How were dead S-type cells able to transform living
R-type cells?

Griffith concluded that in the presence of the dead S-type pneumococcus cells, some of the living R-type cells had been transformed into virulent S cells. These cells were able to grow in the bodies of the mice, causing pneumonia and multiplying in the blood. ... The DNA alone caused bacterial transformation.

Griffith introduced heat-killed 'S'- strain bacteria Streptococcus in to a mouse along with live 'R' strain bacteria. ... Some 'transforming principle' transferred from the heat-killed 'S' strain had enabled the 'R' strain to synthesise a smooth polysaccharide coat and become virulent.

In this experiments, Griffith injected mice in the lab with live R-type of bacteria. They did not suffer from the disease. ... He thus concluded that heat-killed smooth type bacterial caused a transformation of the living rough type bacteria. This experiment suggested that DNA and not proteins are the genetic material.