Plasmide

In bacteria and partly in yeast cells, mostly ring-shaped extrachromosomal DNA molecules that are present in addition to chromosomal DNA (linear plasmids have been discovered in bacteria of the genera Streptomyces and Borrelia, among others). Plasmids comprise 1-2% of the total genome.

Plasmids usually contain only a few genes. They are replicated as an independent genetic unit because they each have at least 1 origin of replication. They may be single or multiple in number. Plasmids of different sizes may also be present.

Plasmids can carry genetic information for virulence factors, but also for antibiotic resistance. By conjugation, plasmids can be transferred within a bacterial species. Sometimes even across species boundaries. Hospitalized bacteria often possess such plasmid-encoded properties.

In genetic engineering, plasmids are used as "tools":

• as cloning vectors to duplicate certain genes: the foreign gene in question is inserted into the plasmid, which multiplies along with it when it divides.
• as "transport vessels" to introduce a foreign gene into cells of other organisms, e.g. plants. The ability of plasmids to penetrate other cells is used in the transformation of plants with the aid of agrobacteria.

1. Hof H et al (2019):Hof H (2019) Special Virology. In: Hof H, Schlüter D, Dörries R, eds Duale Reihe Medizinische Mikrobiologie. 7th, completely revised and expanded edition. Stuttgart: Thieme S 287