Bacterial plasmids are extra-chromosomal, covalently-closed circular (CCC) molecules of DNA that are capable of autonomous replication (1). Plasmids may contain genes for a variety of phenotypic traits, such as antibiotic resistance, virulence, or metabolic activities, although some plasmids comprise genes conferring no detectable phenotype and are said to be "cryptic " Some plasmids (referred to as selftransferable or conjugative plasmids) have the ability to transfer copies of themselves to other bacterial strains or species; this trait is encoded by tra genes, In addition, some other plasmids are incapable of selftransfer, but are able to utilize the tra functions of cojugative plasmids present in the same bacterial cell to ensure that they are also passed to other strains and species; such plasmids are said to be mobilized.
Woodford, N., Johnson, A., Johnson, A. P., & Woodford, N. (2003). Plasmid Analysis. In Molecular Bacteriology (pp. 51–62). Humana Press. https://doi.org/10.1385/0-89603-498-4:51