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.