Frederick Griffith discovered the transforming principle where a non-virulent strain of bacterium was changed into a lethal strain. Griffith concluded that a "transforming factor" altered the genotype of the non-virulent microorganism.
In 1928, Griffith performed a series of experiments using two live strains of pneumonia bacteria: one pathogenic and the other, non-pathogenic. The smooth coat strain was lethal, while the rough coat strain was non-lethal to mice. Griffith killed the pathogenic strain by applying heat. He then injected the dead smooth cell in combination with the live rough cell into mice. The mice contracted pneumonia and died. Griffith's experiments proved that the genetic make-up of the non-pathogenic strain was altered by one of the components of the heat-killed pneumonia-causing bacterium, causing the rough cell to become pathogenic. In 1944, a group of scientists identified the transforming factor as deoxyribonucleic acid or DNA.