Stenotrophomonas maltophilia
Gram-negative bacilli
Identification & Growth
Identification
- Oxidase-negative, DNase-positive, lysine-positive, motile
- Grows well on blood agar with a greyish-green pigment and ammoniacal odour
Growth Conditions
- MacConkey agar, lactose-negative; grows best at 30–35 °C, 24–48 h
Clinical Significance
- Pneumonia in patients on carbapenem therapy, catheter infection, cystic fibrosis
Intrinsic Resistance
- ALL carbapenems (chromosomal L1 metallo-beta-lactamase, EUCAST Expected Resistant Phenotypes v1.2)
- Aminoglycosides, penicillins and most cephalosporins
Bench Alerts
- Never report meropenem/imipenem as susceptible, even with a large zone
- SXT is the reference option; alternatives are levofloxacin or minocycline
- Typically emerges after prolonged carbapenem use
Clinical Notes
Clinical presentations
- Causes ventilator-associated pneumonia, catheter-related bacteremia, and infection in immunocompromised or cystic fibrosis patients.
- Is a low-virulence opportunistic pathogen that rarely causes disease in immunocompetent hosts without invasive devices.
- Can cause ocular infection, including keratitis and endophthalmitis, associated with contact lenses or ocular surgery.
Specimens and collection
- Catheter and peripheral blood cultures help establish a relationship with a colonized intravascular device.
- Respiratory specimens in cystic fibrosis or ventilated patients should be processed on selective media.
- Ocular secretion culture is indicated when device-associated keratitis or endophthalmitis is suspected.
Epidemiology and at-risk populations
- Associated with prior carbapenem exposure, which selects for this organism, which is intrinsically resistant to this class.
- Patients with hematologic malignancies, neutropenia, and long-term central venous catheters are at higher risk.
- Is a common environmental contaminant of water and moist surfaces in the hospital setting.
Resistance and therapeutic implications
- Exhibits intrinsic carbapenem resistance due to constitutive production of L1 metallo-beta-lactamase and L2 serine-beta-lactamase.
- Trimethoprim-sulfamethoxazole is historically considered the drug of choice, with emerging resistance reported in some regions.
- Levofloxacin and minocycline are therapeutic alternatives in resistant strains or sulfa intolerance.
Bench and reporting notes
- Non-fermenting gram-negative bacillus, usually oxidase-negative, which can be mistaken for Pseudomonas on initial screening.
- Report should alert the clinician to intrinsic carbapenem resistance to avoid inappropriate therapeutic escalation.
- Isolation from a respiratory specimen in a patient without clinical infection may represent colonization only.
Sources
- EUCAST — Clinical breakpoints
- CDC — Stenotrophomonas maltophilia
- PubMed — Stenotrophomonas maltophilia clinical review
Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.
FAQ: Frequently Asked Questions
How to identify Stenotrophomonas maltophilia in the lab?
Stenotrophomonas maltophilia is identified through oxidase-negative, dnase-positive, lysine-positive, motile, grows well on blood agar with a greyish-green pigment and ammoniacal odour.
What are the intrinsic resistances of Stenotrophomonas maltophilia?
This organism is naturally resistant to all carbapenems (chromosomal l1 metallo-beta-lactamase, eucast expected resistant phenotypes v1.2), aminoglycosides, penicillins and most cephalosporins. These drugs should not be reported as susceptible.
Where is Stenotrophomonas maltophilia commonly found?
It is typically associated with pneumonia in patients on carbapenem therapy, catheter infection, cystic fibrosis.
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