Plasmids are circular DNA molecules present in the cytoplasm of bacteria, capable of autonomous replication (independent replication). Plasmids consist genes in addition to chromosomal DNA, most bacteria possess extrachromosomal genetic elements. These are not essential for the normal life and functioning of the host bacterium but may confer on its properties like drug resistance and toxigenicity lead to survival advantage under appropriate conditions.
By their ability to transfer genes from one cell to another, plasmids have become important vectors in genetic engineering,
Plasmid have been classified in many ways, depending on where there they are self-transmissible or nontransmissible (nonconjugating), on the property encoded (sex, drug resistance, etc.)
An important method of plasmid classification is by incompatibility typing. Closely related plasmid do not coexist stably in the same bacterial cell, while underrated plasmids can. on this bases plasmids have been classified into different incompatibility groups. They have also been classified based on the type of conjugation tube induced, which determine the susceptibility of the host bacterium to lysis by some virulent bacteriophages.
TextBook Of Microbiology