Distribution of virulence plasmids within Salmonellae.

The virulence region of the Salmonella dublin 50 MDa plasmid shared homology with 678 of 1021 salmonellae tested in colony hybridization experiments. The majority of S. dublin, S. typhimurium and S. enteritidis isolates tested hybridized with the region whereas, with the exception of S. hessarek, S. pullorum and S. gallinarum, other serotypes did not. Homologous virulence regions were plasmid encoded. In S. typhimurium a common 60 MDa plasmid was present in all phage types tested but not in DT4, DT37 and DT170. Smaller plasmids showing partial homology were found in DT12, DT18, DT193 and DT204C. In S. enteritidis a distinct plasmid profile for each of eight phage types was observed. Hybridizing plasmids were found in DT3, DT4, DT8, DT9 and DT11 whereas DT7, which was plasmid free, and DT10 and DT14, which harboured plasmids, did not hybridize. The extent of homology shared between S. dublin, S. typhimurium and S. enteritidis virulence plasmids was about 10 MDa and appeared conserved. Virulence plasmids from S. typhimurium and S. enteritidis did not show homology with a region of the S. dublin 50 MDa plasmid which was not associated with virulence functions whereas plasmids of about 24 MDa and 38 MDa in some S. typhimurium phage types did. The association of conserved virulence regions upon differing plasmids within salmonellae is discussed with reference to possible mechanisms of distribution and evolution of virulence genes.