Stenotrophomonas Maltophilia 

Updated: Sep 16, 2021
Author: Sara Ahmed, MBBS, FCPS, MRCP(UK); Chief Editor: Michael Stuart Bronze, MD 



Stenotrophomonas maltophilia previously known as (Xanthomonas) maltophilia is an aerobic gram-negative bacillus that is found in various aquatic environments. Although an uncommon pathogen in humans, S maltophilia infection in humans, especially nosocomial, has been increasingly recognized.

S maltophilia is an organism of low virulence and frequently colonizes fluids used in the hospital setting (eg, hospital tap water,[1]  irrigation solutions, intravenous fluids) and patient secretions (eg, respiratory secretions, urine, wound exudates). S maltophilia usually must bypass normal host defenses to cause human infection. For example, if an irrigation solution becomes colonized with this organism, irrigating an open wound can cause colonization or infection of the wound. S maltophilia is usually incapable of causing disease in healthy hosts without the assistance of invasive medical devices that bypass normal host defenses.[2]

Risk factors associated with S maltophilia infection have been defined and may include underlying malignancy, immunosuppressant therapy, cystic  fibrosis, COPD, HIV, neutropenia, mechanical ventilation, prior colonization with stenotrophomonas, central venous catheter, genitourinary catheter, continuous ambulatory peritoneal dialysis (CAPD), recent surgery, trauma, prolonged hospitalization, ICU admission, and exposure to broad-spectrum antibiotics, third or fourth generation cephalosporins and carbapenems, and hyperalimentation.[3, 4, 5, 6, 7]




S maltophilia has few pathogenic mechanisms and, for this reason, predominantly results in colonization rather than infection. The main virulence factors that lend to disease pathogenesis include production of a variety of extracellular enzymes, evasion of host immune response, and triggering intense tissue inflammation.

If infection does occur, invasive medical devices are usually the vehicles through which the organism bypasses normal host defenses. Bacterial factors that contribute toward colonization of respiratory tract or prosthetic devices such as intravascular, surgical catheters and endotracheal tubes include cationic surface charge, flagella and fimbriae, which assist adhesion, and biofilm formation.



United States

S maltophilia is a noninvasive organism that has low virulence. It frequently colonizes body fluids but rarely causes infection (eg, intravenous line infections).


S maltophilia frequently colonizes the respiratory tract in patients with cystic fibrosis.[8, 9, 10, 11, 12]

The 2016 Cystic Fibrosis Foundation Patient Registry Annual Data Report shows a 13% prevalence of S maltophilia; this figure changed little over the preceding decade.[13]


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 depend on host factors.

S maltophilia bacteremia is associated with high morbidity and mortality rates and should be considered in patients with recent use of broad-spectrum antibiotics or recent isolation from any other site. The 30-day all-cause mortality rate associated with S maltophilia bacteremia (33.3%) is reported to be more than that of bacteremia caused by Pseudomonas aeruginosa (21.5%; P  =  0.080) and Acinetobacter species (17.3%; P  =  0.041). The independent factor associated with 30-day mortality was the SOFA score.[14]



The course S maltophilia infection depends on the site of the infection, severity, response to antibiotics, and existence of other comorbidities. S maltophilia infections may be life-threatening, especially in immunocompromised patients.[15]





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.

Patients often have a history of indwelling catheters, use of immunosuppressant therapy or broad-spectrum antibiotics, or cystic fibrosis.


Signs and symptoms of S maltophilia infections are related to the organ system involved and are indistinguishable from infections with other pathogens.


S maltophilia is a nonfermentative aerobic gram-negative bacillus formerly classified in the genus 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, intravenous 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
  • Intravenous lines and/or fluids
    • Intravenous solutions
    • Central venous catheters
  • Pressure monitoring devices - Pressure transducer fluids
  • Dialysis machines [16]
  • Urine and/or fluids
    • Indwelling Foley catheters
    • Urometers
    • Irrigation solutions
    • Hospital tap water [1]
    • In a Greek multicenter study on the colonization of municipal water supplies, treated water, and dialysate, S maltophilia accounted for 13.5% of isolates, Pseudomonas for 22.7%, and Chryseobacterium for 14.9%. [17]


Diagnostic Considerations

The main clinical problem presented by S maltophilia is failing to recognize 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 is potentially pathogenic only in those with impaired host defenses, ie, patients with organ transplants, diabetes, systemic lupus erythematosus (SLE), alcoholic cirrhosis, multiple myeloma, and those on steroids or immunosuppressed due to some other reasons.

S maltophilia recovered from blood cultures may have come from contaminated intravenous 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 sites implies a pathogenic role, eg, sacral decubitus ulcer/osteomyelitis. It has been reported as a cause of osteomyelitis.[18]  S maltophilia is a rare cause of nosocomial pneumonia in patients who are ventilated and presumed to have nosocomial pneumonia because of fever, pulmonary infiltrates, and leukocytosis.[19] The recovery of S maltophilia from respiratory secretions almost always represents colonization rather than infection, and, unless proven otherwise, its presence should not be addressed therapeutically.

S maltophilia may cause community-acquired pneumonia (CAP) in individuals with alcoholic cirrhosis.

Other common sites of infection include skin and soft tissues, keratitis (especially in contact lens wearers),endopthalmitis and device related infections.[20, 21, 22]

Stenotrophomonashas been reported as a rare cause of nosocomial meningitis and endocarditis.[23, 24]

Table. Hospital-Acquired S maltophilia Infections (Open Table in a new window)


Predisposing Factor

Catheter-associated bacteriuria

Indwelling urinary catheters

Intravenous line infections

Central intravenous catheters


Urinary tract instrumentation

Primary bacteremia

Arterial monitoring devices


Contamination of blood during collection/processing of blood cultures




Laboratory Studies

Culture of the organism from body fluids and proper identification from the microbiology laboratory confirms the presence of S maltophilia. Usually, the presence of S maltophilia represents colonization. A potential pathogenic role must be evaluated by an infectious disease specialist. The mere recovery of a potential pathogen from any body fluid does not indicate a pathogenic role for the organism.

S maltophilia is an obligate aerobe that grows well on commonly used laboratory media, including blood and MacConkey agars. It is lactose nonfermenting, oxidase-negative, and catalase-positive, DNase positive and can be reliably identified in the laboratory using standard biochemical tests. In addition, it is accurately identified by commercially available automated identification systems.[25]

Histologic Findings

The histology of S maltophilia in the rare situations when it causes infection is indistinguishable from the histology of infections caused by other aerobic gram-negative bacilli.



Medical Care

Colonization of body fluids in hospitalized patients should be minimized if possible. Foley catheters should be used only as long as necessary and should be avoided if at all possible in immunocompromised hosts predisposed to urinary tract infections, eg, patients with diabetes, SLE, or multiple myeloma.

Colonization of respiratory secretions in intubated patients in ICUs is the rule and is difficult to prevent.

Patient-to-patient spread of organisms may be minimized or prevented by effective infection-control measures.


Consultation with an infectious disease specialist is essential for differentiating colonization from infection in patients with S maltophilia isolated from various body fluids.


Because S maltophilia is a common nosocomial colonizer in patients and medical fluids, the recovery of S maltophilia should be considered nonpathogenic unless proven otherwise.

If S maltophilia is recovered from several patients in the same area, sections of an ICU or ward can become the focus of further spread within the hospital setting.

Effective infection control measures can minimize or limit the spread of this and other organisms in the ICU.

Appropriate isolation procedures, rather than antimicrobial therapy, should be used to control the spread of S maltophilia.

Medical personnel, including medical students, housekeeping staff, attending physicians, nursing personnel, and respiratory therapists, are potential carriers of the organism from patient to patient.




Medication Summary

Because S maltophilia is predominantly a colonizer, antimicrobial treatment may be unnecessary and may be potentially harmful. Thus, determination of whether the organism is truly the cause of an infection is necessary to determine the need for antibiotic therapy.

As a general principle, colonization should not be treated with antimicrobial therapy.

S maltophilia, is usually resistant to aminoglycosides, antipseudomonal penicillins, and cephalosporins. S maltophilia is consistently susceptible to trimethoprim-sulfamethoxazole (TMP-SMZ) and is a drug of choice.[26, 27] If TMP-SMZ cannot be used, the organism is usually sensitive to doxycycline, minocycline, respiratory quinolones,or ceftazidime.[28, 29]  Tigecycline may potentially be helpful, but clinical investigation is needed.[26, 30, 31]  Polymyxin B/Colistin may be effective, at least in vitro, though not without frequent adverse effects.[32, 33]

Studies have demonstrated activity of Eravacyclines and Omadacycline in vitro but further studies are required to ascertain the clinical efficacy and potential therapeutic option for multidrug-resistant S maltophilia infections.[34, 35, 36, 37, 38]

Cefiderocol demonstrated activity in vitro however it was found to be resistant in clinical isolates.

The combination of aztreonam with ceftazidime/avibactam has been successfully used for the treatment of a persistent S maltophilia bacteremia in a renal transplant patient.[39]  This synergistic combination suggests a potential therapeutic option for multidrug resistant S maltophilia infections.


Class Summary

Therapy should be selective to avoid covering multiple isolates from a body site in the context of the clinical setting.

Sulfamethoxazole/trimethoprim (Bactrim, Bactrim DS, Septra, Septra DS)

Inhibits bacterial growth by inhibiting synthesis of dihydrofolic acid. Antibacterial activity includes common urinary tract pathogens, except P aeruginosa.

Cefepime (Maxipime)

Fourth-generation cephalosporin with good gram-negative coverage, similar to ceftazidime, but better gram-positive coverage.

Minocycline (Dynacin, Minocin)

Treats infections caused by susceptible gram-negative and gram-positive organisms, in addition to infections caused by susceptible Chlamydia, Rickettsia, and Mycoplasma.

Tigecycline (Tygacil)

A glycylcycline antibiotic that is structurally similar to tetracycline antibiotics. Inhibits bacterial protein translation by binding to 30S ribosomal subunit, and blocks entry of amino-acyl tRNA molecules in ribosome A site. Indicated for complicated skin and skin structure infections caused by E coli, E faecalis (vancomycin-susceptible isolates only), S aureus (methicillin-susceptible and -resistant isolates), S agalactiae, S anginosus group (includes S anginosus, S intermedius, and S constellatus), S pyogenes, and B fragilis.


Questions & Answers