The first evidence of this process was seen in 1928 when Fred Griffith noted that nonencapsulated Streptococcus pneumoniae (“R” strain) could be changed to the capsule- producing form (“S” strain).
This was done by culturing live “R” strain cells in media containing dead capsule-producing streptococci. Today a number of other genera have been identified that may also undergo transformation. E. coli, Haemophilus, Bacillus, and Pseudoinonas are all able to be transformed. However, cells do not simply “take in” extra-chromosomal DNA.
Those microbes with the genetic ability to take up naked DNA and be transformed are called competent cells. Competent cells may operate naturally or they may be artificially stimulated to take in the DNA by altering the environment in which the culture is being grown.
For example, E. coli can only become competent by being cultured in media with a high concentration of calcium ions.
Transformation occurs in three stages. A large segment of DNA is first bound to a special receptor site on the surface of the competent cell. The segment is then cut into smaller, more manageable pieces by a DNAase enzyme released by the recipient.
Finally, the attached segment of DNA is actively moved into the cell where it is prepared for recombination with the endogenote. The transformation process plays an important role in forming new gene combinations and creating genetic variety in microbes; however, conjugation probably plays a more important role.