In the field of biology, plasmids are known as DNA fragments, generally circular and double helix, found in the cytoplasm of bacteria. Its size can vary between 3 and 10 Kb and a single bacterium can present from one to more than 100 copies of a single plasmid.

It is important to highlight that these molecules are not important for the existence of cells, however there are certain moments where their presence can provide essential genetic information to condense some important protein, such as antibiotics.

Certain plasmids can make up chromosomal DNA, these are known as integrative plasmids. Once they are integrated into the DNA, they are modified and are called an episome. The episomes are usually duplicated in each cell fragmentation, integrating the elemental genetic information of the bacterium.

Plasmids can be passed between different bacteria through a process called bacterial conjugation. Through this process, the plasmids are transferred from a cell called “donor” to another cell that plays the role of recipient. The conjugation requires a direct interaction between both cells.

Plasmids can be classified according to:

  • Their conjugation capacity: conjugative and non-conjugative plasmids. The former are characterized by having sufficient capacity to transfer between cells. While the latter lack the essential genetic information to start the transfer process.
  • Resistance plasmids, so called because they contain the genetic data necessary to confer resistance to certain antibiotics, which would otherwise kill the host cell. These plasmids are responsible for most bacteria showing resistance to all antibiotics that are used daily.
  • Fertility plasmids, known as F11 factors, are those that have genes associated with conjugation capacity. In addition to this, they encompass segments known as insertion sequences, which are responsible for facilitating the binding of the plasmid to the host’s chromosomal DNA.
  • Bacteriocinogenic plasmids, these are elements that are secreted by the bacteria itself to kill other bacteria. This type of plasmid gives bacteria the protection it needs against certain types of bacteriocin.