This plasmid is the great choice in medical importance as it leads to the spread of multiple drug resistance among bacteria.
The extra chromosomal mechanism of drug resistance was first reported by Japanese workers (1959), while investigation the sudden increase in infections caused by shigella strains, streptomycin, chloramphenicol and tetracycline.
The plasmid consist of two components – the transfer factor (RTF) which is responsible for conjugation transfer, and a resistance determinant (r) for each of the several drugs.
An R factor can have several r determinants, and resistance to as many as eight and more drugs can be transferred simultaneously.
Some times the RTF may dissociate from the r determinants, the two components existing as separate plasmids. In such case resistance is not transferable. The RTF can have attached to it determinants other than those for drug resistance. Enterotoxin and hemolysis production in some enteropathogenic E. Coli are transmitted by this factor.
Transferable drug resistance is now universal in distribution and involves all antibiotics in common use of antibiotics in the area. Bacteria carrying R factors can be transmitted from animals to man. Hence excess use of antibiotics in veterinary practice or in animal feeds can also lead to an increase of multiple drug resistance in the community.