In a series of experiments , Griffith infected mice with nonvirulent R -strain bacteria and highly virulent S -strain bacteria . Heat -killed S bacteria were nonvirulent . However , the nonvirulent heat -killed bacteria and the nonvirulent R bacteria together produced a virulent mixture . What did this demonstrate ? A. A transforming principle in the bacteria allowed the nonvirulent R strain to become virulent . B. The molecules present on the outside of bacteria determine whether it's virulent or nonvirulent . C. The S -strain bacteria had not been killed completely . D. The translation of proteins , carbohydrates , lipids , and DNA is important in determining bacterial virulence .