Retrospective analysis of plasmid patterns in a study of burn unit outbreaks of infection due to Enterobacter cloacae.

To evaluate the usefulness of plasmid pattern analysis as an epidemiologic tool and to determine the mechanism of resistance to silver sulfadiazine and mafenide acetate, silver sulfadiazine-resistant, mafenide acetate-resistant Enterobacter cloacae isolates from a 1976 outbreak of infections in a burn unit (group I) were analyzed retrospectively. These strains were compared with various E cloacae isolates (groups II-V), including E cloacae isolates from a more recent (1982) burn unit outbreak (group V). Agarose gel electrophoresis (AGE) of DNA isolated from "epidemic" strains of E cloacae revealed a uniform pattern of four plasmid DNA bands, ranging in molecular size from 66 to 2 megadaltons. No such plasmid pattern was seen in silver sulfadiazine-, mafenide acetate-susceptible isolates of E cloacae in groups II, III, and V. Analysis of non-E cloacae coisolates suggested that silver sulfadiazine resistance was associated with the 55-megadalton plasmid. Plasmid pattern analysis easily separated "epidemic" E cloacae isolated in 1976 from those isolated in 1982.