Evaluation of pulsed-field gel electrophoresis and midi sherlock to characterize a cluster of Stenotrophomonas Maltophila isolates recovered from a medical center

Over the past few years, an escalation in the number of nosocomial infections due to Stenotrophomonas maltophilia (Sm) has been noted in our institution. To ascertain if the infections were nosocomially acquired, pulsed-field electrophoresis and gas chromatography with the MIDI Sherlock system were utilized to characterize Sm isolates recovered from patients over an eighteen month period. Sherlock uses gas chromatography to qualitatively and quantitatively analyze the cellular fatty acid composition of organisms and creates a two-dimensional plot based on principal-component analysis to define groups of closely related organisms. A total of 24 isolates from 13 patients were examined. The sources and numbers of the isolates were as follows: blood cultures (12), catheter tip (5), urine (3), bile (1), sputum (2), and throat (I). XbaI digested Sm DNA restriction fragments were separated using contour-clamped homogeneous electric field (CHEF) electrophoresis. Thirteen chromosomal patterns were obtained. One bone marrow transplant patient had multiple isolates from blood (3), catheter tip (3), and urine (1) cultures. The isolates recovered from blood and catheter tip had identical patterns; whereas, the isolate recovered from urine was different. One patient admitted to the Burn Trauma Unit at the same time as the above patient had a pattern identical to the patient's blood culture isolate. An indistinguishable strain was recovered 14 months later from a patient admitted to a different unit. The isolates from the remaining ten patients had distinct patterns. Despite some evidence of nosoconmial spread, our results indicate that there is great diversity among isolates of Stenotrophomonas maltophilia recovered in our institution. Additionally, excellent correlation was obtained between Sherlock and pulsed-field electrophoresis. The methods were complementary in discriminating among the clinical Sm isolates.