R factors determining multiple resistance including both gentamicin and carbenicillin have been identified in high incidence among hospital isolates of Pseudomonas aeruginosa. The factors are readily transmitted to other P. aeruginosa but not to Escherichia coli strains K-12 or C, or to Proteus mirabilis. R factor-containing isolates are predominantly immunotype 7 isolated from urinary sources.Pseudomonas aeruginosa, although resistant to many antibiotics, has been generally susceptible to gentamicin and carbenicillin. Recently, descriptions have appeared of R factors in this organism, including reports of factors determining resistance to one or the other of these antibiotics (3,4,8,9,11,12,14,15, 17 with L-amino acids at 20 ,ug/ml as required. Antibiotics were used in selection plates at the following concentrations (;g/ml): gentamicin, 1 to 20; carbenicillin, 50, 500, or 750; streptomycin, 100; tetracycline, 100; kanamycin, 50; and rifampin, 100.Antibiotic resistance. Antibiotic disk susceptibilities were determined by the Bauer-Kirby method (2) with Mueller-Hinton agar (Difco). For determination of minimal inhibitory concentrations, single drops with 101 organisms from ovemight culture were spotted on similar agar media containing serial twofold supplements of antibiotic. In cases where strain interactions were likely, single samples per culture dish were examined with 35 by 10 mm dishes.Bacterial strains. All clinical strains studied were isolated and identified at Veterans Administration Hospital, Cincinnati. These and other locally derived strains are designated with the PL prefix of this laboratory. The origins and pertinent characteristics of the major mating strains are presented in Table 1. Strain PL1 is a leu mutant of strain 280 met rit. 'Ihe mutant was isolated from a stock of the parental strain which had been exposed to diethylsulfate and subjected to carbenicillin selection (1.3 mg/ml). All strains were maintained at room temperature on complex medium solidified with agar (0.75%) in sealed, screw-capped glass vials.Bacterial matings and selection of transcipients.

Properties ofa population ofcarbenicillin-and gentamicin-resistant, tobramycin-susceptible Pseudomonas aeruginosa at Veteran's Administration Hospital, Cincinnati, Ohio, have been followed during a 16-month period. As originally described, the strains were isolated from patients with urinary tract colonizations and were predominantly Parke-Davis immunotype 7. For the majority of these organisms, antibiotic resistance was correlated with the presence of a selfconjugative plasmid of incompatibility group P-2. The source and relative incidence of multiply resistant isolates have remained constant during the current study, but the immunotype has shifted form type 7 to type 2. Concomitantly, the population has lost the property of conjugative transfer of resistance, and resistant strains are now compatible with P-2 plasmids. A group P-2 R plasmid, pMG5, will mobilize resistance markers, demonstrating that the multiple resistance of the nonconjugative strains is mediated by R plasmids. Additionally, gentamicin resistance due to either conjugative or nonconjugative plasmids is correlated with the presence of similar gentamicin acetyltransferase activity. pMG5-mobilized plasmids are shown to be incompatible with pMG5. pMG5 is also shown to mobilize resistance markers from nontransferring antibioticresistant strains representing populations from Parkland Memorial Hospital, Dallas, Texas, and Cleveland Clinic Foundation, Cleveland, Ohio.