Pseudomonas

Pseudomonas is a non-lactose-fermenting, gram-negative bacillus that produces pyocyanin, which gives it a characteristic blue-green color. Pseudomonas is found ubiquitously in the environment, as well as in moist reservoirs, such as hospital sinks and respiratory equipment. Pseudomonas has a sweet, grape-like odor. The most clinically relevant species is Pseudomonas aeruginosa (P. aeruginosa), which has a wide array of clinical manifestations from benign diseases, such as swimmer’s ear and “hot tub” folliculitis, to disseminated bacteremia and osteomyelitis. Risk factors for infections include: neutropenia, cystic fibrosis, asplenia, burn injuries, and indwelling catheters/endotracheal intubation. Management is primarily with piperacillin/tazobactam.

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General Characteristics

  • Gram-negative bacilli
  • Encapsulated
  • Motile: 1–3 motile flagella
  • Obligate aerobe
  • Oxidase positive
  • Catalase positive
  • Non-lactose fermenting
  • Produces a blue-green pigment: due to pyocyanin and fluorescein production
  • Produces a characteristic fruity, grape-like odor
  • Clinically relevant species: Pseudomonas aeruginosa (P. aeruginosa)

Photograph depicts the colonial growth pattern displayed by Pseudomonas aeruginosa bacteria.

Image: “6688” by the CDC/Dr. Theo Hawkins. License: Public domain.

Pathogenesis

Reservoir and transmission

Reservoir: 

  • Ubiquitous in the environment
  • Moist reservoirs: 
    • Sinks
    • Respiratory and dialysis equipment

Transmission:

  • Person-to-person transmission from infected reservoirs

Risk factors

  • Neutropenia
  • Burn injuries
  • Asplenia
  • Cystic fibrosis
  • Endotracheal intubation
  • Chronic, indwelling catheters

Pathogenesis of P. aeruginosa 

Table: Virulence factors
Virulence factorEffect
Polysaccharide capsule
  • Antiphagocytic
  • Adherence to tracheal epithelium
  • Aids in prolonged colonization/biofilm production
Pili
  • Adherence to respiratory epithelium
Phospholipase C
  • Degrades cell membranes
Exotoxin A
  • Ribosylates and inactivates EF-2, which causes cell death
Pyocyanin
  • Mediates tissue damage through production of reactive oxygen species
Type III secretion system
  • Facilitates direct delivery of toxins to host cell
In vivo biofilm formation
  • Allows organism to persist in airways of patients with cystic fibrosis
ꞵ-lactamase and efflux pumps
  • Contributes to multi-drug resistance

Mechanisms of pathogenesis of Pseudomonas aeruginosa

Image by Lecturio.

Diseases Caused by P. aeruginosa 

Table: Diseases caused by P. aeruginosa
Type of conditionCharacteristics
Urinary tract infection
  • Especially in patients with indwelling catheters
  • Common cause of nosocomial UTIs
Burn wound infections Associated with burn injuries, causing:
  • Vascular damage
  • Tissue necrosis
  • Bacteremia
  • Ear infections Otitis externa:
    • Benign, but painful
    • Often associated with swimmers (“swimmer’s ear”)
    Malignant external otitis:
    • Pain, swelling, and purulent discharge from the external auditory canal
    • May lead to:
      • Cranial nerve damage
      • Bacteremia
      • Sepsis
    Skin infections Ecthyma gangrenosum:
    • Rapidly progressive, focal, black, necrotic skin lesions
    • Associated with neutropenia
    “Hot tub” folliculitis:
    • Infection in regions of apocrine sweat glands (external ear, areola, nipple)
    • Named as such due to the source of infection, which is often from pools/hot tubs
    Pulmonary infections
    • Causes nosocomial pneumonia and ventilator-associated pneumonia
    • Most common cause of chronic pulmonary infection in patients with cystic fibrosis
    Eye infections Occurs in contact lens wearers or minor eye trauma:
    • Corneal ulcers
    • Keratitis
    Disseminated infections Occurs in immunocompromised hosts:
    • Transplant patients
    • Neutropenic patients
    Osteomyelitis
    • Often in patients with IV drug use or diabetes 
    • Associated with puncture wounds

    Diagram of common sites of Pseudomonas infection

    Image by Lecturio.

    Antimicrobials

    Treatment

    • P. aeruginosa has varying resistance to antibiotics:
      • Multi-drug resistant: resistant to at least 1 agent in > 3 antibiotic categories
      • Extensively drug resistant: resistant to at least 1 agent in all but 2 antibiotic categories
      • Pan-drug resistant: resistant to all antibiotic categories
    • Antibiotic choice is tailored to local resistance patterns (combination antibiotic therapy): 
      • 1st line: 
        • Combination penicillin/beta-lactamase inhibitors (piperacillin/tazobactam)
        • Cephalosporins (ceftazidime)
        • Monobactam (aztreonam)
        • Fluoroquinolones (ciprofloxacin) (only oral)
        • Carbapenems (meropenem)
      • Multi-drug-resistant strains are treated with newly developed antibiotics:
        • Beta-lactam-beta-lactamase inhibitor combinations (ceftolozane/tazobactam) 
        • Cephalosporins (cefiderocol)
        • Carbapenem-beta-lactamase combination (imipenem-cilastatin-relebactam)
        • Polymyxins
      • Adjunct antibiotics: aminoglycosides (tobramycin, gentamicin, amikacin)

    Prevention

    • Hygiene within the hospital:
      • Use of appropriate sterile technique
      • Careful cleaning of communal sinks and showers
    • Periodic monitoring, replacement, and cleaning of:
      • Respiratory support equipment
      • Intravenous (IV) lines (especially long-term lines)
      • Catheters 

    Mnemonic

    To help remember the many clinically relevant facts about Pseudomonas, use the mnemonic “PSEUDOMONAS”:

    • Pneumonia, pyocyanin
    • Sepsis
    • Ecthyma gangrenosum
    • UTIs
    • Diabetes, drug use
    • Osteomyelitis 
    • Mucoid, polysaccharide capsule
    • Otitis externa (“swimmer’s ear”)
    • Nosocomial infections (catheters, endotracheal tubes)
    • Exotoxin A
    • Skin infections (“hot tub” folliculitis and burns)

    References

    1. Riedel, S., & Hobden, J.A. (2019). In Riedel S, Morse SA, et al (Eds.), Jawetz, Melnick, & Adelberg’s Medical Microbiology (28th ed.)
    2. Hohmann, E.L., & Portnoy, D.A. (2018). In Jameson JL, et al (Eds.), Harrison’s Principles of Internal Medicine (20th ed. Vol 2, pp. 2676–2683).
    3. Baron, S. (1996). Medical microbiology. University of Texas Medical Branch at Galveston. https://www.ncbi.nlm.nih.gov/books/NBK7627/

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