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Plasmids And Drug Resistance

	Plasmids And Drug Resistance -Antibiotic resistance plasmids (R factors) are found in several bacterial genera, both gram negative and gram positive. Because of their wide distribution in bacteria, and because they confer multiple resistance, R factors are of medical importance. R plasmids consist of two major genetic elements, the transfer factor (T) and the element containing the information for drug resistance. The transfer factor is responsible for the autonomy of the plasmid as a replicon and its transferability during conjugation. Plasmids carrying the transfer factor are called conjugative R plasmid.
They have the capacity of self transmission. Plasmids carrying the drug resistance markers, but not possessing the ability to be transferred during conjugation, are called nonconjugative plasmids(r). Plasmids in gram positive bacteria are generally nonconjugative. Mitsuhashi et.al (1973) proposed that nonconjugative plasmids may have evolved from bacteriophages by recombination that resulted in aquisition of drug resistance genes, but caused a loss of phage virulence.
Drug resistance markers can be transferred from both Rand r plasmids to the bacterial chromosome, the F factor, the F-lac episome, the T plasmid DNA and the genome of bacteriophages. Specific insertion sequences (the IS elements) enable the drug resistance determinants to dissociate from plasmids and recombine with different DNA molecules. The IS elements enable the drug resistance determinants to be transposed as well defined units from one site to another. Many resistance genes are unstable, and are rapidly lost from most cells in a bacterial population in the absence of antibiotics. Some bacterial strains can, however, maintain and inherit resistance plasmids for generations. This suggests that plasmid genes are neutral, or at least not disadvantageous, in a stable environment, but enhance the survival of the bacterial host under adverse conditions.
The bacterial host is provided with extra genetic capacity to cater for entirely new biochemical reactions. These genes could not have been obtained by mutation of chromosomal genes.

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