Antibiotic resistance and R plasmids in endemic strains of Salmonella typhimurium isolated in Sicily.

The characteristics of R plasmids in strains of Salmonella typhimurium isolated in Sicily during 1983 from apparently isolated cases of human infection have been determined. A plasmid of the Inc group 10-B-0, characterized by ApSm resistance markers and with a molecular weight of 70 Md, was found in strains belonging to different phage types (2, 3, 117, 135, NT). The data obtained suggest that strains of the same phage type harboring such plasmids come from the same bacterial clone which has continued to be endemic through interhuman contagion. Instead, the presence of the same plasmid in strains of different phage type could indicate its spreading in different bacterial clones.