Effect of an R plasmid on the virulence of a hospital strain of Escherichia coli.

The virulence of a multiple antibiotic-resistant isolate of Escherichia coli was compared to that of the same isolate 'cured' of its resistance by acridine orange, and to that of the cured bacteria transformed back to resistance by the plasmid DNA extracted from the original resistant isolate. There were virtually no differences in the mouse LD 50 of these three variants of the same organism.