Transformation is one of the three methods of genetic recombinant in bacteria. The other two are transduction and conjugation. Transformation occurs when the bacterial cells take up the DNA and which in turn changes their phenotype.
Transformation requires that there should not be any cell to cell contact. In the experiments performed by Griffith, the heat-killed virulent S strains were mixed with live R strains. The R strain took the DNA of heat-killed S strains which in turn converted them into the virulent strains. Since this horizontal gene transfer did not include any cell-to-cell contact, he called the process transformation.
