Frequently Asked Questions
How to identify Proteus mirabilis in the lab?
Proteus mirabilis is identified through oxidase-negative, strongly urease-positive, h₂s-positive on tsi (k/a h₂s+), indole-negative (differentiates from p. vulgaris).
What are the intrinsic resistances of Proteus mirabilis?
This organism is naturally resistant to colistin/polymyxin b (eucast expected resistant phenotypes v1.2), nitrofurantoin and tetracyclines (including tigecycline). These drugs should not be reported as susceptible.
Where is Proteus mirabilis commonly found?
It is typically associated with uti, especially in catheterized patients; struvite stones from urine alkalinization.
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