Recognition of the cryptic plasmid, pSLT, by restriction fingerprinting and a study of its incidence in Scottish Salmonella isolates.

The plasmid pSLT is a cryptic plasmid of 60 megadaltons (Md) present in Salmonella typhimurium LT2. We present evidence that it has a characteristic fingerprint when digested with the restriction enzymes PstI and SmaI. Among a representative collection of S. typhimurium isolates it was present in 67% of strains and was widely distributed amongst different phage types (DT) with the exception of DT10 and U285. Furthermore, its prevalence among veterinary isolates was significantly higher than among human isolates. It was not found among any of the 96 strains representative of other salmonella serotypes currently prevalent and thus appears to be serotype-specific.