Plasmids Are Molecular Parasites

Plasmids are autonomous DNA molecules that are molecular parasites. They are the major DNA vehicles that move antibiotic resistance genes among bacteria. Each plasmid contains its own origin of replication and a gene encoding a specific replicator protein. These two elements give plasmids control over their own DNA replication, over their own “lives.” Plasmids can be small (on the order of 3,000 base pairs) or large (chromosome-size or about 3,000,000 base pairs). Many are circular, but linear forms are found. Some plasmids are present at only one or a few copies per cell, whereas others maintain hundreds of copies. The low-copy-number plasmids have special mechanisms for assuring that after cell division each daughter cell receives a plasmid copy. One mechanism, based on addiction modules, is briefly discussed in Box 6-1. Many plasmids contain a variety of antibiotic-resistance genes, which enables them to transfer several types of resistance at once. Plasmids create serious resistance problems because they can move from cell to cell at high frequency.