Transforming principle was an early name for DNA. In 1928, scientists didn't know yet that DNA carried genetic information, but they knew that there was something that could cause bacteria to transform from one type to another.
Transformation occurs when one bacterium (you know, those microscopic, single-celled creatures) picks up free-floating DNA and incorporates it into its own genome. The idea of the transforming principle was discovered during an experiment by Frederick Griffith.
