INTRODUCTION	Section 2 of 10
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Background: Stenotrophomonas (Pseudomonas) maltophilia is an aerobic gram-negative bacillus that is an infrequent pathogen in humans and is found in a variety of aquatic environments. S maltophilia is an organism of low virulence and is a frequent colonizer of fluids used in the hospital setting, ie, irrigation solutions and intravenous (IV) fluids, and of patient secretions, ie, respiratory secretions, urine, or wound exudates. S maltophilia usually must bypass normal host defenses to cause human infection. For example, if fluid in an irrigation solution becomes colonized with this organism, irrigating an open wound can cause colonization or infection of the wound. S maltophilia usually is not capable of causing disease in healthy hosts without the assistance of invasive medical devices that bypass normal host defenses.

Pathophysiology: S maltophilia has few pathogenic mechanisms and, for this reason, predominantly results in colonization rather than infection. If infection does occur, invasive medical devices usually are the vehicles by which the organism bypasses normal host defenses. Otherwise, the pathophysiology of this nonfermentative aerobic gram-negative bacillus is not different from other nonfermentative aerobic organisms.

Frequency:

• Internationally: In the ambulatory setting, S maltophilia is a frequent colonizer in the respiratory tract in patients with cystic fibrosis.

Mortality/Morbidity: Mortality and morbidity relate to the inoculum of S maltophilia that is able to bypass normal host defense mechanisms.

• If an intravenous infusion contains large numbers of S maltophilia, then direct injection into the bloodstream may result in the signs and symptoms associated with gram-negative bacteremia.

• Similarly, in the urinary tract, if urological irrigation fluids that contain large numbers of S maltophilia are used during an invasive urological procedure, eg, cystoscopy, then gram-negative bacteremia may occur with its attendant mortality and morbidity, which are dependent on host factors.

	CLINICAL	Section 3 of 10
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History:

• Because S maltophilia infections are extremely uncommon, no specific patient history suggests its presence other than contact with other colonized individuals.

• Obtaining a history of the use of irrigant solutions that could potentially contain S maltophilia is important in an epidemiological setting rather than in a clinical setting.

Physical: Signs and symptoms are related to the organ system involved.

Causes:

• S maltophilia is a nonfermentative aerobic gram-negative bacillus formerly classified as Pseudomonas. Unlike Pseudomonas aeruginosa, S maltophilia is an organism of low virulence with limited ability to cause infection in humans.

• S maltophilia is a water organism, and it survives and multiplies in aqueous environments, particularly respiratory secretions, urine, IV fluids, and irrigant solutions.

• S maltophilia may persist in an aquatic environment for extended periods.

• Sources of S maltophilia colonization include the following:

• Personnel
  • Hands

  • Antiseptic soaps

  • Hand lotion

• Respiratory equipment and/or fluids
  • Ultrasonic nebulizers

  • Inhalation medications

  • Respirator tubing condensate

• IV lines and/or fluids
  • IV solutions

  • Central venous catheters

• Pressure monitoring devices - Pressure transducer fluids

• Urine and/or fluids
  • Indwelling Foley catheters

  • Urometers

  • Irrigation solutions

	DIFFERENTIALS	Section 4 of 10
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Other Problems to be Considered:

The main clinical problem presented by S maltophilia is not recognizing its significance in the clinical context. The recovery of S maltophilia from respiratory secretions or from the urine of catheterized patients should be regarded as colonization until proven otherwise.

S maltophilia is a common cause of catheter-associated bacteruria in hospitalized patients. S maltophilia commonly colonizes the urine and potentially is pathogenic only in those with impaired host defenses, ie, patients with diabetes, systemic lupus erythematosus (SLE), cirrhosis, multiple myeloma, and those on steroids.

S maltophilia recovered from blood cultures may have come from contaminated IV fluids or from a distant infected source, eg, secondary bacteremia from the urinary tract in a patient who recently underwent instrumentation during a genitourinary (GU) procedure.

S maltophilia recovered from a wound with a clear or a serosanguineous discharge is of no clinical significance. If recovered from a purulent wound, S maltophilia may be the cause of the patient's wound infection.

The most common cause of confusion regarding S maltophilia is assuming that its recovery from body fluids implies a pathogenic role.

S maltophilia virtually never causes nosocomial pneumonia in patients who are ventilated and presumed to have nosocomial pneumonia because of fever, pulmonary infiltrates, and leukocytosis. The recovery of S maltophilia from respiratory secretions invariably represents colonization rather than infection, and its presence should not be addressed therapeutically.

Table 1. Hospital-Acquired S maltophilia Infections

Infection	Predisposing Factor
Catheter-associated bacteriuria	Indwelling urinary catheters
IV line infections	Central IV catheters
Urosepsis	Urinary tract instrumentation
Primary bacteremia	Arterial monitoring devices
Pseudobacteremia	Contamination of blood during collection/processing of blood cultures