Evidence for Related Virulence Sequences in Plasmids of Salmonella dublin and Salmonella typhimurium

Abstract

Transposon-insertion mutants were prepared from virulent field isolates of S. dublin and S. typhimurium. Detailed restriction-enzyme mapping of the single sites of TnA insertion in 2 mutants (M51 and M173) of S. dublin that showed diminished virulence in a mouse assay indicated that these sites were .apprx. 5 kbp (kilobase pairs)apart on the .apprx. 70 kbp plasmid harbored by the isolate. A Tn10-insertion mutant (M242) of S. typhimurium that showed diminished virulence was also identified. A single copy of Tn10 was inserted into the .apprx. 90 kbp plasmid harbored by this isolate. Hybridization studies indicated that homology existed between the region encompassing the sites of TnA insertion of M51 and M173, and that encompassing the site of Tn10 insertion in M242. Restriction mapping indicated that the 2 regions were very similar and could even be identical and, if so, the Tn10 insertion in M242 could be mapped to a point 1.5 kbp from the TnA insertion in M51 and 6.5 kbp from that in M173. It appeared that the maximal extent of the putative similarity/identity was between 13 and 23 kbp. It is proposed that this stretch of high homology could represent a virulence sequence that was conserved during the evolutionary divergence of the 2 Salmonella serotypes.