Plasmids

 

Plasmids are small, circular molecules of DNA that are capable of replicating independently. They do not depend on chromosomal DNA of the organism for replication so are also referred to as extra-chromosomal DNA

Plasmids are made up of circular double chains of DNA and measure between a few kilobases and several hundred kilobases. Some plasmids have a linear structure and do not form a circular shape.

 

Plasmids provides certain properties to bacteria and can get transferred from one bacteria to another through a process known as conjugation (contact between cells that is followed by transfer of DNA content). There are different types of plasmids like,

Resistance Plasmids-Also referred to as antimicrobial resistance plasmids, resistance plasmids are a type of plasmids that carry genes that play an important role in antibiotic resistance. They are also involved in bacterial conjugation by producing conjugation pili which transfer the R plasmid from one bacterium to another.

Degradative Plasmids-These enable the host organism to degrade/break down xenobiotic compounds. Also referred to as recalcitrant substances, xenobiotic compounds include a range of compounds released into the environment as a result of human actions and are therefore not naturally occurring or common in nature. Degradative plasmids are found in Burkholderia spp, Escherichia coli, Pseudomonas fluorescens etc

Fertility Plasmids- Fertility plasmids (F plasmid) plays an important role in reproduction and they contain genes that code for the production of sex pilus as well as enzymes required for conjugation. F plasmid also contains genes that are involved in their own transfer.

Col Plasmids-Col plasmids confer to bacteria the ability to produce toxic proteins known as colicins. Bacteria as E. coli, Shigella and Salmonella use these toxins to kill other bacteria.

Virulence Plasmids -Pathogenic bacteria carry genes for virulence factors that allow them to invade and infect their respective hosts. The virulence factors are the result of the organisms' own genetic material or due to extra-chromosomal DNA such as transposons, plasmids etc

Most of these plasmids can be transmitted from one bacterium to another.

Some of the other types of plasmids include:

• Recombinant plasmids - Plasmids that have been altered in the laboratory and introduced into the bacteria for the purposes of studies
• Cryptic plasmids - No known functions
• Metabolic plasmids - Enhance metabolism of the host
• Conjugative plasmids - Promote self-transfer
• Suicide plasmids - Fail to replicate when transferred from one cell to another