[The plasmid heterogeneity of Shigella sonnei phase-I and -II strains]

The plasmid composition of S. sonnei standard strains has been studied by the method of electron microscopy of the preparations of plasmid DNA. In S. sonnei cells I-941-HP, phase I, plasmids of 2,500; 5,000; 5,600; 6,100 and 6,800 base pairs, as well as plasmids of 85,000-117,000 and 170,000-235,000 base pairs have been detected. In S. sonnei cells, phase II, plasmids of 2,500; 4,900 and 6,100 base pairs, as well as plasmids of 85,000-109,000 base pairs, have been found. Thus, virulent S. sonnei in phase I contain additional plasmids of 5,600; 6,800; 110,000-117,000 and 170,000-237,000 base pairs. The range of plasmid lengths between 85,000-117,000 and 170,000-237,000 base pairs exceeds the usual background of electron-microscopic studies, which makes it possible to come to the conclusion on the intrastrain heterogeneity of these classes of plasmids. The suggestion has been made that the transition of S. sonnei from phase I to phase II is linked with the loss of fragments of the genetic material, limited by inverted DNA repetitions.