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Resistance Transfer Factor RTF - Bacteriocin Plasmids

	Resistance Transfer Factor RTF - Bacteriocin Plasmids The discovery of RTF was first made in Japan during an outbreak of bacterial dysentery when a strain of Shiegella dysentriae was found to be simultaneously resistant to sulphonamides, streptomycin, chloram phenicol and tetracycline. Subsequently it was found that this simultaneous resistance to four drugs could also be lost by a single mutation. Since then, multiple drug resistant bacteria have been isolated from every part of the world and it know well established that this property is controlled by the RTF. The number of inhibitory substances for which resistance may be mediated by R-factors has now grown to ten or more antibiotics and several heavy metals such as Hg, Cd, Ni, and Co. It bas also been found that different R factors have different combinations or resistance genes ranging from 1-8.
The RTF consists of two components, one called the RTF.(resistance transfer factor) carrying genes for replication and transmission of the plasmid and the second consisting of one or more sequentially linked R determinants (resistance determinants). The RTF region of several R-factors has been characterized and found to have molecular weights as large as 60 million daltons while the R-determinants have molecular weights of about ten million.
These two segments of the RTF have been found to have different G+C content. Several R-factors are also capable of dissociating into independently replicating RTFs and R-determinants. The latter, appear to be replicons but arc incapable of being transferred unless they are associated with the RTF. The stability of the association has been found to vary with the R-factor and the host. Some R-factors are stable in one host but dissociate readily in another. In nature, bacteria may contain either the R-determinants or only the RTF. Such strains can be detected by simple techniques which Anderson and his coworkers used to find Salmonella strains carrying RTF. In this a tester strain was produced by early interruption of mating between a R-factor donor which contained an R-determinant but not RTF. This tester was then mated with a potential RTF donor or with a drug sensitive recipient.
The transfer of an RTF from the donor was detected by the acquired ability of the tester to transfer its drug resistance to another recipient. Some R-determinants are capable of autonomous replication but not of self transfer. The genes controlling the conjugation process are located only on RTF since only RTF carries genes for replication and transmission, of the plasmid.
	Plasmids isolated from bacteria fall into a number of compatibility groups. A compatibility group is defined as a group of plasmids which can stably establish in the same cell and is not related to each other. That is, only plasmids representing different compatibility groups can co exist and replicate side by side in the same host. This phenomenon of incompatibility reflects competition between members of a given group for common host cell functions for replication. As a Consequence even if a cell has two incompatible plasmids to begin with later as replication proceeds only one type is selected and established. Compatibility and incompatibility is therefore, more of a reflection of interaction between the plasmids and the cell. Naturally occurring bacteria can have as many as 7-8 different plasmids

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