Frederick Griffith described the conversion of a non-pathogenic pneumococcal bacteria to a virulent strain. In this experiment, Griffith mixed the living non-virulent bacteria with a heat inactivated virulent form. This was at 1928 and by then, people thought that the genetic material was in the proteins, so the DNA had phosphorus compounds (where later MacLeod, McCarthy and Avery experiment would explain that it was really the DNA that caused the genetic material to change and therefore it was the one who stored it). But Griffith's great discovery was the understanding that the union of live R (rough) and dead S (smooth) streptococcus pneumoneae could kill, even though the type of bacteria it killed was S and was dead.
The differentiation of the two pneumonia bacteria is important, as the R-type ones only infect, but the S-type ones are the ones that kill. This is because the type that kills has a gelatinous polysaccharide on the membrane that does not make it possible for it to die (it is a kind of barrier against our antibodies). Once again with this information we can see the picture that tells us that when the two types separately are dead given the boiling do not affect the individual. But when the R-type (does not kill) joins with the S-type (it is dead, but it is the type that kills) they end up killing the individual. This is because there is a "transformation" in the S type.
So, this great "transformation" of live R-type batteries into S was his great discovery.
