R
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Plasmids -
It was long known that repeated exposure of bacteria to antibiotics results in the development of a resistant strain.
Some bacterial cells undergo gene mutation which makes them resistant to the antibiotic.
They live and multiply 10 the presence of the drug, giving rise to a resistant strain.
This type of resistance develops gradually, and can be treated by administering antibiotics before a large number of resistant cells are formed.
Another type of resistance was observed with respect to the antibiotic gentamycin.
Bacterial strains developed resistance to gentamycin only, a few months after its introduction.
The gentamycin-resistant strain was to also found to be resistant to three other antibacterial agents, chloramphenicol, ampicillin and sulfonamide
Growth of the strain with any susceptible strain of bacteria conferred antibiotic resistance to the latter within a few hours. Thus the antibiotic resistance was infectious.
The infectious antibiotic resistance factor or R factor consists of genes producing resistance to the four antibiotics and genes that facilitate the, transfer of the R factor.
They are carried in plasmids called the R (for resistance) plasmids. When a strain carrying R plasmids conjugates with other bacterial cells, a copy of the
R plasmid is transferred to the latter, giving it the property of drug resistance.
R plasmids were first discovered in a strain of Shigella in the late 1950s during an epidemic of dysentery in Japan.
Shigella acquired drug resistance with the introduction of antibiotic therapy in 1946.
By 1964, 50% of shigellas were resistant to all the four major drugs (sulfonamides, streptomycin, tetracycline and chloramphenicol) against dysentery.
Plasmid-mediated drug resistance has serious medical implications. Firstly contact with a drug-resistant non-pathogenic member of the normal gut flora can make a drug-sensitive pathogen resistant.
Secondly, a pathogen may acquire new R plasmid resistances during epidemics.
Plasmid DNA occurs in two forms called co-integrates and aggregates.
The plasmid co-integrate contains all the resistance genes, together with the genes required for transfer, in one circular molecule, e. g. RIGO (Cm, Sm Tu, Tc) isolated from Shigella flexneri.
In plasmid aggregates the various genes are located, at least for some time, on separate molecules, e. g. in one isolate of Salmonella typhimurium type 29.
