**4.2.1 Plasmid profiling**

Plasmid profile analysis was one of the earliest DNA-based subtyping schemes. It is particularly important, since most of the plasmids harbour virulence and antimicrobial resistance properties in *Salmonella*. Plasmid content of the host within the same serotype reveals the differentiation according to the profile (the number and molecular sizes of plasmids) obtained. The different plasmid profiles within a serotype points the lateral transfer by gaining or loosing the plasmid(s). The plasmids found in *Salmonella* differ in size 2 – 200 kb with different functionalities (Rychlik et al., 2006).

The detection method is based on the isolation of plasmids followed by agarose gel electrophoresis. Different protocols can be used (Helmuth et al., 1985). To view the plasmid pattern, agarose gel must be stained with ethidium bromide solution and then visualised under UV light.

Plasmid analysis has several limitations. Plasmids can rapidly be acquired or lost. Also, single predominant plasmids have become endemic within various serotypes. In sporadic isolates of *S. enteritidis* from Maryland, 88% of isolates contained a single 36-Mda plasmid (Morris et al., 1992). Similarly, only 1 of 56 *S. typhimurium* isolates failed to encode a 90 kb plasmid, which is thought to be a serotype specific virulence plasmid. Despite the ubiquitous nature of the 90 kb plasmid, profiling of the entire complement of plasmids in each strain was able to discriminate *S. typhimurium* strains isolated from a single poultry flock or closely related flocks (Millemann et al.,1995).

Plasmid analysis was also able to identify a multi-state outbreak of chloramphenicol resistant *S. newport* in humans that could be traced back to contaminated beef and to dairy farms (Riley et al.,1983). In a testament to the power of combining a strong traditional epidemiological analysis with serological and genotypic tests, a peak of *S. muenchen* was noted in Ohio, Michigan, Georgia and Alabama. Epidemiological studies failed to identify a common food source responsible for this outbreak, but a strong correlation with marijuana use was identified. Marijuana obtained from affected households was contaminated with *S. muenchen* and the isolates from the different states showed a similar plasmid fingerprint suggesting interstate transfer of the contaminated drug (Taylor et al., 1982).

Plasmid profiling is most useful in an outbreak setting that is limited temporally and geographically (Mendoza & Landeras, 1999). Furthermore, this technique will only be successful if the serotype of interest carries multiple plasmids of differing sizes.
