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Griffith's Experiment
In the 1920's Griffith discovered on one of his petri plates that among the many shiny colonies of pneumococcus that there were a few that were dry and shriveled. At first he thought the odd ones were contaminants, but, no, they passed all the classic tests for being pneumococcus. So he wondered if the loss of smooth shine to become dry and rough might have any impact on their pathogenicity in mice. He ran these two tests: he injected them into mice to see what would happen and got these results the next day.
→ 
→ Lots of Smooth
→ → a few RoughJust to be sure, he cultured the two mice, and found, as shown above, lots of smooth bacteria in the dead mouse, and only a few rough bacteria in the live mouse.
He then wanted to know if the smooth type wore something toxic. So he boiled a culture of smooth to kill it and injected that into a mouse. And just for another control, he boiled some rough and injected that also into a fourth mouse:
→ → no bacteria
→ → no bacteriaOf course no living bacteria were expected as he had only injected dead bacteria in the first place.
Now comes the big "cross-over" experiment! He made mixtures of boiled bacteria and their opposite live types, and injected them into mice:
→ → Smooth
→ → SmoothExperiment 5 showed that Rough did not confer any protective effects, and Experiment 6 showed that Smooth conferred pathogenicity.
Thus Griffith found that the dead Smooth form contained something that would convert - or transform - the Rough form into a smooth and lethal form. He called this the "transforming principle," which, because it bred true, was thus a genetic factor.
P.S.: "pneumococcus" is today called Streptococcus pneumoniae.