What were Griffith's transformation experiments are?

Griffith's transformation experiments, conducted in 1928, involved studying two strains of Streptococcus pneumoniae in mice: a virulent smooth strain (S) and a non-virulent rough strain (R). He discovered that when he injected mice with heat-killed S strain bacteria mixed with live R strain bacteria, the mice developed pneumonia and died. This indicated that some "transforming principle" from the dead S strain had converted the live R strain into a virulent form. Griffith's work laid the groundwork for later discoveries about DNA as the genetic material.