Bacterial Transformation

Back in the early 20th century, a British bacteriologist named Frederick Griffith was working with the pneumonia-causing bacteria Streptococcus pneumoniae. He performed a simple experiment using two different strains. One strain is known as the S strain because of a protective capsule that makes the colonies or clumps it forms appear smooth, and also makes it virulent or harmful. The second was the R strain, a version of the bacteria lacking the protective capsule, giving the colonies a rough appearance and rendering it non-virulent.

First, Griffith took some of the S strain bacteria and heated it, producing a heat-killed version of the S strain. Then, he gathered some mice and divided them into four groups. He injected the first group with the virulent S strain, and the second with the non-virulent R strain. He dosed the third group with the heat-killed S strain, and finally, be combined the heat-killed S strain and the R strain together, and injected this mix into the fourth group. As expected, the mice in the first group died, and the ones in the second and third group lived. But to Griffith's surprise, the mice in the last group also died.

When he published his study in 1928, h