- Plasmids are extrachromosomal DNA substances.
- They are typically very smaller than the bacterial chromosome, varying from less than 5 to more than several 100 kbp.
- However, plasmids as giant, as 2 million base pairs can occur in some bacteria.
- Plasmids are circular and double stranded Deoxyribonucleic acid molecules that encode traits that are not essential for bacterial viability.
- Plasmids are capable of replicating independently of the bacterial chromosomes.
- The plasmids can also present as integrated with bacterial chromosomes, and plasmids integrated with host chromosome are referred to as episomes.
- Plasmids are present in both Gram-negative and Gram-positive bacteria.
Types of plasmids
Plasmids depending on their transmissibility and nature of the factor can be of the different types:
Transmissibility of plasmids:
Plasmids, depending on transmissibility are of two types:
- Transmissible plasmids
- Non transmissible plasmids.
- They can be transferred from cell to cell by a process of genetic transfer known as conjugation.
- They are large (mol. wt. 40–100 million) plasmids.
- They contain quite a dozen of genes responsible for synthesis of the sex pilus and for the synthesis of enzymes required for their transfer.
- Usually, 1 to 3 copies of the plasmid are present in a cell.
Non transmissible plasmids:
- These cannot be transferred from cell to cell, because they do not contain the transfer genes.
- They are very tiny (mol. wt. 3–20 million), typically non conjugative, and have high copy numbers (typically 10–60 per chromosome).
- They depend upon their bacterial host to provide some functions needed for replication and are distributed randomly between daughter cells at division.
Nature of factors:
Depending on the nature of factors, plasmids are of these 3 types:
- The F factor
- The R factor
- The Col factor
The F factor:
- The F plasmid, also called F factor, is a transfer factor that contains the genetic information, essential for controlling mating process of the bacteria during conjugation.
- The F plasmid of E.coli is the prototype for fertility plasmids in Gram negative bacteria.
- Strains of Escherichia coli with an extrachromosomal F plasmid are called F+ and function as donors, whereas strains that lack the F plasmid are F− and behave as recipients.
- The conjugative functions of the F plasmid are determined by a cluster of at least twenty five (25) transfer (tra) genes.
These genes determine
- Expression of pili.
- Synthesis and transfer of DNA during mating.
- Interference with the power of F+ bacteria to serve as recipients.
- The F plasmid in E. coli can occur as an extrachromosomal genetic element or be integrated into the bacterial chromosome.
- Both the F plasmid and also the bacterial chromosome are circular Deoxyribonucleic acid molecules (DNA).
- Hence, reciprocal recombination between them produces a bigger DNA circle consisting of F− plasmid Deoxyribonucleic acid molecules (DNA) inserted linearly into the chromosome.
The R factor:
- Resistance factors, also known as R factors, are extrachromosomal plasmids.
- They are circular with double-stranded Deoxyribonucleic acid (DNA).
- R factors occur in 2 sizes: large or big plasmids (mol. wt. 60 million) and small plasmids (mol. wt. 10 million).
- The large plasmids are conjugative “R” factors, that contain extra DNA to code for the conjugation process.
- The small plasmids contain only the “r” genes and don’t seem to be conjugative.
- R factor consists of 2 components: the resistance transfer factor (RTF) and resistant determinant (r).
- The RTF is responsible for conjugational transfer, whereas every r determinant carries resistance for one of the many antibiotics.
Functions of R factor:
- The R factors are responsible for spread of multiple-drug resistance among bacterium.
- They carry the genes for a variety of enzymes that can destroy antibiotics and modify membrane transport system.
- The R factor may carry one antibiotic resistance gene or may carry two or more of these genes.
- The R factor carrying more than two genes has many clinical implications:
- First and foremost, a bacterium carrying such genes will show resistance to more than one type of antibiotics, such as penicillins and aminoglycosides.
- The second importance is that the use of an antibiotic that selects an organism for a bacterium resistant to one antibiotic (such as penicillin) will select for a bacterium resistant to other antibiotics (such as tetracyclines, aminoglycosides, chloramphenicol, erythromycin, etc.).
- They may carry genes for resistance to metal ions.
- For example, the genes code for an enzymes that reduce mercuric ions to elementary mercury thereby making the bacteria resistant to action of mercuric ions.
- They also carry resistance to certain bacteriophages by coding for the enzymes, e.g., restriction endonucleases that degrade the DNA of the infecting bacteriophages.
Colicinogenic (Col) factor:
Col factor is a plasmid that resembles the F factor in promoting conjugation, leading to self-transfer and also at times transfer of segments of chromosomes.
- The Col factor encodes for production of colicins, that are antibiotics like substances which is specifically and selectively lethal to other enteric bacteria.
- They also encode for production of diphthericin and antibiotic drugs produced by Corynebacterium diphtheriae and Pseudomonas pyocyanea, which are just like colicins.
Functions of plasmids
Many plasmids can control or manage medically important properties of pathogenic bacteria.
- Resistance to one or several antibiotics.
- Production of toxins.
- Synthesis of cell surface structures needed for adherence or colonization.
- Some plasmids are cryptic and don’t have recognizable effects on the microorganism cells that harbor them.
- Comparing plasmid profiles is a useful or helpful method for assessing possible relatedness of individual clinical isolates of a selected bacterial or any microorganism species for epidemiological studies.
Plasmids: Types and Function