Resistance plasmids

Examples of Resistance plasmids in the following topics:

• Types of Plasmids and Their Biological Significance

  • The gene to be replicated is inserted into copies of a plasmid containing genes that make cells resistant to particular antibiotics.
  • Only bacteria that take up copies of the plasmid survive, since the plasmid makes them resistant.
  • Just as the bacterium produces proteins to confer its antibiotic resistance, it can also be induced to produce large amounts of proteins from the inserted gene.
  • Fertility F-plasmids, which contain tra genes.
  • Resistance plasmids, which contain genes that provide resistance against antibiotics or poisons.

• Agrobacterium and Crown Gall Disease

  • Argobacterium causes Crown Gall Disease by transferring a DNA plasmid to the host plant, causing the host to make nutrients for it.
  • A. tumefaciens can transfer part of its DNA to the host plant, through a plasmid - a bacterial DNA molecule that is independent of a chromosome.
  • For example, plasmids can confer on bacteria the ability to fix nitrogen, or to resist antibiotic compounds.
  • Typically bacteria transfer plasmids through conjugation: a donor bacteria creates a tube called a pilus that penetrates the cell wall of the recipient bacteria and the plasmid DNA passes through the tube.
  • The other bacteria either integrates the plasmid into its chromosomes, or it remains free-floating in the cytoplasm.

• Introduction to Plasmids

  • Plasmid sizes vary from 1 to over 1,000 kbp.
  • The term plasmid was first introduced by the American molecular biologist Joshua Lederberg in 1952.
  • Similar to viruses, plasmids are not considered by some to be a form of life.
  • Plasmids may carry genes that provide resistance to naturally occurring antibiotics in a competitive environmental niche, or the proteins produced may act as toxins under similar circumstances.
  • The upper half of the image shows a bacterium with its chromosomal DNA and plasmids dividing into two identical bacteria, each with their chromosomal DNA and plasmids.

• Plasmids and Lysogeny

  • In addition, plasmid DNA provides a mechanism by which horizontal gene transfer can occur, contributing to antibiotic resistance.
  • Horizontal gene transfer is a major mechanism promoting bacterial antibiotic resistance, as the plasmid DNA can transfer genes from one species of bacteria to another.
  • The plasmid DNA which is transferred often has developed genes that encode for resistance against antibiotics.
  • The ability to transfer this resistance from one species to another is increasingly becoming an issue in clinics for treatment of bacterial infections.
  • The advantages of plasmid DNA transfer allow for survival advantages.

• Genetically Modified Organisms (GMOs)

  • Manipulating the DNA of plants (or creating genetically modified organisms called GMOs) has helped to create desirable traits, such as disease resistance, herbicide and pesticide resistance, better nutritional value, and better shelf-life.
  • In nature, the disease-causing A. tumefaciens have a set of plasmids, called the Ti plasmids (tumor-inducing plasmids), that contain genes for the production of tumors in plants.
  • DNA from the Ti plasmid integrates into the infected plant cell's genome.
  • Researchers manipulate the Ti plasmids to remove the tumor-causing genes and insert the desired DNA fragment for transfer into the plant genome.
  • The Ti plasmids carry antibiotic resistance genes to aid selection and can be propagated in E. coli cells as well.

• Molecular and Cellular Cloning

  • A plasmid (also called a vector) is a small circular DNA molecule that replicates independently of the chromosomal DNA.
  • In cloning, the plasmid molecules can be used to provide a "folder" in which to insert a desired DNA fragment.
  • Plasmids are usually introduced into a bacterial host for proliferation.
  • Plasmids occur naturally in bacterial populations (such as Escherichia coli) and have genes that can contribute favorable traits to the organism such as antibiotic resistance (the ability to be unaffected by antibiotics).
  • Not all recombinant plasmids are capable of expressing genes.

• Genetic Analysis

  • Bacterial chromosomes contain double stranded molecules of DNA arranged in a circular form called plasmids .
  • Plasmids are located in the cytoplasm of bacteria, are capable of autonomous replication, and transfer genes from parent cell to daughter cell.
  • Bacteria possess extra chromosomal genetic elements that encode for antibiotic resistance, toxins, virulence determining genes, and reduced sensitivity to mutagens such as heavy metals.
  • Plasmid profiling using molecular, biochemical, and microbial techniques is essential to understanding the mechanism of pathogenicity and to fuel genetic engineering.

• Plasmids as Cloning Vectors

  • Plasmids may be conjugative/transmissible or non-conjugative.
  • Nonconjugative plasmids do not mediate DNA through conjugation.
  • Antibiotic resistance: Vectors with antibiotic-resistance allow for survival of cells that have taken up the vector in growth media containing antibiotics through antibiotic selection.
  • The pGEX-3x plasmid is a popular cloning vector.
  • The various elements of the plasmid are labelled.

• Shuttle Vectors and Expression Vectors

  • An expression vector is generally a plasmid that is used to introduce a specific gene into a target cell.
  • An expression vector, otherwise known as an expression construct, is generally a plasmid that is used to introduce a specific gene into a target cell .
  • Cloning vectors, which are very similar to expression vectors, involve the same process of introducing a new gene into a plasmid, but the plasmid is then added into bacteria for replication purposes.
  • The E. coli component of a yeast shuttle vector includes an origin of replication and a selectable marker, such as an antibiotic resistance like beta lactamase.
  • The pGEX-3x plasmid is a popular cloning vector.

• Pili and Pilus Assembly

  • Perhaps the most well-studied is the F pilus of Escherichia coli, encoded by the F plasmid or fertility factor .
  • Typically, the DNA transferred consists of the genes required to make and transfer pili (often encoded on a plasmid), and is a kind of selfish DNA; however, other pieces of DNA often are co-transferred, and this can result in dissemination of genetic traits, such as antibiotic resistance, among a bacterial population.
  • Conjugation diagram 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked, and a single strand of DNA is then transferred to the recipient cell. 4- Both cells recircularize their plasmids, synthesize second strands, and reproduce pili; both cells are now viable donors.