Griffith's Transformation Experiment
Pneumococcus bacteria include two strains, a virulent S strain with a Smooth coat that kills mice (left), and a non-virulent R Rough strain that does not (middle). Heating destroys the virulence of S (right). When heat-killed S is mixed with live R and injected into mice, the mouse dies. It's tissue are found to contain living bacteria with smooth coats like S, and these bacteria are subsequently virulent to other mice. Griffith concluded that something in the heat-killed S bacteria 'transformed' the biological and especially the hereditary properties of the R bacteria. The nature of this 'transforming principle' was unknown.