Griffith thought some "transforming principle" from the heat-killed strain was responsible for making the harmless strain virulent. In 1944 this transforming principle was identified as being genetic by Oswald Avery, , and Maclyn McCarty. They isolated DNA from a virulent strain of "S. pneumoniae" and using just this DNA were able to make a harmless strain virulent. They called this uptake and incorporation of DNA by bacteria 'transformation'. See Avery–MacLeod–McCarty experiment.
ɡ ɹ ɪ f ɪ θ WORD_BOUNDARY θ ɔ t WORD_BOUNDARY s ʌ m WORD_BOUNDARY t ɹ æ n s f ɔ ɹ m ɪ ŋ WORD_BOUNDARY p ɹ ɪ n s ɪ p ə l WORD_BOUNDARY f ɹ ʌ m ð ə WORD_BOUNDARY h iː t k ɪ l d WORD_BOUNDARY s t ɹ eɪ n WORD_BOUNDARY w ʌ z WORD_BOUNDARY ɹ ɪ s p ɑ n s ɪ b ə l WORD_BOUNDARY f ɔ ɹ WORD_BOUNDARY m eɪ k ɪ ŋ WORD_BOUNDARY ð ə WORD_BOUNDARY h ɑ ɹ m l ə s WORD_BOUNDARY s t ɹ eɪ n WORD_BOUNDARY v ɪ ɹ ə l ə n t WORD_BOUNDARY ɪ n WORD_BOUNDARY n aɪ n t iː n h ʌ n d ɹ ɪ d WORD_BOUNDARY f ɔ ɹ t i WORD_BOUNDARY f ɔ ɹ WORD_BOUNDARY ð ɪ s WORD_BOUNDARY t ɹ æ n s f ɔ ɹ m ɪ ŋ WORD_BOUNDARY p ɹ ɪ n s ɪ p ə l WORD_BOUNDARY w ʌ z WORD_BOUNDARY aɪ d ɛ n t ɪ f aɪ d WORD_BOUNDARY æ z WORD_BOUNDARY b iː ɪ ŋ WORD_BOUNDARY d̠ʒ ɪ n ɛ t ɪ k WORD_BOUNDARY b aɪ WORD_BOUNDARY ɑ s w ə l d WORD_BOUNDARY eɪ v ə ɹ ɹ i WORD_BOUNDARY æ n d WORD_BOUNDARY m ə k l ɪ n WORD_BOUNDARY m ə WORD_BOUNDARY k ɑ ɹ t i WORD_BOUNDARY ð eɪ WORD_BOUNDARY aɪ s ə l eɪ t ɪ d WORD_BOUNDARY d iː ɛ n eɪ WORD_BOUNDARY f ɹ ʌ m WORD_BOUNDARY ʌ WORD_BOUNDARY v ɪ ɹ ə l ə n t WORD_BOUNDARY s t ɹ eɪ n WORD_BOUNDARY ʌ v WORD_BOUNDARY ɛ s WORD_BOUNDARY n uː m oʊ n ɪ iː WORD_BOUNDARY æ n d WORD_BOUNDARY j uː z ɪ ŋ WORD_BOUNDARY d̠ʒ ʌ s t WORD_BOUNDARY ð ɪ s WORD_BOUNDARY d iː ɛ n eɪ WORD_BOUNDARY w ɜː ɹ WORD_BOUNDARY eɪ b ə l WORD_BOUNDARY t ə WORD_BOUNDARY m eɪ k WORD_BOUNDARY ʌ WORD_BOUNDARY h ɑ ɹ m l ə s WORD_BOUNDARY s t ɹ eɪ n WORD_BOUNDARY v ɪ ɹ ə l ə n t WORD_BOUNDARY ð eɪ WORD_BOUNDARY k ɔ l d WORD_BOUNDARY ð ɪ s WORD_BOUNDARY ʌ p t eɪ k WORD_BOUNDARY æ n d WORD_BOUNDARY ɪ ŋ k ɔ ɹ p ə ɹ ɹ eɪ ʃ ə n WORD_BOUNDARY ʌ v WORD_BOUNDARY d iː ɛ n eɪ WORD_BOUNDARY b aɪ WORD_BOUNDARY b æ k t ɪ ɹ i ə WORD_BOUNDARY t ɹ æ n s f ɔ ɹ m eɪ ʃ ə n WORD_BOUNDARY s iː WORD_BOUNDARY eɪ v ə ɹ ɹ i WORD_BOUNDARY m æ k WORD_BOUNDARY l aʊ d WORD_BOUNDARY m ə WORD_BOUNDARY k ɑ ɹ t i WORD_BOUNDARY ɛ k s p ɛ ɹ ɪ m ə n t WORD_BOUNDARY