Review
Stenotrophomonas maltophilia: the significance and role as a nosocomial pathogen

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Abstract

Stenotrophomonas maltophilia is a significant pathogen that primarily affects patients with co-morbid illnesses, usually malignancies, who have been hospitalized for prolonged periods and received broad-spectrum antibiotic therapy. The organism has been associated with an expanding spectrum of clinical manifestations associated with high case to fatality ratios. Therapy of these infections presents a significant challenge both for the clinician and the microbiologists because of its high-level antibiotic resistance to most of the currently used agents and methodological difficulties in susceptibility testing with this organism.

Introduction

Stenotrophomonas maltophilia (previously known as Pseudomonas maltophilia or Xanthomonas maltophilia) is a non-fermentative, Gram-negative bacillus, which has assumed an increasingly important role as a nosocomial pathogen in compromised patients.1 Although it was considered previously to have limited pathogenicity, reports indicate that infection with this organism is associated with significant morbidity and mortality particularly in severely compromised patients.2 Treatment of S. maltophilia infections is also complicated by the fact that isolates are inherently resistant to many of the currently available broad-spectrum agents including carbapenems.3 In recent years several trials have elucidated risk factors for S. maltophilia infection, which include neutropenia, the presence of a central venous catheter (CVC), prolonged hospitalization and previous therapy with broad-spectrum antibiotics.4 However, the clinical consequences of infection and the consensus to the most appropriate is not clearly established.

This review will focus on the emerging role and clinical significance of S. maltophilia as a nosocomial pathogen. Also current knowledge of therapeutic considerations important for the management of S. maltophilia infections will be reviewed.

Section snippets

Microbiological background

S. maltophilia is a motile, non-fermentative, oxidase-negative, aerobic Gram-negative bacillus which is widespread in the environment. Originally included in the genus Pseudomonas, it was then transferred to the genus Xanthomonas. Recently it has been reclassified as the single species of the new genus Stenotrophomonas.5

S. maltophilia grows on most bacteriological media, typically producing pigmented (yellow or lavender–green) colonies. The organism can be identified in the clinical laboratory

Epidemiology

S. maltophilia is ubiquitous in the environment with wide geographic distribution. The organism can readily be isolated from water, soil, plants, animal sources and sewage. The wide range of nosocomial sources from which the bacterium has been isolated are summarized in Table I. It is also common in the hospital setting, usually as a commensal, contaminant or part of the patient's endogenous flora.1 Isolation from human faeces was first noted in 1961 by Hugh.7 After that few studies have

Predisposing factors

Studies analysing predisposing factors have elucidated several risk factors associated with S. maltophilia infection, and these are summarized in Table II. Among these, prior exposure to antimicrobial agents has been consistently associated with S. maltophilia infection. Particularly prior therapy with carbapenems, to which the bacterium is inherently resistant, has been cited by several authors. In a study Elting et al.22 found prior therapy with imipenem was 10 times more frequent among cases

Clinical manifestations

S. maltophilia is associated with an expanding spectrum of clinical syndromes; these are summarized in Table III. Bacteraemia and pneumonia are the two most common manifestations and both are associated with high mortality. An increase in the incidence of bacteraemia has been observed, especially during the last two decades.7., 11., 13., 14. Although the portal of the entry was unknown in more than half of the episodes, evidence indicates the importance of intravascular devices for bacteraemia.

Treatment of S. maltophilia infection

One striking characteristic of S. maltophilia is its inherent resistance to many of the currently available broad-spectrum antibiotics. S. maltophilia strains are considered resistant to virtually all classes of beta-lactams including extended-spectrum penicillins, third-generation cephalosporins and carbapenems and to variety of other antibiotics, including aminoglycosides (AGA), with variable susceptibility to fluoroquinolones.6., 7.

Multiple mechanisms are involved in the high-level

Conclusion

Despite the recognition of S. maltophilia as an important nosocomial pathogen, little is known about the epidemiology of this organism in the hospital setting. Also there is no clear consensus as to the most appropriate therapy. Large, controlled studies evaluating the role of antimicrobial combinations in clinical practice and correlating the susceptibility testing results with clinical outcomes are needed.

Recent evidence indicates that the incidence and importance of S. maltophilia infections

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