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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Gram negative bacteria classification flowchart

Gram-negative bacteria:
Most bacteria can be classified according to a lab procedure called Gram staining.
Bacteria with cell walls that have a thin layer of peptidoglycan do not retain the crystal violet stain utilized in Gram staining. These bacteria do, however, retain the safranin counterstain and thus appear as pinkish-red on the stain, making them gram negative. These bacteria can be further classified according to morphology (diplococci, curved rods, bacilli, and coccobacilli) and their ability to grow in the presence of oxygen (aerobic versus anaerobic). The bacteria can be more narrowly identified by growing them on specific media (triple sugar iron (TSI) agar) where their enzymes can be identified (urease, oxidase) and their ability to ferment lactose can be tested.
* Stains poorly on Gram stain
** Pleomorphic rod/coccobacillus
*** Require special transport media

Image by Lecturio.

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)
Pseudomonas aeruginosa bacteria

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

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


Reservoir and transmission


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


  • 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
  • Adherence to respiratory epithelium
Phospholipase C
  • Degrades cell membranes
Exotoxin A
  • Ribosylates and inactivates EF-2, which causes cell death
  • 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 Pseudomonas aeruginosa

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
    • Often in patients with IV drug use or diabetes 
    • Associated with puncture wounds
    Sites of Pseudomonas infection

    Diagram of common sites of Pseudomonas infection

    Image by Lecturio.



    • 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)


    • 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 


    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)


    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.

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