Why are plasmids useful for bacteria


Plasmids are small rings made of double-stranded DNA that, although not essential for life, carry genes that are beneficial, often for resistance to certain antibiotics. Bacteria can take up plasmids, copy them independently of the main DNA and transfer them to other bacteria. In this way, antibiotic resistance can easily spread. The transfer from one bacterial species A to another bacterial species B is also possible (conjugation). This type of transfer of genetic information is also known as horizontal gene transfer - in contrast to vertical gene transfer from parents to children or from a bacterial cell to its two daughter cells.

Restriction endonucleases can be used to cut open strands of DNA precisely

In genetic engineering, plasmids play a very important role as vectors for the transfer of passenger DNA. Incidentally, the term "plasmid" was coined in 1952 by LEDERBERG, who received the Nobel Prize for Medicine in 1958 for his discovery in 1946.

Types of plasmids

There are three types of plasmids:

  • Resistance plasmids. These are the plasmids of interest for genetic engineering. These plasmids contain one (or more) genes for antibiotic resistance, for example resistance to ampicillin or tetracycline.
  • Virulence plasmids. These are plasmids that are interesting for medicine. These plasmids contain genes that can trigger diseases in the infected host organism. Usually virulence plasmids contain one or more genes that encode enzymes that produce toxins. The toxins (poisonous substances) then damage the infected host cell.
  • Fertility plasmids. These are plasmids that carry the so-called F-factor (fertility factor). A bacterial cell with an F plasmid is able to form a plasma bridge and thus transfer its plasmids to another bacterial cell. This process is also called conjugation.

Recent findings

According to a report by the Max Planck Society, the transfer of plasmids for bacteria can "backfire". This is because plasmids can contain not only genes that are useful for bacteria, but also harmful genes, for example for toxins. One of these toxins, the zeta toxin, for example, converts a sugar that is important for the structure of the bacterial cell wall into a poisonous molecule.

Source: "Bacteria poison themselves from within", Max Planck Society of March 22, 2011.