Plasmid

Definition

A plasmid is a small double-stranded unit of DNA, usually circular but sometimes linear, that exists independent of the chromosome and is capable of self-replication. Each plasmid carries only a few genes.

This shows an electon microscope image of DNA of the pSC101 plasmid, which was used as a vector in the first successful DNA cloning experiments, published in SN Cohen, ACY Chang, HW Boyer, RB Helling. 'Construction of biologically functional bacterial plasmids in vitro', PNAS USA, 70/11 (1973), 3240–4. Credit: Stanley N Cohen.

Connections Stanley Cohen

Importance

Plasmids come in many different sizes and are used for many different purposes in biotechnology. They first made their mark in the field of recombinant DNA in the 1970s, being used as a tool to insert genes into bacteria to encourage their production of therapeutic proteins such as human insulin.

In more recent years plasmid DNA has begun to be investigated as a therapeutic platform for treating infectious, genetic and acquired diseases. Plasmid DNA is seen, for example, as a promising tool for the development of DNA vaccines against HIV-AIDS, Ebola, Malaria, enteric pathogens, and influenza. The plasmid is genetically modified to produce one or two specific proteins from a pathogen and then purified for immunisation. DNA vaccines offer several advantages over traditional vaccines. Firstly, they eliminate the need to inject infectious agents. Secondly, they stimulate both B- and T-cell immune responses. Thirdly, they are more stable in different temperatures so are easier to store and transport. Lastly, they can be manufactured on a large-scale and at low cost.

The development of plasmids as vaccines poses significant challenges. Most DNA vaccines tested in experimental trials have so far evoked too weak an immune response in humans to protect against disease. Positive results, however, were announced in 2006 from preliminary clinical trials conducted with a DNA vaccine developed against H5N1 avian flu. Plasmids are also being explored for the development of DNA vaccines for non-treatable neurological disorders, such as ischemic stroke, Parkinson's disease, Alzheimer's disease and multiple sclerosis. A preliminary study of DNA vaccination against multiple sclerosis, completed in 2007, indicates the approach could be effective for this disease. More extensive trials, however, are needed to confirm the viability of DNA plasmids and their use for DNA vaccines.

Discovery

Independent strands of DNA were first found in bacterial cells in the late 1940s by researchers investigating how bacteria become resistant to antibiotics and how traits are passed on to offspring by phages (viruses of bacteria) and DNA structures other than chromosomes. Various names were applied to these DNA strands during this time, including pangenes, bioblasts, plasmagenes, plastogenes, choncriogenes, cytogenes, proviruses, and episomes.

The word 'plasmid' was first coined by Joshua Lederberg in 1952. He used it to describe 'any extrachromosomal hereditary element'. Lederberg first used the term in a paper he published describing some experiments he and his graduate student Norton Zinder conducted on Salmonella bacteria and its virus P22. During the course of this work they observed that virus particles could somehow pick up bacterial genes and transfer them to another host, a process they called transduction. Yet, how this phenomenon worked and what lay behind remained poorly understood for many years. This changed following the unveiling of the double-helix structure of DNA in 1953, which proved that DNA was made up of genetic material. Soon after scientists began to determine that plasmids were made up of small sequences of DNA which helped them to pass on particular traits.

By the 1960s a number of plasmids had been identified. This included fertility plasmids first observed in the late 1940s by Esther Lederberg, Joshua's wife, which carry the fertility genes necessary for bacterial conjugation. Another group were resistance (R) plasmids which carry the genes that encode resistance to antibiotics or poisons. It was an R plasmid (pSC101) that proved instrumental to the generation of the first recombinant DNA molecule.

Application

A plasmid is a strand or loop of DNA that is typically found in bacteria as well as archae (single-cell organisms) and eukarya (organisms of complex cell structure). Plasmids carry only a few genes and exist independently of chromosomes, the primary structures that contain DNA in cells. Able to self-replicate, plasmids can be picked up from the environment and transferred between bacteria.

Plasmids are used by their host organism to cope with stress-related conditions. Many plasmids, for example, carry genes that code for the production of enzymes to inactivate antibiotics or poisons. Others contain genes that help a host organism digest unusual substances or kill other types of bacteria.

Several characteristics of plasmids make them easy to modify genetically. Firstly, they have relatively small DNA sequences, between 1,000 and 20,000 DNA base pairs. Secondly, they are easy to cut open, without falling apart, and snap back into shape. This makes it easy to insert new DNA into plasmids. Once a new DNA is inserted, the modified plasmid can be grown in bacteria for self-replication to make endless copies.

The ease of manipulation and reproduction of plasmids, as well as their long-term stability, has made them indispensable tools in genetics and biotechnology laboratories. One of their most important functions is as a delivery vehicle, or vector, to introduce foreign DNA into bacteria, a fundamental step for genetic engineering and many other biotechnology applications.

Plasmid: timeline of key events

Datta was a microbial geneticist who showed that multi-antibiotic resistance was transferred between bacteria by plasmids. She first made the connection in 1959 after investigating a severe outbreak of Salmonella typhimurium phage-type 27 at Hammersmith Hospital where she worked. This involved an examination of 309 cultures, of which she found 25 were drug resistant, eight of which were resistant to Streptomycin which had been used to treat the patients. She concluded that the antibiotic resistance developed over time because the earlier cultures of the salmonella typhimurium infection (from the start of the outbreak) were not drug resistant. 1922-09-17T00:00:00+0000Lederberg was an American geneticist who helped discover the mechanism of genetic recombination in bacteria. This was based on some experiments he performed with Edward Tatum in 1946 which involved mixing two different strains of bacteria. Their experiments also demonstrated for the first time that bacteria reproduced sexually, rather than by cells splitting in two, thereby proving that bacterial genetic systems were similar to those of multicellular organisms. Later on, in 1952, working with Norton Zinder, Lederberg found that certain bacteriophages (viruses that affect bacteria) could carry a bacterial gene from one bacterium to another. In 1958 Lederberg shared the Nobel Prize for Medicine for 'discoveries concerning genetic recombination and the organisation of the genetic material of bacteria.' 1925-05-23T00:00:00+0000Falkow was a microbiologist who made his scientific mark by showing how bacteria develop resistance to antibiotics. During the 1960s he demonstrated that bacteria could acquire resistance by swapping genetic material via plasmids, small microbial DNA molecules. He studied a wide variety bacteria, from diarrhoea-causing E. coli Salmonella to bacteria that cause whooping cough and bubonic plague. 1934-01-24T00:00:00+0000The rise in resistance was observed in cultures of Salmonella typhimurium taken from both humans and animals sent to the Enteric Reference Laboratory in London. Research conducted by Naomi Datta and E S Anderson showed that the R transfer factor (plasmid) contributed to the rise in resistance. 1963-01-01T00:00:00+0000The phenomenon of plasmid-mediated resistance was first observed by a team of Japanese scientists led Tsutomu Watanabe investigating a series of outbreaks of dysentery caused by Shigella bacteria following the Second World War. T Watanabe (1963) 'Infective heredity of multiple drug resistance in bacteria', Bacteriology Reviews, 27/1: 87-115.1963-03-01T00:00:00+0000Drug resistant bacteria were first identified in Japan and then in Britain. Some of the earliest observations of this phenomenon were made by Naomi Datta who in 1962-63 showed that structures with some similarity to phages could transfer drug-resistance genes. Ephraim Anderson, director of the Enteric Reference Laboratory in Colindale, London, subsequently showed that genetic factors endowing resistance to major drugs used against human disease could be transferred by plasmids from minor pathogens. A summary of the work was published in ES Anderson, 'Origin of transferable drug-resistance in the enterobacteriaceae', British Medical Journal, 27 Nov 1965, 1289-91. 1965-11-27T00:00:00+0000Lederberg was an American geneticist who helped discover the mechanism of genetic recombination in bacteria. This was based on some experiments he performed with Edward Tatum in 1946 which involved mixing two different strains of bacteria. Their experiments also demonstrated for the first time that bacteria reproduced sexually, rather than by cells splitting in two, thereby proving that bacterial genetic systems were similar to those of multicellular organisms. Later on, in 1952, working with Norton Zinder, Lederberg found that certain bacteriophages (viruses that affect bacteria) could carry a bacterial gene from one bacterium to another. In 1958 Lederberg shared the Nobel Prize for Medicine for 'discoveries concerning genetic recombination and the organisation of the genetic material of bacteria.' 2008-02-02T00:00:00+0000Datta was a British microbial geneticist who showed that multi-antibiotic resistance was transferred between bacteria by plasmids. She first made the connection in 1959 after investigating a severe outbreak of Salmonella typhimurium phage-type 27 at Hammersmith Hospital where she worked. This involved an examination of 309 cultures, of which she found 25 were drug resistant, eight of which were resistant to Streptomycin which had been used to treat the patients. She concluded that the antibiotic resistance developed over time because the earlier cultures of the salmonella typhimurium infection (from the start of the outbreak) were not drug resistant. 2008-11-30T00:00:00+0000Falkow was an American microbiologist who made his scientific mark by showing how bacteria develop resistance to antibiotics. During the 1960s he demonstrated that bacteria could acquire resistance by swapping genetic material via plasmids, small microbial DNA molecules. Thereafter he focused his attention on how pathogens cause disease and in 1985 helped to identify a single genetic locus in Yersinia pseudotuberculosis, a Gram-negative bacteria, that accounts for its ability to infect cultured animal cells. He later showed that a sub-type of E. coli caused a life-threatening diarrhoea prevalent in many low-income countries. Known as the founder of molecular pathogenesis, Falkow's work paved the way to the development of new vaccines, including for whooping cough. He also helped to devise a uniform nomenclature for bacterial plasmids.2018-05-05T00:00:00+0000
Date Event People Places
17 Sep 1922Naomi Datta was born in London, UKDattaHammersmith Hospital
23 May 1925Joshua Lederberg was born in Montclair, NJ, USAJoshua LederbergUniversity of Wisconsin
24 Jan 1934Stanley Falkow was born in Albany, New York, USAFalkowGeorgetown University School of Medicine, University of Washington School of Medicine, Stanford University
1963 - 1965Increasing use of antibiotics noted to contribute to rapid rise of antimicrobial resistance in enterobacteria Naomi Data, ES AndersonEnteric Reference Laboratory
March 1963Concerns about antimicrobial resistance mount after discovery that bacteria can pick up resistance genes from small self-replicating DNA in the environment known as plasmids. WatanabeKeito University
27 Nov 1965Plasmids noted to carry genes conveying antibiotic resistance in bacteriaAnderson, Datta 
2 Feb 2008Joshua Lederberg diedJoshua LederbergUniversity of Wisconsin
30 Nov 2008Naomi Datta diedDattaHammersmith Hospital
5 May 2018Stanley Falkow diedFalkowGeorgetown University School of Medicine, University of Washington School of Medicine, Stanford University

17 Sep 1922

Naomi Datta was born in London, UK

23 May 1925

Joshua Lederberg was born in Montclair, NJ, USA

24 Jan 1934

Stanley Falkow was born in Albany, New York, USA

1963 - 1965

Increasing use of antibiotics noted to contribute to rapid rise of antimicrobial resistance in enterobacteria

Mar 1963

Concerns about antimicrobial resistance mount after discovery that bacteria can pick up resistance genes from small self-replicating DNA in the environment known as plasmids.

27 Nov 1965

Plasmids noted to carry genes conveying antibiotic resistance in bacteria

2 Feb 2008

Joshua Lederberg died

30 Nov 2008

Naomi Datta died

5 May 2018

Stanley Falkow died

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