EXPERIMENT Bacteria of the S (smooth) strain of Streptococcus pneumoniae are pathogenic because they have a capsule that protects them from an animals 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.