Frederick griffith was one of the leading scientists looking at the genetic role of dna. he was working on two strains of streptococcus pneumoniae: the rough strain (r) did not cause pneumonia, but the smooth strain (s) did. when griffith heat-killed the s strain and gave both the heat-killed s strain and the r strain to mice, the mice developed pneumonia and died. what happened in this experiment that caused the mice to develop pneumonia?

Transformation. Some bacterial strains have the ability to incorporate other bacteria's genetic material if it is present in the environment. What happened in this case is that the smooth strain bacteria's DNA, even after heat treatment that killed the bacteria themselves, persisted in the medium shared with the rough strain. The rough strain bacteria incorporated the genes responsible for virulence and themselves became virulent. As a result, the mice injected with this mixture of heat-killed smooth and still living rough bacteria still died, as the latter strain gained the genetic qualities of their conspecifics.