English bacteriologist whose studies of pneumococci, the bacteria that cause pneumonia, suggested that bacteria could undergo some kind of genetic transformation. The virulence of pneumococci depends on the presence of a polysaccharide capsule. Virulent strains with a polysaccharide coat were known as type S; bacteria without the capsule were type R. When mice were inoculated with a mixture of living R and heat-killed S bacteria, they became infected. Griffith was able to isolate living S bacteria from these animals. Later researchers proved that the transforming factor was DNA.