The organism originally known as Pseudomonas maltophilia was placed in the genus Xanthomonas in 1983.⁵ In 1993, it was reclassified Stenotrophomonas maltophilia and remains the only member of the genus Stenotrophomonas.⁶ S. maltophilia is ubiquitous and propagates in moist environments. Environmental isolates have been found in water sources (bottled water, wells, rivers), sewage, soil, plants, and food.^7,8 Hospital sources of S. maltophilia include ice-making machines, sinks, shower heads, disinfectant solutions, intravenous infusates, the hands of health care personnel, and cardiac, dialysis, and ventilator equipment.^9-14 Human sites of isolation include respiratory secretions, blood, urine, wounds, and pericardial and cerebrospinal fluid.⁸

The pathogen is opportunistic and displays low virulence. S. maltophilia infection can cause bacteremia, endocarditis, pneumonia, mastoiditis, peritonitis, meningitis, or infections of the eye, bone, joints, urinary tract, soft tissue, and wounds.^1,15-22 The risk factors for infection with S. maltophilia are admission to the intensive care unit (ICU), mechanical ventilation, immune deficiency, malignancy, cystic fibrosis, indwelling catheters, and debilitation.^8,10,14 An increased risk of infection also occurs in individuals exposed to multiple antibiotics or long-term antibiotic therapy.¹⁴ It is rare for healthy individuals to have clinically significant infection. Mortality rates from S. maltophilia infections are high. It is difficult to discern the relationship between mortality and S. maltophilia infection because other factors, such as the poor medical condition of the patient and the presence of other pathogens, also contribute to the high mortality rate.

Differentiating genuine infection from colonization is difficult because S. maltophilia is often cultured as part of a polymicrobial infection. In this circumstance, the diagnosis is often based on clinical judgement. A pure culture of S. maltophilia with clinical signs and symptoms of infection, or isolation from a normally sterile site (e.g., cerebrospinal fluid, blood), require treatment.

S. maltophilia is resistant to many currently available antimicrobial agents. Resistance to b-lactam drugs (penicillins, cephalosporins, imipenem) is thought to occur via two mechanisms. The first mechanism is a relatively impermeable outer membrane; the second involves two types of inducible b-lactamases designated as L1 and L2.^23-25 L2 appears to be active against the monobactam aztreonam. The mechanism of resistance to aminoglycosides is poorly understood. Changes in the outer membrane proteins that reduce outer membrane permeability may be partially responsible for moderate resistance to quinolones.²⁶ Some strains of S. maltophilia have been reported to transfer antibiotic resistance to other bacteria such as Pseudomonas aeruginosa, Enterobacteriaceae, and Proteus mirabilis.^27-29 This is of great concern clinically when S. maltophilia is present in polymicrobial infections.

The selection of agents used to treat S. maltophilia is primarily based on in vitro susceptibility data studies because there is a lack of controlled clinical trials to determine optimum therapy. Retrospective studies and anecdotal reports also provide some basis for treatment recommendations. The treatment of choice for S. maltophilia infection is trimethoprim-sulfamethoxazole (TMP-SMZ) alone or in combination with ticarcillin-clavulanate.²² TMP-SMZ is bacteriostatic for most isolates, hence high doses (12–15 mg/kg/day based on TMP) are usually recommended.³⁰ High doses may be especially difficult for elderly patients with poor renal function to tolerate because of severe skin reactions, bone marrow suppression, and thrombocytopenia. Combination therapy may be attempted when the patient is seriously ill, clinical response is poor, or when in vitro testing demonstrates resistance to all agents tested. For patients allergic to TMP-SMZ, a combination of ticarcillin-clavulanate and ciprofloxacin may be useful. Patients with a documented penicillin allergy may be treated with TMP-SMZ and ciprofloxacin.^31,32 Minocycline, doxycycline, and the newer quinolones (clinafloxacin, sparfloxacin), appear to have reasonably good in vitro activity and may be considered as an option.^26,33 Despite permeability problems and b-lactamase production, some strains show in vitro susceptibility to ceftazidime, cefoperazone, and moxalactam.⁸ Considering the high potential for resistance, antibiotic susceptibility testing should always be used to guide clinicians toward the most appropriate therapy. Tetracycline and the aminoglycosides demonstrate consistently poor in vitro activity and should not be used as monotherapy.

It is difficult to reach a firm conclusion when comparing in vitro susceptibility studies, due to differing methods of testing and strain-to-strain variation. The criteria for defining susceptibility and resistance vary between studies. Because of the pathogen’s slow growth rate and rapid mutation rate, there may be discordance between in vitro susceptibility results and clinical outcome. In vitro data that demonstrate synergy between combination drugs may prove to be a failure clinically because in vitro and in vivo concentrations differ. The synergy of antibiotic combinations varies between strains.

Nondrug strategies to prevent infection with S. maltophilia include prompt removal of infected indwelling devices and avoidance of prolonged implantation, when possible. Strict adherence to infection control practices, such as wearing gloves while in contact with body secretions and hand washing before and after patient contact, is recommended by the Centers for Disease Control. Sterilization/disinfection of equipment (cardiac, pulmonary, hemodialysis, ice machines) is needed when nosocomial outbreaks occur.

Consultant pharmacists should serve as a valuable educational resource to patients and other health care professionals to prevent, control, and treat outbreaks of S. maltophilia. Consultant pharmacists should be mindful that residents who are debilitated, immunocompromised, catheterized, diagnosed with malignancy, or have received antibiotics possess risk factors for acquiring S. maltophilia infection. The pharmacist should be vigilant while monitoring antibiotic usage and recommend discontinuation of all inappropriate antibiotics. While general recommendations regarding empiric treatment are available, the most appropriate choice of antibiotic therapy is based on susceptibility testing of the actual strain isolated from the patient. Education of health care personnel regarding the rationale for stringent adherence to infection control practices is essential.

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