Analysis of epidemic and endemic isolates of Xanthomonas maltophilia by contour-clamped homogeneous electric field gel electrophoresis.

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, hospital wide 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 XbaI CHEF patterns differed by a single band were differentiated clearly by SspI. 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.