- They are small, circular, self-replicating and double-stranded DNA molecule present in bacterial cell in addition to bacterial chromosome.
- So, plasmids are genetic elements that replicate independently of the host chromosome.
- Unlike the viruses plasmids do not have an extracellular form and exist inside cells simply as free and typically circular DNA.
- Plasmids carry gene that provide special characteristics to bacterial cells. E.g., antibiotic resistance.
- Most plasmids are circular, but many linear plasmids are also known. E.g., Borellia burgdoferi contains 17 different circular and linear plasmids.
- They are approximately 1 kb to 1 mb in size.
- Different plasmids are present in cells in different numbers; this is called copy number.
- Some plasmids are present in cell in only 1-3 copies called low copy number plasmids whereas others may be present in over 100 copies called high copy number plasmids.
- Most plasmids in gram negative bacteria replicate in a manner similar to that of chromosome i.e. bidirectional replication of theta mode.
- Most plasmid of gram positive bacteria replicate by rolling circle mechanism similar to that used by ф x 174.
- When a plasmid is transferred into a cell that already carries another plasmid, the 2nd plasmid may not be maintained and is lost during subsequent replication.
- If this occurs, the two plasmids are said to be incompatible.
- The inability of the two plasmids with the same origin to be maintained in the same cell is known as plasmid incompatibility.
- A number of incompatibilities (Inc) have been recognized.
- The plasmid of one incompatibility group excludes each other from replicating in the cell but can co-exist with plasmids from other groups.
- Some plasmids called episomes can integrate into chromosome, and under such conditions their replication comes under control of the chromosomes.
- Plasmids can sometime be eliminated from host cells by various treatments.
- This removal of plasmid from the host cell is called curing.
- Curing may occur spontaneously, but it is greatly increased by treatments with certain chemicals such as acridine dyes.
- Plasmids carry genes that have profound influence on the cell’s phenotype.
- On the basis of the properties conferred by plasmids to the cell, plasmids are classified as follows. They are:
Image source: microbenotes
A) Sex factor or fertility(F) factor
- F factor was discovered in E. coli for the first time.
- Male cells containing F factor are called F+ and are donors in mating.
- Female cells lack this factor and are labeled F– and are recipient cells.
- F factor confers the capability to transmit chromosomal markers on their host cells.
- F factor mediates host conjugation and its own transfer.
Image source: sciencedirect
B) Resistance plasmids (R plasmids)
- Among the most wide spread and well-studied group of plasmids are R plasmids which confer resistance to antibiotics, drugs, heavy metals, and other growth inhibitors.
- They can be categorized as :
i) Antibiotic resistance plasmids
- R plasmids were first discovered in Japan in strains of enteric bacteria that had acquired resistance to number of antibiotics (multiple-resistance).
- It was correlated with the increasing use of antibiotics for the treatment of the infectious diseases.
- Later, it was shown that the resistant strains could transfer resistance to sensitive strains via conjugation.
- The resistance genes are associated in various combinations as parts of transmissible plasmids known as R factor.
- An R factor consists of two segments of DNA, one the resistance transfer factor (RTF) and the 2nd resistance determinants (r determinants).
- RTF contains genes for replication and transmission of plasmids and r determinants replicate autonomously.
- Several antibiotic resistance genes can be carried by an R plasmid.
- The genes encode proteins that either inactivates the antibiotics or prevents it’s uptake into the cell.
- E.g., plasmid R 100 carries encoding resistance to sulfonamides, streptomycin, spectinomycin, fusidic acid, chloramphenicol, tetracycline, etc.
- Many drug resistant elements on R plasmids such as those on R100 are also transposable elements.
Image source: sciencedirect
ii) Heavy metals resistance plasmids
- There are several bacterial strains that contain genetic determinants of resistance to heavy metals viz- Hg+, Ag+, Cd++, Cu++, Pb++, Zn++
- These determinants for resistance are often found on plasmids and transposons.
- In 1970, in Tokyo, both heavy metal resistance and antibiotic resistance were found.
- Antibiotic resistance plasmids were found in high frequency in E. coli isolated from hospital patients whereas heavy metal resistance plasmids without antibiotic resistance determinants were found in E. coli from an industrially polluted river.
- E.g., plasmid R100 carries several genes conferring resistance to mercury.
C) Bacteriocinogenic plasmids
- They are also called virulence or toxin inducing plasmids.
- Bacteriocins are proteins that inhibit or kill closely related species or even different strains of the same species.
- They have a narrower spectrum of activity than antibiotics and the genes encoding bacteriocins are often carried on a plasmid or transposons.
- E.g., E. coli produces colicins by Col plasmids, Bacillus subtilis produces subtilisin.
- Some of the virulence characteristics are carried on plasmids.
- E.g., Enteropathogenic strain of E. coli has the ability to colonize the small intestine and to produce a toxin.
- Colonization requires presence of a cell surface protein called colonization factor antigen, encoded by plasmid.
- Toxins like hemolysin and enterotoxin in enteropathogenic coli are known to be encoded by a plasmid.
D) Degradative plasmids
- Plasmids have been isolated from a number of cultures of Pseudomonas putida which could degrade a number of organic chemicals which are known as degradative plasmids.
- Special genes present on different plasmids confer degradation capacity to the species of
- E.g., camphor (CAM) plasmid-encode enzyme to degrade camphor.
- Octane plasmid (OCT) degrades octane.
- XYL plasmid degrades xylene and toluene.
- NAH plasmid degrades naphthalene.
- SAL plasmid degrades salicylate.
- TOL plasmid degrades toluene.
- These plasmids are transmissible between the strains of species of putida conjugation.
- Superbug (oil eating bug) was created by transferring the four plasmids (OCT, XYL, CAM, NAH) into one strain of putida.
E) Penicillinase plasmid of Staphylococcus aureus.
- An enzyme penicillinase was found in strains of Staphylococcus aureus which degrades penicillin by hydrolyzing its β- lactam ring.
- It led to the development of several strains of penicillin-resistant aureus.
- The enzyme was encoded by plasmid and not chromosome.
- Penicillinase plasmids may be of type alpha, beta or gamma etc. on the basis of marker’s present on them and production of chemically different penicillinase.
- Penicillinase plasmids also confer resistance to some other antibiotics.
Image source: frontiers
F) Cryptic plasmids
- They are non-functional plasmids.
- Every bacterium carries a low molecular weight DNA as plasmid which does not carry any gene.
- Presence of plasmid seems to be a rule and not an exception.
G) Ti plasmids
- Agrobacterium tumifaciens is oncogenic and causes tumors in dicot commonly called as crown gall disease.
- It is due to the presence of Ti plasmids- tumor inducing plasmids.
- It consists of T-DNA of 20 kb which encodes enzymes for synthesis of auxins and cytokinins which develop tumor and enable the infected plant to produce nitrogenous compound called opines.
- Ti plasmids also contain several genes such as vir, ori, tra, noc (for nopaline catabolism in nopaline plasmid), arc (arginine catabolism), and occ (octapine catabolism in octapine plasmid) gene.
H) Ri plasmids
- Agrobacterium rhizogens causes hairy root disease in plants.
- It is due to presence of Ri plasmid.
- Ri plasmids are closely related to Ti plasmid.
- They are large sized plasmids of 190-240 kb.
- Ri plasmids are put in 3 groups. They are: mannopine Ri plasmid, agropine Ri plasmid, and cucumopine Ri plasmids.
Plasmids and its types