Avery's experiments showed that bacteria are transformed by:

Avery's experiments showed that bacteria are transformed by "transforming principle" - DNA.

When Griffit in his experiments showed that R-strain bacteria (non-virulent) could be transformed by the heat-killed S bacteria, and become virulent, scientists assumed that there is some kind of "transforming principle" that is involved.

Avery and his colleagues cultured heat-killed S cells and tried t separate cellular components that could possibly be that "transforming principle". Molecule of DNA gave positive result in a chemical test and also when enzymes able to degrade DNA were introduced, the transforming activity was eliminated.