Transformation occurs when a cell takes up genetic material, usually exogenous DNA, from its surroundings and undergos genetic alteration. It can occur naturally or be induced artificially. The most common known incidences of transformation occurs in bacteria, such as Escherichia coli
Bacterial transformation was first discovered by Frederick Griffith, a British biologist, in 1928. He experimented with a rough strain and a smooth strain of Streptococcus pneumoniae.The rough strain of the bacteria could be killed by the immune system of a mouse. The smooth strain had a outer coat which prevented it from being killed by the immune system, it could infect and kill mice. He discovered that when he injected mice with a rough strain of S. pneumoniae and a heat-killed smooth strain of S. pneumoniae. This "transforming principle" was later discovered by Oswald Avery, Colin MacLeod, and Maclyn McCarty in 1944. They used the enzymes protease, RNase, or DNase on the rough and heat-killed smooth strains before injecting the strains into three saparate groups of mice. They discovered that the the strains that had protease or RNase used on them still killed the mice, but the strains that had DNase used on it did not infect the mice. They concluded that DNA was the cause of bacteria transformation.