By definition transformation is a process of genetic alteration in a bacterium associated with an uptake of a naked exogenous DNA. To uptake a naked DNA a bacterium must be in a special state called ‘competent’. Usually a bacterium is not competent for transformation. Competency is induced by treating bacteria grown to logarithmic phase with divalent cation at low temperature. The exact mechanism of competence of bacteria is not understood. It is thought that the competent bacteria carry a special conformation in phospholipid bilayer in the outer membrane which enables bacteria to uptake DNA when incubated with naked DNA and briefly treated with heat shock at 42ºC. Because not all competent cells can uptake the DNA a selection step following the treatment is necessary to select the transformants.