This study examined the distribution of virulence plasmids among 1,021 Salmonella isolates collected in 1986. Using a 4 kilobase DNA probe derived from the virulence region of Salmonella dublin's 50 megadalton (MDa) plasmid, researchers performed colony hybridization experiments to identify which bacterial strains carried homologous virulence sequences. The virulence region was widespread, found in 678 isolates including most S. dublin (219/233), S. enteritidis (67/72), and S. typhimurium (333/404) strains tested. However, most other serotypes lacked this virulence region. Southern blot analysis revealed that despite carrying similar virulence sequences, different serotypes harbored these genes on plasmids of varying sizes: approximately 60 MDa in S. typhimurium, 35-60 MDa in S. enteritidis, and 50 MDa in S. dublin. Within specific phage types, virulence plasmids showed consistent restriction patterns, suggesting clonal spread. DNA sequence analysis demonstrated significant conservation of the virulence region across these three major Salmonella species, particularly in fragments of approximately 1.8, 4.0, and 8.5 kilobases in size. These findings suggest that virulence genes have been distributed among different Salmonella species, possibly through horizontal gene transfer mechanisms.

Key findings

• Virulence plasmid sequences from S. dublin's 50 MDa plasmid were found in 67% of 1,021 Salmonella isolates tested, predominantly in S. dublin, S. typhimurium, and S. enteritidis species
• The same virulence region is carried on plasmids of different sizes across species: 60 MDa in most S. typhimurium, 35-60 MDa in S. enteritidis, and 50 MDa in S. dublin
• Virulence regions showed high DNA sequence conservation across the three species, particularly in 1.8, 4.0, and 8.5 kilobase fragments, indicating functional importance
• Different S. enteritidis phage types possessed distinct plasmid profiles despite carrying the same virulence sequences, suggesting independent acquisition or evolution
• Virulence plasmids were largely absent from 43 other Salmonella serotypes tested, indicating serotype-specific distribution patterns