Plasmids

Bacteria, Archaea, and some eukaryotic microbes possess extrachromosomal DNA molecules called plasmids.

Plasmids are small, circular DNA molecules, usually with fewer than 30 genes, though some may be linear.

Copy number refers to the number of plasmid copies in a cell, ranging from one in single-copy plasmids to over 100 in multicopy plasmids.

Plasmids usually replicate independently, while some, such as episomes, integrate into the chromosomal DNA to replicate along with it.

The episome F factor mediates DNA transfer via conjugation by encoding genes for sex pili and plasmid transfer.

Plasmids, though nonessential, aid survival. Some Pseudomonas species metabolize environmental pollutants via plasmid-encoded enzymes.

Plasmids confer ecological benefits, such as enabling Rhizobium to form nitrogen-fixing nodules.

They can also encode bacteriocins to eliminate competing bacteria.

Certain plasmids enhance pathogenicity, such as those in E. coli strains that encode toxins and attachment factors, causing diarrhea.

Resistance plasmids, like R100, confer resistance to multiple antibiotics.