Pseudomonas aeruginosa and other members of this group of gram-negative bacilli are opportunistic pathogens that frequently cause hospital-acquired infections, particularly in ventilator patients, burn patients, and patients with neutropenia or chronic debility. Many sites can be infected, and infection is usually severe. Diagnosis is by culture. Antibiotic choice varies with the pathogen and must be guided by susceptibility testing because resistance is common.
Epidemiology
Pseudomonas is ubiquitous and favors moist environments. In humans, P. aeruginosa is the most common pathogen, but infection may result from P. paucimobilis, P. putida, P. fluorescens, or P. acidovorans. Other important hospital-acquired pathogens formerly classified as Pseudomonas include Burkholderia cepacia and Stenotrophomonas maltophilia. B. pseudomallei causes a distinct disease known as melioidosis that is limited mostly to the southeast Asia and northern Australia.
P. aeruginosa is present occasionally in the axilla and anogenital areas of normal skin but rarely in stool unless antibiotics are being given. In hospitals, the organism is frequently present in sinks, antiseptic solutions, and urine receptacles. Transmission to patients by health care practitioners may occur, especially in burn and neonatal intensive care units, unless infection control practices are meticulously followed.
Diseases Caused by Pseudomonas
Most P. aeruginosa infections occur in hospitalized patients, particularly those who have neutropenia or who are debilitated or immunocompromised. P. aeruginosa is a common cause of infections in intensive care units. HIV-infected patients, particularly those in advanced stages, and patients with cystic fibrosis are at risk of community-acquired P. aeruginosa infections.
Pseudomonas infections can develop in many anatomic sites, including skin, subcutaneous tissue, bone, ears, eyes, urinary tract, lungs, and heart valves. The site varies with the portal of entry and the patient’s vulnerability. In hospitalized patients, the first sign may be overwhelming gram-negative sepsis.
Skin and soft-tissue infections
In burns, the region below the eschar can become heavily infiltrated with organisms, serving as a focus for subsequent bacteremia—an often lethal complication.
Deep puncture wounds of the foot are often infected by P. aeruginosa. Draining sinuses, cellulitis, and osteomyelitis may result. Drainage from puncture wounds often has a sweet, fruity smell.
Folliculitis acquired in hot tubs is often caused by P. aeruginosa. It causes an itchy pustular rash around hair follicles.
Acute external otitis (swimmer's ear), which is common in tropical climates, is the most common form of Pseudomonas infection involving the ear. A more severe form, referred to as malignant external otitis, can develop in diabetic patients. It is manifested by severe ear pain, often with unilateral cranial nerve palsies, and requires parenteral therapy.
Ecthyma gangrenosum is a skin lesion that occurs in neutropenic patients and is usually caused by P. aeruginosa. It is characterized by erythematous, centrally ulcerated, purple-black areas about 1 cm in diameter occurring most often in moist areas such as the axillary, inguinal, or anogenital areas. Ecthyma gangrenosum typically occurs in patients with P. aeruginosa bacteremia.
Respiratory tract infections
P. aeruginosa is a frequent cause of ventilator-associated pneumonia. In HIV-infected patients, Pseudomonas most commonly causes pneumonia or sinusitis.
Pseudomonas bronchitis is common late in the course of cystic fibrosis. Isolates from patients with cystic fibrosis have a characteristic mucoid colonial morphology and result in a worse prognosis than nonmucoid Pseudomonas.
Other infections
Pseudomonas is a common cause of nosocomial urinary tract infection, especially in patients who have had urologic manipulation or obstructive uropathy. Pseudomonas commonly colonizes the urinary tract in catheterized patients, especially those who have received broad-spectrum antibiotics.
Ocular involvement generally manifests as corneal ulceration, most often after trauma, but contamination of contact lenses or lens fluid has been implicated in some cases.
Rarely, Pseudomonas causes acute bacterial endocarditis, usually on prosthetic valves in patients who have had open-heart surgery or on natural valves in IV drug abusers.
Bacteremia
Many Pseudomonas infections can cause bacteremia. In nonintubated patients without a detectable urinary focus, especially if infection is due to a species other than P. aeruginosa, bacteremia suggests contaminated IV fluids or drugs or antiseptics used in placing the IV catheter.
Diagnosis
Culture
Diagnosis of Pseudomonas infections depends on culturing the organism from the site of infection: blood, skin lesions, drainage fluid, urine, cerebrospinal fluid, or eye. Susceptibility testing is also done.
Localized infection may produce a fruity smell of newly mown grass, and pus may be greenish.
Treatment
Various antibiotics depending on site and severity of infection and susceptibility testing
Localized infection
Hot-tub folliculitis resolves spontaneously and does not require antibiotic therapy.
External otitis
Focal soft-tissue infection may require early surgical debridement of necrotic tissue and drainage of abscesses in addition to antibiotics.
Small corneal ulcers
Asymptomatic bacteriuria
Systemic infection
Right-sided endocarditis can be treated with antibiotics, but usually the infected valve must be removed to cure an infection involving the mitral, aortic, or prosthetic valve.
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Key Points
Most P. aeruginosa infections occur in hospitalized patients, particularly those who are debilitated or immunocompromised, but patients with cystic fibrosis or advanced HIV may acquire the infection in the community.
Infection can develop in many sites, varying with the portal of entry (eg, skin in burn patients, lungs in patients on a ventilator, urinary tract in patients who have had urologic manipulation or obstructive uropathy); overwhelming gram-negative sepsis may occur.
Surface infections (eg, folliculitis, external otitis, corneal ulcers) may develop in healthy people.
Diagnose using cultures.
Treat systemic infection with parenteral therapy using a single drug (eg, an antipseudomonal beta-lactam, a fluoroquinolone).
