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Chapter: Microbiology and Immunology: Bacterial Genetics

Extrachromosomal DNA Substances

Plasmids are extrachromosomal DNA substances. They are replicons that are maintained as discrete, extrachromosomal genetic elements in bacteria.

Extrachromosomal DNA Substances


Plasmids are extrachromosomal DNA substances. They are replicons that are maintained as discrete, extrachromosomal genetic elements in bacteria. They are usually much smaller than the bacterial chromosome, varying from less than 5 to more than several 100 kbp. However, plasmids as large, as 2 million base pairs can occur in some bacteria. Plasmids are circular and double-stranded DNA molecules that encode traits that are not essential for bacterial viability. They are capable of replicating independently of the bacterial chromosomes. The plasmids can also be present as integrated with bacterial chromosomes, and plasmids integrated with host chromosome are known as episomes. Plasmids are present in both Gram-positive and Gram-negative bacteria.

Types of plasmids

Plasmids depending on their transmissibility and nature of the factor can be of the following types:

Transmissibility of plasmids

Plasmids, depending on transmissibility are of two types: (a) transmissible plasmids and (b) nontransmissible plasmids.

    Transmissible plasmids: They can be transferred fromcell to cell by a process of genetic transfer known as conju-gation. They are large (mol. wt. 40–100 million) plasmids. They contain more than a dozen genes responsible for synthesis of the sex pilus and for the synthesis of enzymes required for their transfer. Usually, one to three copies of the plasmid are present in a cell.

      Nontransmissible plasmids: These cannot be transferredfrom cell to cell, because they do not contain the transfer genes. They are small (mol. wt. 3–20 million), usually non-conjugative, and have high copy numbers (typically 10–60 per chromosome). They depend on their bacterial host to provide some functions required for replication and are distributed randomly between daughter cells at division.

Nature of factors

Depending on the nature of factors, plasmids are of the following types: (a) the F factor, (b) the R factor, and (c) the Col factor.

      The F factor: The F plasmid, also called F factor, is atransfer factor that contains the genetic information, essential for controlling mating process of the bacteria during conjugation.

      The F plasmid of Escherichia coli is the prototype for fertility plasmids in Gram-negative bacteria. Strains of E.coli with an extrachromosomal F plasmid are called F1andfunction 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 25 transfer (tra) genes. These genes determine (a) expression of pili, (b) synthesis and transfer of DNA during mating, (c) interference with the ability of F1 bacteria to serve as recipients, and (d) other functions.

      The F plasmid in E. coli can occur as an extrachromosomal genetic element or be integrated into the bacterial chromo-some. Both the F plasmid and the bacterial chromosome are circular DNA molecules. Hence, reciprocal recombination between them produces a larger DNA circle consisting of F-plasmid DNA inserted linearly into the chromosome.

        The R factor: Resistance factors, also called R factors,are extrachromosomal plasmids. They are circular with double-stranded DNA. R factors occur in two sizes: large plasmids (mol. wt. 60 million) and small plasmids (mol. wt. 10 million). The large plasmids are conjugative “R” factors, which contain extra DNA to code for the conjugation process. The small plasmids contain only the “r” genes and are not conjugative. R factor consists of two components: the resistance transfer factor (RTF) and resistant determinant (r). The RTF is responsible for conjugational transfer, while each r determinant carries resistance for one of the several antibiotics.

Colicinogenic (Col) factor: Col factor is a plasmid thatresembles 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, which are antibiotics-like substances that are specifically and selectively lethal to other enteric bacteria.

·           They also encode for production of diphthericin and pyocyanin produced by Corynebacterium diphtheriae and Pseudomonas pyocyanea, respectively, which are sub-stances similar to colicins.

Functions of plasmids

Many plasmids control medically important properties of pathogenic bacteria. These include (a) resistance to one or several antibiotics, (b) production of toxins, and (c) synthesis of cell sur-face structures required for adherence or colonization. Some plasmids are cryptic and have no recognizable effects on the bacterial cells that harbor them. Comparing plasmid profiles is a useful method for assessing possible relatedness of individual clinical isolates of a particular bacterial species for epidemiological studies.

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