A self-replicating extrachromosomal DNA molecule, often carrying genes responsible for antibiotic resistance and virulence.

Plasmids are most commonly found in bacteria as circular, double-stranded DNA molecules that can replicate independently. Many carry antibiotic resistance genes and provide a mechanism by which such genes can be transferred between strains or between different species of bacteria. They can range in size from 1 Kb to more than 500 Kb. They are largely used in biotechnology approaches.