EXPERIMENT   Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsule and are nonpathogenic.

 RESULTS   When Griffith mixed heat-killed pathogenic bacteria with living bacteria of the nonpathogenic strain, some of the living cells became pathogenic.

 CONCLUSION   Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by an unknown, heritable substance from the dead S cells.