Plasmid macroevolution in a nosocomial environment: demonstration of a persistent molecular polymorphism and construction of a cladistic phylogeny on the basis of restriction data.

Descendants of a gentamicin resistance plasmid first isolated from the Minneapolis Veterans Administration Hospital in 1975 persisted at the hospital until 1983. During this extended period of time several macroevolutionary variants arose and two classes of variants were shown to persist with the initial plasmid. It was possible to construct a cladistic phylogeny on the basis of restriction endonuclease cleavage patterns of the initial and variant plasmids. Additionally, a plasmid polymorphism comprised of members of the plasmid classes was shown to persist at the hospital throughout the endemic period.