In the field of biology, a plasmid is known as a DNA fragment, generally circular and with a double helix, which are 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 note that these molecules are not important for the existence of cells, however there are certain moments, in which their presence can provide essential genetic information to condense some important protein, such as antibiotics.
Certain plasmids can form chromosomal DNA, these are known as integrative plasmids. Once they are integrated into the DNA, they are modified and are called episomes. The episomes usually duplicate in each cellular fragmentation, integrating to the elemental genetic information of the bacterium.
Plasmids can be transmitted between different bacteria through a process called bacterial conjugation. Through this process, plasmids are transferred from a cell called "donor" to another cell that plays the role of receptor. Conjugation requires a direct interaction between both cells.
Plasmids can be classified according to:
- Its 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 grant resistance to certain antibiotics, which if not, would kill the host cell. These plasmids are responsible for the majority of bacteria showing resistance to all the antibiotics that are usually used daily.
- Fertility plasmids, known as F11 factors, are those that have genes related to the conjugation capacity. In addition to this, they encompass segments known as insertion sequences, which are responsible for facilitating the binding of the plasmid with the host's chromosomal DNA.
- Bacteriocinogenic plasmids, these are elements that are secreted by the bacteria itself to kill other bacteria. This type of plasmids provide bacteria with the protection they need against certain types of bacteriocin.
