Use of plasmid profiles in the investigation of a patient with Helicobacter pylori infection and peptic ulcer disease. Plasmids may effect bacterial virulence and antibiotic resistance, and serve as epidemiologic markers. In this study, plasmid DNA profiles of serial isolates of Helicobacter pylori showed persistence of an identical strain of this organism in a patient with duodenal ulcer disease. Three control strains of H. pylori isolated from other patients contained plasmids different from each other and from that of the original patient; two of these strains had two plasmids each. These data have important implications for further study of the epidemiology and pathogenesis of H. pylori-related diseases. 