Molecular divergence of the serotype-specific plasmid (pSLT) among strains of Salmonella typhimurium of human and veterinary origin and comparison of pSLT with the serotype specific plasmids of S. enteritidis and S. dublin.

Molecular variants of the serotype-specific plasmid (SSP) of Salmonella typhimurium (pSLT) were recognised in clinical and veterinary isolates by restriction enzyme fingerprinting. Three had undergone minor DNA rearrangements, whereas two had acquired resistance determinants to a wide range of antimicrobial agents including gentamicin, trimethoprim, tetracycline, streptomycin, ampicillin (Ap) and kanamycin (Km). One of the latter was the result of co-integrate formation with an IncX, conjugative R-plasmid that specified ApKm resistance. The co-integrate plasmid (pOG669) was incompatible with, and displaced, pSLT and its molecular variants. The restriction fingerprints of SSPs of S. enteritidis and S. dublin were compared with pSLT. All were related at the 35% level on the basis of a Dice coefficient of similarity. The SSPs of S. enteritidis and S. dublin were incompatible with the co-integrate plasmid pOG669. Whereas in S. enteritidis this resulted from incompatibility with the pSLT component (the SSP was compatible with the IncX component), the converse was found with S. dublin.