Retrospective Analysis of Plasmid Patterns in a Study of Burn Unit Outbreaks of Infection Due to Enterobacter cloacae

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Abstract
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.