Proteus

Overview and Classification of Proteus

Medical Importance: Proteus species, particularly Proteus mirabilis and Proteus vulgaris, are significant opportunistic Gram-negative pathogens. They are primary causes of severe and complicated human urinary tract infections (UTIs) and are characterized by their ability to induce the formation of mineral stones.
Classification: Proteus belongs to the family of Gram-negative enteric bacilli.
Taxonomic Distinctions:
- Proteus: Possess $>80$ O-antigen strains and $19$ H-antigen strains. They lack K (capsular) antigens but possess fimbrial antigens (adhesins).
- Comparison with other Enterics:
  - Escherichia: $>180$ O strains, $53$ H strains, and $>80$ K strains.
  - Salmonella: $>60$ O strains, diphasic H antigens (phases $1$ and $2$ ), and the Vi (capsular) antigen in S. typhi.
  - Shigella: $4$ major serogroups; lacks H and K antigens; possesses Invasion Plasid Antigens (Ipa).
  - Klebsiella: $9$ to $12$ O antigens; lacks H antigens; $77$ to $79$ K antigens (capsular polysaccharide).
  - Citrobacter: $43$ O antigens, $>20$ H antigens, and Vi antigen.

Nature and Morphological Characteristics

Microscopic Morphology:
- Shape: Short, straight, rod-like bacilli.
- Gram Stain: Gram-negative (stains pink to red due to the safranin counterstain on a thin cell wall).
- Arrangement: Arranged as single cells or in random small structures.
Cell Envelope Structure:
- Outer Membrane: Comprised of Lipopolysaccharide (LPS). The Lipid A portion serves as an endotoxin that can trigger severe inflammation and sepsis. The variable O-antigen chain is used for serotyping.
- Periplasmic Space: Contains a thin layer of peptidoglycan that maintains the rod geometry during the "swimmer" state.
- Inner Membrane: A symmetric phospholipid bilayer that manages metabolism and powers flagellar motors.
Appendages and Organelles:
- Peritrichous Flagella: Composed of flagellin protein. In the "swarmer" state, thousands of these flagella allow bacteria to move in unified rafts across surfaces.
- Fimbriae (Pili): Includes distinct types like MR/P (Mannose-Resistant Proteus-like) fimbriae and PMF fimbriae. These are engineered to adhere to the urinary tract lining and plastic surfaces of indwelling catheters.
- Nucleoid and Plasmids: Features a single circular chromosome. Extrachromosomal plasmids often carry genes for multidrug resistance and specialized beta-lactamase enzymes.
- Urease Enzyme: The cytoplasm is densely packed with active urease complexes, which define the bacterium's physiological impact by altering the surrounding chemical environment.

Cultural and Biochemical Properties

Cultural Properties:
- MacConkey Agar: Produces pale, colorless, or transparent colonies (non-lactose fermenter).
- CLED Agar: Produces isolated, blue-green colonies.
- XLD Agar: Produces yellow or translucent colonies characterized by a black center.
- SS Agar: Produces colorless colonies with a black center.
- Nutrient/Blood Agar: Exhibits swarming motility, creating a wave-like film rather than isolated colonies.
Physiologic and Biochemical Identification:
- Oxidase Test: Negative.
- Catalase Test: Positive.
- Urease Test: Positive (Rapid and intensive). Causes a pH spike that shifts indicators from pale yellow to vibrant fuchsia pink within hours.
- Triple Sugar Iron (TSI) Slant: Alkaline/Acid ( $K/A$ ) with heavy $H_2S$ (hydrogen sulfide) production. The slant turns red, the butt turns yellow, and the base turns jet black.
- Phenylalanine Deaminase (PAD) Test: Positive (indicated by a deep hunter green color on the slant).
- Gelatin Liquefaction: Positive.
- Methyl Red (MR): Positive.
- Voges-Proskauer (VP): Negative.

Epidemiology and Distribution

Distribution in the Philippines: Proteus is a top cause of complicated UTIs and surgical site infections (SSIs) in medical and surgical wards.
Morbidity: It accounts for approximately $46\%$ of all nosocomial urinary tract infections when grouped with other coliforms.
Mortality: Urosepsis mortality rates range from $15\%$ to $30\%$ , with higher risks associated with Extended-Spectrum Beta-Lactamase (ESBL) producing strains.
High-Risk Populations:
- Patients with long-term indwelling urinary catheters.
- Individuals with structural urinary tract abnormalities.
- Post-operative patients (specifically abdominal or bowel surgeries).

Microbiological Pathogenesis

Swarming Motility and Adherence:
- Contact with solid surfaces (catheters or bladder walls) triggers a transformation into elongated, hyper-flagellated "swarm cells."
- Swarming allows the bacteria to travel up the urethra against the flow of urine.
- Upon reaching a destination, the bacteria revert to short cells and utilize MR/P fimbriae to lock onto surface cells of the kidneys or bladder.
Urease Production:
- Massive quantities of urease hydrolyze urea into ammonia ( $NH_3$ ) and carbamate.
- Ammonia binds to hydrogen ions to form ammonium ( $NH_4^+$ ) and free hydroxyl ions ( $OH^-$ ).
- This reaction causes a urine pH spike from acidic ( $pH \approx 6.0$ ) to highly alkaline ( $pH > 7.2$ , often reaching $8$ or $9$ ).
- Ammonia levels are toxic, chemically stripping the protective mucus layer of the bladder wall.
Crystal Precipitation and Stone Formation:
- Alkaline conditions precipitate Struvite (Magnesium ammonium phosphate: $MgNH_4PO_4 \cdot 6H_2O$ ) and Carbonate Apatite (Calcium phosphate: $Ca_{10}(PO_4)_6CO_3$ ).
- These minerals combine with bacterial biofilms to form jagged, rapidly growing urinary stones (Staghorn calculi).
The Biofilm Shield:
- Bacteria trap mineral crystals in a sticky matrix of sugars and proteins.
- This crystalline biofilm on catheters seals bacteria away from antibiotics and the immune system.

Host Immunologic Response

Cytokine Alarm: Toll-Like Receptor $4$ (TLR4) detects the O (somatic) antigen. This triggers bladder cells and macrophages to release cytokines ( $IL-6$ , $IL-8$ , and $TNF-\alpha$ ). $IL-8$ recruits neutrophils, leading to pyuria (pus/dead neutrophils in urine).
Immune Evasion: Neutrophils cannot engulf large struvite stones. The stones physically irritate the tissue, causing chronic inflammation, tissue damage, and hematuria (blood in urine).
Systemic Adaptive Immunity: In cases of bacteremia or pyelonephritis, B-cells produce antibodies to neutralize H (flagellar) antigens (halting movement) and the urease enzyme (blocking the active site).

Clinical Manifestations and Complications

Clinical Manifestations:
- Complicated UTIs: Cystitis, dysuria, urgency, frequency, suprapubic pain, and a strong ammonia odor in urine.
- Urinary Stones: Renal colic and persistent infection associated with staghorn calculi.
Complications and Emergencies:
- Pyelonephritis: Characterized by high fever, chills, Costovertebral Angle (CVA) tenderness, nausea, and vomiting.
- Catheter-Associated UTIs (CAUTI): Formation of thick crystalline crusts inside drainage tubes, leading to catheter blockage and potentially autonomic dysreflexia.
- Extraintestinal Infections: Includes wound infections (diabetic foot ulcers, pressure sores, burn wounds), bacteremia, and septic shock.

Diagnostic Procedures

Specimen Collection: Mid-stream clean-catch urine, catheterized urine, wound swabs (from deep ulcers), or blood culture bottles.
Urinalysis: Distinctive finding of alkaline pH ( $7.5$ to $8.5$ or higher), positive for nitrites and leukocyte esterase.
Phenomenon Identification: Culture on Blood Agar or Nutrient Agar shows unmistakable swarming motility (concentric wave-like target rings) accompanied by a strong ammonia odor.
Serology (Weil-Felix Test):
- O antigens of specific non-motile Proteus strains (OX-2, OX-19, and OX-K) share structural components with Rickettsia.
- Testing involves mixing patient serum with dead Proteus OX suspensions.
- Positive clumping indicates Rickettsial infections (e.g., Typhus or Rocky Mountain Spotted Fever).
Advanced Assays: VITEK 2, MicroScan, and MALDI-TOF Mass Spectrometry.

Management and Prevention

Targeted Antimicrobial Therapy:
- First-line: Ampicillin, Cephalexin, or Ciprofloxacin.
- ESBL Strains: Carbapenems or Piperacillin-Tazobactam.
Surgical Intervention: Necessary for stone eradication via Percutaneous Nephrolithotomy (PCNL) or Extracorporeal Shock Wave Lithotripsy (ESWL).
Urease Inhibitors: Acetohydroxamic Acid (AHA) is used for chronic, un-resectable stones.
Preventive Interventions:
- Catheter Stewardship: Maintaining continuous closed drainage.
- Dietary Measures: Hydration and acidification of urine using ascorbic acid or proanthocyanidin.
- Hygiene: Rigorous hospital hand hygiene and contact precautions.

Reference

Source: Jawetz, Melnick & Adelberg's Medical Microbiology, $28^{th}$ Edition.
Authors: Stefan Riedel, Stephen A. Morse, Timothy Mietzner, Steve Miller.
Section: Chapter $15$ , pp $231$ - $243$ .