Physical Characterization of Ten R Plasmids Obtained from an Outbreak of Nosocomial Klebsiella pneumoniae Infections

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Abstract
Gentamicin resistance in Klebsiella pneumoniae involved in an outbreak at the Minneapolis Veterans Administration Hospital was due to a transmissible R plasmid. In addition to gentamicin, this plasmid conferred resistance to tobramycin, kanamycin, ampicillin, carbenicillin, cephalothin, chloramphenicol, and sulfathiazole. R plasmids which transferred this complex antibiogram were identified in several clinical isolates, including four different serotypes of K. pneumoniae, Escherichia coli, Enterobacter cloacae , and Proteus morganii . The covalently closed circular form of all R plasmids isolated had a sedimentation coefficient of 76S to 77S, corresponding to a molecular weight of 58 × 10 6 . The possibility that a single R plasmid was responsible for the dissemination of multiple drug resistance among all of these different clinical strains was examined by characterizing the plasmids by using Eco RI restriction endonuclease. The same 15 fragments were obtained from each of the 10 plasmids analyzed. Their molecular weights ranged from 4 × 10 5 to 11 × 10 6 . Thus, we conclude that each of the 10 plasmids present in the various clinical strains isolated from the hospital over a 7-month period originated from a common source and that R plasmid transfer was important in their spread.