Thirty-five isolates of from cattle or sheep were screened for the presence of plasmids and for resistance to a range of antibiotics. Eight strains (four of serotype A1, three of serotype A2 and one untypable) contained plasmid DNA and isolates of the same serotype had similar plasmid profiles, which were different from those of the other serotypes. All but one of the plasmid-bearing strains were isolated from pneumonic animals or from animals in contact with pneumonic cattle or sheep. In A2 and untypable strains, there was no obvious correlation between antibiotic resistance and the presence of a specific plasmid. In contrast, all plasmid-bearing A1 strains exhibited ampicillin resistance (Ap), which was shown by transfer studies to be plasmid-mediated. Plasmid DNA prepared from transformants was not routinely detected on ethidium-bromide-stained agarose gels, but could be amplified to detectable levels by treatment of cultures with chloramphenicol (Cm) or by modifying the growth conditions. The Ap plasmids from were identical by restriction enzyme analysis. Restriction analysis and hybridization data indicated that these plasmids were closely related to the prototype ROB-1 β-lactamase-encoding plasmid, originally isolated from . From substrate profiles and isoelectric focusing data, the β-lactamases encoded by the plasmids were indistinguishable from the ROB-1 β-lactamase.


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