Demonstration of an R plasmid in a strain of Pasteurella haemolytica isolated from feedlot cattle.

A strain of Pasteurella haemolytica isolated from the lungs of feedlot cattle with pneumonia was shown to be resistant to ampicillin, tetracycline, and streptomycin. Two small, nontransmissible plasmids were shown to be responsible, for tetracycline resistance, a 5.7 X 10(6) dalton plasmid, and for streptomycin resistance, a 5.5 X 10(6) dalton plasmid. Ampicillin resistance passed by transformation to Escherichia coli could not be shown conclusively to be on a plasmid. Ampicillin resistance was shown to be mediated by means of a beta-lactamase with high activity against cephaloridine.