To determine risk factors for patients whose cultures grew Stenotrophomonas maltophilia.

Retrospective case-control study of 60 patients with cultures positive for S maltophilia, matched by specimen site to 120 controls whose cultures grew other gram-negative aerobic bacteria.

University medical center.

S maltophilia was identified from the following sites: respiratory (36), wound (13), urinary (6), blood (4), and cerebral spinal fluid (1). By univariate analysis, cases had a higher risk of exposure than controls for ampicillin (P<.001), gentamicin (P<.001), vancomycin (P=.001), metronidazole (P=.003), piperacillin (P=.007), cefotaxime (P=.014), ceftazidime (P=.017), ciprofloxacin (P=.030), tobramycin (P=.040), and chronic respiratory disease (P=.024). Length of time foreign objects were in place prior to positive culture differed significantly between cases and controls only for endotracheal tubes in patients with respiratory isolates (median number of days: 12.5 for cases, 5 for controls; P=.007). For patients with urinary tract infections, having a urinary catheter increased the odds of infection 10 times over controls. Exposures found by multivariate analysis to be significantly more prevalent in cases than controls included ampicillin, cefotaxime, erythromycin, gentamicin, metronidazole, piperacillin, tobramycin, chronic respiratory disease, and female gender. Odds ratios were >1 indicating higher risk for cases, except for erythromycin, which had an odds ratio <1.

The primary risk factor associated with isolation of S maltophilia was antibiotic use. For patients with pulmonary infections, chronic respiratory disease and length of time an endotracheal tube was in place also contributed to the risk. This suggests that judicious use of antibiotics may prevent some cases of S maltophilia infection.

Xanthomonas maltophilia is increasingly a cause of nosocomial infections. The mode of transmission of this organism is not well known.

To investigate clonality of X maltophilia isolates in epidemic and endemic settings.

An outbreak of X maltophilia was noted in the Intensive Care Nursery (ICN). Over the ensuing 9 months, hospitalwide isolates of X maltophilia were analyzed using contour-clamped homogeneous electric field (CHEF) gel electrophoresis of chromosomal DNA. This method was compared with the antibiogram for detecting differences and similarities among strains.

X maltophilia was recovered from 76 sites in 72 patients; 65 isolates from 61 patients and the hands of one nurse were available for analysis. CHEF demonstrated differences between most epidemiologically unrelated strains and similarity between most epidemiologically related strains. Several strains, initially presumed to be related because of temporal and spatial proximity of the patients involved, were determined by CHEF analysis to be independent infections. One pair of isolates whose Xbal CHEF patterns differed by a single band were differentiated clearly by Sspl. There was enough variation in the minimum inhibitory concentrations of selected antibiotics to allow typing of some strains. The antibiogram, however, did not group all of the ICN outbreak isolates with others found to be genetically identical by CHEF and it grouped 39 of 56 isolates with others not genetically the same.

Although it is a convenient and economical tool, the antibiogram has limitations. Analysis by CHEF should help to elucidate the epidemiological spread of X maltophilia in the hospital.