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Gram-negativeBacilliEnterobacterales

Proteus mirabilis

A Gram-negative, motile, urease-producing rod of the family Enterobacteriaceae, commonly causing urinary tract infections with characteristic alkaline urine and struvite staghorn calculi.

Organism Card

DomainMust know
Identity
  • G−ve rod, Enterobacterales family [1]
  • Facultative anaerobe; from normal bowel flora [2]
  • Highly motile — classic "swarming" growth on agar
Lab discriminator
  • Urease-positive → splits urea to ammonia → alkalinises urine [3][4]
  • Swarming motility on blood agar (spreading film, concentric rings)
  • Lactose non-fermenter on MacConkey (colourless colonies)
  • H₂S producer on TSI/XLD
  • vs E. coli: lactose +, non-swarming, urease usually −
  • vs Klebsiella: non-motile, urease +, mucoid colonies
Reservoir / transmission
  • Normal GI flora; colonises perineum
  • Ascending route from rectum → urethra → bladder ± kidney [2]
  • Facilitated by indwelling catheters, urinary tract obstruction, VUR [2]
  • Associated with complicated & nosocomial UTI [2]
Key virulence
  • Urease → hydrolyses urea to NH₃ + CO₂ → alkaline urine → struvite (MgNH₄PO₄) stone formation [3][4]
  • Flagella → swarming motility → ascending colonisation
  • IgA protease → evades mucosal immunity
  • Fimbriae (MR/P pili) → adherence to uroepithelium
  • Haemolysin → tissue damage
Clinical syndromes
  • UTI: ~2% uncomplicated, higher in complicated/nosocomial [2]
  • Classically causes staghorn calculi (struvite/triple phosphate stones) due to urease
  • Catheter-associated UTI (CAUTI) — important in elderly/hospitalised
  • Magnesium ammonium phosphate (struvite) crystals in alkaline urine = Proteus or Klebsiella clue [3]
  • Less common: wound infections, secondary bacterial peritonitis (polymicrobial) [5], pneumonia (HAP), bacteraemia
Diagnosis
  • MSU or catheter urine for C/ST (> 10⁵ CFU/mL MSU or > 10⁴ catheter) [2]
  • Dipstick: nitrite + (Proteus reduces nitrate), LE + [2]
  • Urine pH alkaline (> 7) + struvite crystals on R/M → strongly suggests urease-producer [3]
  • Blood cultures if systemic sepsis suspected
  • Imaging: CT KUB for staghorn/struvite calculi
Treatment
  • 1st-gen cephalosporins (e.g. cephalexin) active against P. mirabilis [6]
  • Also susceptible to fluoroquinolones, 2nd/3rd-gen cephalosporins, carbapenems [6]
  • Intrinsically resistant to nitrofurantoin — never use for Proteus UTI [7]
  • ESBL-producing strains → carbapenem is drug of choice [1]
  • Stone removal required for struvite stones (antibiotics alone insufficient)
Prevention
  • Minimise catheter duration (strongest modifiable risk for CAUTI)
  • Prophylactic Abx for recurrent UTI: ciprofloxacin / TMP-SMX / cephalexin; avoid amoxicillin & cephalosporin that change faecal flora [8]
  • Cranberry juice may ↓ bacterial adherence (modest evidence) [8]
  • No vaccine available
Classic traps
  • Proteus UTI + alkaline urine + struvite crystals = classic vignette [3]
  • Alkaline urine ≠ always contamination — think urease-producers
  • Nitrofurantoin is a trap answer for Proteus (intrinsic resistance) [7]
  • P. mirabilis vs P. vulgaris: both urease +, both nitrofurantoin-resistant, but P. vulgaris → indole +, more resistant
  • Do NOT confuse with Providencia/Morganella — also urease + but different resistance profiles [7]
  • Polymicrobial growth with Proteus in peritoneal fluid → think secondary (not spontaneous) bacterial peritonitis [5]

Exam Intelligence

  1. GC 105: Proteus is listed under Enterobacterales with disease associations including UTI and abdominal infections. Key teaching point: ESBL-producing Enterobacterales → carbapenems are the antibiotic of choice [1]. This is a favourite MCQ distractor — students who pick ceftriaxone for ESBL Proteus lose the mark.

  2. GC 210 (UTI lecture): Prophylaxis for recurrent UTI includes ciprofloxacin, septrin, trimethoprim, cephalexin, or nitrofurantoin 50 mg OD. Critical caveat: nitrofurantoin is avoided in pyelonephritis (poor renal tissue levels) and must be avoided entirely in Proteus (intrinsic resistance) [8]. This double-pitfall is highly examinable.

  3. Gen Clerk Microbiology Summary: Proteus mirabilis and P. vulgaris both have intrinsic resistance to nitrofurantoin [7]. Listed alongside Morganella morganii and Providencia spp. in the "classical intrinsic resistance" table — expect this to appear as an MCQ option where nitrofurantoin is the wrong answer for a Proteus UTI.

  4. Felix PY Lai notes (Medicine + Paediatrics): "Magnesium ammonium phosphate crystals occur in alkaline urine with increased NH₃ production by urease-producing organisms such as Proteus or Klebsiella, suggestive of UTI" [3]. This is a classic data-interpretation question — a urine R/M showing coffin-lid crystals in alkaline urine should prompt you to answer Proteus (or Klebsiella) UTI.

  5. Ryan Ho Urogenital: P. mirabilis accounts for ~2% of uncomplicated UTI but is proportionally more important in complicated/nosocomial UTI [2]. In exam vignettes, Proteus UTI will typically be set in a patient with a catheter, structural urinary anomaly, or recurrent stone disease — not a young healthy woman with simple cystitis (that's E. coli).

  6. Felix PY Lai antibiotic tables: 1st-gen cephalosporins cover E. coli, P. mirabilis, and K. pneumoniae; fluoroquinolones also cover Proteus mirabilis [6]. Know these spectra for the "best empirical antibiotic" MCQ format.

  • Urease-positive UTI organism: Proteus vs Klebsiella vs Ureaplasma. In a stone-forming UTI vignette, Proteus is the default answer unless specifically clued otherwise. Klebsiella is urease + but less classically linked to staghorn calculi.
  • Nitrofurantoin trap: Nitrofurantoin is an excellent drug for uncomplicated lower UTI by E. coli, but it is intrinsically resistant in Proteus, Morganella, and Providencia. If the stem names Proteus, nitrofurantoin is always wrong.
  • Swarming on agar: If a question describes "swarming colonies" or "spreading film of growth," think Proteus immediately — this is the most classic culture description.
  • Alkaline urine + crystals: Coffin-lid–shaped struvite crystals in alkaline urine = urease-producer. Calcium oxalate (envelope-shaped, acidic urine) and uric acid (rhomboid, acidic urine) are distractors.
  • Secondary bacterial peritonitis: Proteus can appear in polymicrobial peritoneal culture — polymicrobial = secondary peritonitis (bowel leak), NOT spontaneous bacterial peritonitis (monomicrobial) [5].

No past paper questions directly examining Proteus mirabilis as the named organism or primary answer were identified in the indexed 2016–2025 Fourth Summative MCQ, SAQ, or Minicase papers. The organism may appear as a distractor option in UTI or antibiotic-choice stems; however, no specific question stem with a Proteus-focused answer could be faithfully quoted from the provided past paper context.

[1] Lecture slides: GC 105. Medically important microbes what every doctor should know.pdf [2] Senior notes: Ryan Ho Urogenital.pdf; Adrian Lui Pediatrics Notes.pdf [3] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf; MBBS Final MB (Pediatrics) (Felix PY Lai).pdf [4] Senior notes: Adrian Lui Pediatrics Notes.pdf (urease virulence factor) [5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf; MBBS Final MB (Surgery) (Felix PY Lai).pdf (secondary bacterial peritonitis) [6] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (antibiotic spectrum tables) [7] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (intrinsic resistance table) [8] Lecture slides: GC 210. Urinary tract infection.pdf

Klebsiella pneumoniae

Encapsulated, non-motile Gram-negative rod of the Enterobacteriaceae family that is a major cause of nosocomial pneumonia, urinary tract infections, and bacteremia, particularly in immunocompromised and hospitalized patients.

Salmonella typhi

Facultative intracellular, flagellated Gram-negative bacillus of the family Enterobacteriaceae that causes typhoid (enteric) fever, characterized by sustained bacteremia, fever, and potential intestinal perforation.

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