Microbiology

P. aeruginosa is a motile gram-negative rod that commonly produces green or bluish pigment and may have a mucoid appearance (which is particularly common in isolates from pts with cystic fibrosis). P. aeruginosa differs from enteric GNB in that it has a positive reaction in the oxidase test and does not ferment lactose.

Epidemiology

Because P. aeruginosa is found in most moist environments (e.g., in soil, in tap water, and on countertops), people routinely come into contact with the organism. The many factors that predispose to P. aeruginosa infection include disruption of cutaneous or mucosal barriers (e.g., due to burns or trauma), immunosuppression (e.g., due to neutropenia, AIDS, or diabetes), and disruption of the normal bacterial flora (e.g., due to broad-spectrum antibiotic therapy).

• P. aeruginosa is no longer a major cause of life-threatening bacteremia among pts with neutropenia or burn injury.
• P. aeruginosa bacteremia is currently most common among pts in the ICU.

Clinical Manifestations

P. aeruginosa can infect virtually all sites in the body but has a strong predilection for the lungs.

• Pneumonia: P. aeruginosa is considered a major cause of ventilator-associated pneumonia, although colonization may be difficult to distinguish from true infection.
  • Clinically, most pts have a slowly progressive infiltrate, although progression is rapid in some cases. Infiltrates may become necrotic.
  • It is unclear whether an invasive procedure (e.g., bronchoalveolar lavage, protected-brush sampling of distal airways) is superior to tracheal aspiration in obtaining samples for culture.
  • Chronic respiratory infection with P. aeruginosa is associated with underlying or predisposing conditions (e.g., cystic fibrosis, bronchiectasis).
• Bacteremia: The presentation of P. aeruginosa bacteremia resembles that of sepsis in general but may be more severe, with attributable mortality rates of 28-44%.
  • Pathognomonic skin lesions (ecthyma gangrenosum) that at first are painful, reddish, and maculopapular and later become black and necrotic may develop in pts with marked neutropenia or HIV infection.
  • Endovascular infections occur mostly in IV drug users and pts with prosthetic valves.
• Bone and joint infections: P. aeruginosa is an infrequent cause of bone and joint infections.
  • Injection drug use (associated with sternoclavicular joint infections and vertebral osteomyelitis) and UTIs in the elderly (associated with vertebral osteomyelitis) are risk factors.
  • - Pseudomonas osteomyelitis of the foot most often follows puncture wounds through sneakers and most commonly affects children.
• CNS infections: CNS infections due to P. aeruginosa are relatively rare and are almost always secondary to a surgical procedure or head trauma.
• Eye infections: Keratitis and corneal ulcers can occur, usually resulting from trauma or surface injury by contact lenses. These infections are rapidly progressing entities that demand immediate therapeutic intervention. P. aeruginosa endophthalmitis secondary to bacteremia is a fulminant disease with severe pain, chemosis, decreased visual acuity, anterior uveitis, vitreous involvement, and panophthalmitis.
• Ear infections: In addition to mild swimmer's ear, Pseudomonas ear infections can result in malignant otitis externa, a life-threatening infection that presents as severe ear pain and decreased hearing.
  • Pts may develop cranial-nerve palsies or cavernous venous sinus thrombosis.
  • Most ear infections due to P. aeruginosa occur in elderly diabetic pts.
• UTIs: UTIs due to P. aeruginosa usually result from a foreign body in the urinary tract, an obstruction in the genitourinary system, or urinary tract instrumentation or surgery.
• Skin and soft tissue infections: P. aeruginosa can cause a variety of dermatitides, including pyoderma gangrenosum in neutropenic pts, folliculitis, and other papular or vesicular lesions. Multiple outbreaks have been linked to whirlpools, spas, and swimming pools.
• Infections in pts with fever and neutropenia: P. aeruginosa is always targeted in empirical treatment of these pts, given high rates of infection in the past and high associated mortality rates.
• Infections in pts with AIDS: P. aeruginosa infections in pts with AIDS can be fatal even though the clinical presentation is not particularly severe.
  • Pneumonia is the most common type of infection, with a high frequency of cavitary disease.
  • Since the advent of antiretroviral therapy, P. aeruginosa infection has declined in incidence among these pts but still occurs.

Stenotrophomonas Maltophilia

S. maltophilia is an opportunistic pathogen. Most infections occur in the setting of prior broad-spectrum antimicrobial therapy that has eradicated the normal flora in immunocompromised pts.

• S. maltophilia causes pneumonia, especially ventilator-associated pneumonia, with or without bacteremia.
• Central venous line infection (most often in cancer pts) and ecthyma gangrenosum in neutropenic pts have been described.

Burkholderia Cepacia

This organism can colonize airways during broad-spectrum antimicrobial treatment and is a cause of ventilator-associated pneumonia, catheter-associated infection, and wound infection.

• B. cepacia is recognized as an antibiotic-resistant nosocomial pathogen in ICU pts.
• B. cepacia can cause a rapidly fatal syndrome of respiratory distress and septicemia (the “cepacia syndrome”) in pts with cystic fibrosis.