Griffith's experiment facts for kids
Griffith used two strains of Streptococcus pneumoniae. He then uses the bacteria to infect the mice, which have many similar characteristics to humans. He used a type III-S (smooth) and type II-R (rough) strain. The III-S strain covers itself with a polysaccharide capsule that protects it from the host's immune system. This means that the host will die. The II-R strain does not have that protective shield around it and is killed by the host's immune system.
In this experiment, bacteria from the III-S strain were killed by heat, and their remains were added to II-R strain bacteria. While neither harmed the mice on their own, the blend of the two was able to kill mice.
Griffith was also able to get both live II-R and live III-S strains of S. pneumoniae from the blood of these dead mice. He concluded that the type II-R had been "transformed" into the lethal III-S strain by a "transforming principle" that was somehow part of the dead III-S strain bacteria.
Today, we know that the "transforming principle" Griffith saw was the DNA of the III-S strain bacteria. While the bacteria had been killed, the DNA had survived the heating process and was taken up by the II-R strain bacteria. The III-S strain DNA contains the genes that form the shielding polysaccharide part from attack. Armed with this gene, the former II-R strain bacteria were now protected from the host's immune system and could kill the host.
Griffith's experiment Facts for Kids. Kiddle Encyclopedia.