When was the first bacterial transformation experiment done?

In 1928, Frederick Griffith demonstrated transformation using pneumococcal bacteria in mice. He found that harmless bacteria could be changed into virulent ones when mixed with heat-killed pathogenic cells. Although the chemical nature of the transforming substance was not yet known, the experiment suggested that a transferable hereditary material existed. This work became a cornerstone in genetics and molecular biology, eventually helping lead to the discovery that DNA is the genetic material.