Bacterial plasmids are closed circular molecules of double-stranded DNA that range in size from 1 to >200 kb. They are found in a variety of bacterial species, where they behave as additional genetic units inherited and replicated independently of the bacterial chromosome. However, they rely upon enzymes and proteins provided by the host for their successful transcription and replication.
Plasmids often contain genes that code for enzymes that can be advantageous to the host cell in some circumstances. The encoded enzymes may be involved in resistance to, or production of, antibiotics, resistance to toxins found in the environment (e.g., complex organic compounds), or the production of toxins by the bacteria itself.
Once purified, plasmid DNA can be used in a wide variety of downstream applications such as sequencing, PCR, expression of proteins, transfection, and gene therapy.