Introductory Microbiology

Plasmids

• Plasmids are replicons that are maintained as discrete, extra chromosomal genetic elements in bacteria. They are usually much smaller than the bacterial chromosome, varying from less than 5 to more than several hundred KbP(kilo base pairs), though plasmids as large as 2 Mbp occur in some bacteria. Plasmids usually encode traits that are not essential for bacterial viability, and replicate independently of the chromosome. Most plasmids are super coiled, circular, double stranded DNA molecules, but linear plasmids have also been demonstrated in Borrelia and Streptomyces.
• Many plasmids control medically important properties of pathogenic bacteria, including resistance to one or several antibiotics, production of toxins, and synthesis of cell surface structures required for adherence or colonization. Plasmids that determine resistance to antibiotics are often called R plasmids (or R factors) and the plasmids responsible for conjugation are F plasmids.