Plasmid specification of resistance to antibacterial compounds in environmental Citrobacter freundii

Citrobacter freundii from an environmental source displayed resistance to a wide variety of antibacterials. Some of these resistances could be transferred by conjugation with frequencies as high as 10(-2). The complexity of plasmid involvement in the antibacterial resistance of this species was demonstrated by isolation of plasmid DNA from two isolates. In one strain, resistance to beta-lactam antibiotics was specified by a 55 x 10(6) molecular weight plasmid, whereas kanamycin and neomycin resistance was specified by a 7 x 10(6) molecular weight plasmid. Resistance to chloramphenicol and tetracycline was not plasmid mediated. The other strain contained plasmids of molecular weight 113 x 10(6) and 80 x 10(6), which specified resistance to chloramphenicol, neomycin, kanamycin, tetracycline, and mercury.