Extrachromosomal deoxyribonucleic acid in different enterobacteria.

Eighty-seven different enterobacteria and pseudomonas strains were examined for the presence of extrachromosomal deoxyribonucleic acid (DNA). Thirty-four strains contained closed circular DNA by the ethidium bromide CsCl density technique. Extrachromosomal DNA was most frequent in Escherichia and Klebsiella strains. The extrachromosomal DNA was isolated and characterized by analytical ultracentrifugation and electron microscopy. All the extrachromosomal DNA-containing bacteria contained circular DNA molecules of small size (0.5-4 mum). Most of these bacteria also contained larger circles (20-40 mum). The number of different size classes of circular DNA in each strain varied from one to five. The buoyant density of the extrachromosomal DNA ranged from 1.692 to 1.721 g/cm(3). Many bacteria contained extrachromosomal DNA of more than one density