Correlation between the presence of a fifty-megadalton plasmid in Salmonella dublin and virulence for mice

All of the Salmonella dublin organisms isolated from cattle in Japan (20 strains) and in Europe (2 strains) harbored one plasmid of approximately 50 megadaltons (Mdal). The biochemical reactions of a derivative strain (5230P-) cured of the 50-Mdal plasmid were those of its parental strain 5230P+. However, the strain 5230P- proved to be 100- to 1,000-fold less virulent for mice compared with that of the strain 5230P+, suggesting that the virulent phenotypes of S. dublin may be mediated by the 50-Mdal plasmid.