The methods employed to isolate, modify, and transfer genes from one organism to another depend on rDNA technology. This technology takes advantage of the fact that some bacteria species have extra-chromosomal DNA, called plasmids.
Plasmids contain only a few genes and they can be exchanged between individual bacteria.
In nature, plasmids carry genes that are beneficial to the bacterium. For example, some plasmid genes make the bacterium resistant to antibiotics.