Transformation of nonvirulent bacteria to virulent bacteria by DNA. When injected into mice, (a) the encapsulated strain of pneumococcus (Streptococcus pneumoniae), producing smooth colonies, is lethal, whereas (b) the nonencapsulated strain, producing rough colonies, and (c) the heat-killed encapsulated strain are harmless. (d) Griffith’s research had shown that adding heat-killed virulent bacteria to a live nonvirulent strain (each harmless to mice on their own) permanently transformed the live strain into lethal, virulent, encapsulated bacteria. (e) Avery and his colleagues extracted the DNA from heat-killed virulent pneumococci, removing RNA and protein as completely as possible, and added this DNA to nonvirulent bacteria, which were permanently transformed into a virulent strain.