Plasmid profiles

Most of the genetic information necessary for a functional bacterial cell is contained in the bacterial chromosome. Plasmids are additional DNA sequences that are separate from the chromosome. Stringent plasmids depend on cell replication, and plasmid replication is synchronized with replication of the bacterial chromosome. All plasmids contain gene sequences that are necessary for their own replication, and may also contain tra genes necessary for self-transmission. Plasmids generally code for a variety of phenotypic traits, and it is often of interest to ascribe a particular phenotypic trait to a specific plasmid. Plasmid profiles have been used to identify soil bacteria at the strain level. Such identification is based on the premise that a particular set of plasmids is unique to that strain with respect to size and number of plasmids. Soil bacteria such as rhizobia often contain several megaplasmids, and plasmid profiles of these bacteria have been used to identify rhizobia at the strain level.