Thirteen independent isolates of Serratia marcescens associated with nosocomial urinary tract infections were obtained from the clinical microbiology laboratory at Hines Veterans Administration Hospital. The isolates were resistant to at least ampicillin, carbenicillin, gentamicin, and tobramycin. They could be divided into two groups on the basis of their antibiotypes. Group I (9 strains) showed resistance to 13 antibiotics, including 3 beta-lactams, 6

SUMMARY.Of 3952 clinical isolates of Enterobacteriaceae, 246 exhibited resistance to at least carbenicillin, gentamicin and tobramycin. All these isolates, representing eight genera, were resistant to at least nine antimicrobial agents in common, including the three key antibiotics and streptomycin, kanamycin, sisomycin, ampicillin, cephalothin and sulphonamide. The strains could be subdivided into seven groups depending upon additional resistance traits and some were resistant to as many as 15 antibiotics. When mated with a standard strain of Escherichia coli, 85% of 1 23 randomly selected donors transferred resistance to at least the nine core antibiotics. Some donors occasionally transferred resistance to two additional antibiotics, neomycin and tetracycline, while one Citrobacter freundi donor always transferred linked resistance to all 11 drugs. Although many donors were found to harbour more than one species of plasmid DNA, all except a strain of C. freundi contained at least a plasmid of mol. wt 89 x lo6. Analysis of E. coli transconjugants showed this plasmid to be responsible for transferable resistance to the nine core antibiotics. Restriction-endonuclease analysis indicates that the 89 x lo6 plasmids originating from different isolates were essentially identical with each other. These results show that a particular R plasmid has established itself among the Enterobacteriaceae at Hines VA Hospital. This R plasmid appears to be the predominant genetic element responsible for linked resistance to carbenicillin, gentamicin and tobramycin among these hospitalassociated bacteria.

No abstract