Plasmid

The term plasmid was introduced in 1952 by the American molecular biologist Joshua Lederberg, who used the term to refer to "any extrachromosomal heritable determinant". In 1968, the term plasmid was redefined as a term "for extrachromosomal genetic elements". To distinguish it from viruses, the definition was narrowed to: "genetic elements that exist exclusively or predominantly outside the chromosome and can replicate autonomously".

A plasmid is a small, usually ring-shaped, rarely linear, extrachromosomal, double-stranded DNA molecule that is physically separated from the chromosomal DNA and occurs in bacteria and other organisms. Plasmids are most commonly found in bacteria; however, they are also present in other prokaryotes (e.g., archebacteria) as well as and in eukaryotic organisms. Plasmids can be present in a cell in multiple copies (number between 1 and 1000).

In contrast to viruses, which enclose their genetic material in a protective protein shell (capsid), plasmids consist of so-called "naked" DNA. They do not encode genes that are required to encapsulate the genetic material for transfer to a new host. Plasmid size varies from 1 to over 200 kbp.

Plasmids are exchanged between different cells under natural conditions. They exist in addition to the genetic information of the main chromosome and are capable of autonomous replication. For a prokaryote, a plasmid is not existential. However, plasmids often contain genes that help bacteria gain a selection advantage, for example, in developing resistance to antibiotics. However, plasmid-mediated gene transfer plays an important role not only in the mobilization and spread of antibiotic resistance genes, but also in the spread of degradation pathways and pathogenicity determinants of pathogens (Smalla K et al. 2015).

According to the function of the genes, the plasmids can be divided into:

• Fertility plasmids
• resistance plasmids
• virulence plasmids and
• Metabolic plasmids

In genetic engineering, plasmids are used as "tools", e.g. as a "transport system" suitable for introducing a foreign gene into cells of other organisms, e.g. plants. For example, the ability of plasmids to enter other cells is used in the genetic transformation of plants with the help of agrobacteria.

Artificial plasmids are used as vectors in molecular cloning and are used to study the replication of recombinant DNA sequences in host organisms. For example, plasmid DNA is an efficient tool used in vaccine development (Rahimi Pet al. 2018). Furthermore, transfection of plasmid DNA can be used to specifically investigate the functions of cell lines (e.g. macrophages - Maeß MB et al. 2014).

Plasmids have proven to be highly efficient molecular biology tools. Plasmid DNA in GMP quality (Good Manufacturing Practice) is absolutely essential for direct gene transfer into humans. The same applies if the drug substance contains a genetically modified cell, for example a chimeric antigen receptor (CAR) T cell (Schmeer M et al. 2017).

1. Maeß MB et al. (2014) Highly efficient transfection of human THP-1 macrophages by nucleofection. J Vis Exp 91:e51960.
2. Rahimi Pet al. (2018) Comparison of transfection efficiency of polymer-based and lipid-based transfection reagents. Bratisl Lek Listy 119:701-705.
3. Schmeer M et al (2017) Plasmid DNA manufacturing for indirect and direct clinical applications. Hum Gene Ther 28:856-861.
4. Smalla K et al (2015) Plasmid detection, characterization, and ecology. Microbiol Spectr. 3:PLAS-0038-2014.