Molecular analysis of R-factors from multiresistant nosocomial isolates

During an epidemic of infections at the Seattle Veterans Hospital, Washington, due to a multiresistant strain of Serratia marcencens, other enteric species were isolated that had antibiograms nearly identical to those of the epidemic S. marcescens. In 11 instances, these multiresistant species were isolated from specimens that also contained the epidemic serratia strain. All isolates of the epidemic serratia strain contained a conjugative 45-megadalton R-factor (pLST1000) coding for intermediate resistance to three amino-glycosides (minimal inhibitory concentrations, 4--8 micrograms/ml) and high-level resistance to ampicillin, carbenicillin, cephalothin, streptomycin, and sulfonamide. With the use of agarose gel electrophoresis and restriction endonuclease cleavage patterns after digestion with EcoRI, BamH1, and HindIII, it was determined that eight different enteric strains of six different species isolated from the patients contained an R-factor that was molecularly identical to the one isolated from the epidemic strain of S. marcescens. Thus, the epidemic of multiresistant infections at this hospital was caused both by the spread of an epidemic strain and an "epidemic plasmid." The molecular characteristics of pLST1000 appear to be different from previously described multiresistant plasmids.