<P> Pneumonia was a serious cause of death in the wake of the post-WWI Spanish influenza pandemic, and Griffith was studying the possibility of creating a vaccine . Griffith used two strains of pneumococcus (Streptococcus pneumoniae) bacteria which infect mice--a type III - S (smooth) which was virulent, and a type II - R (rough) strain which was nonvirulent . The III - S strain synthesized a polysaccharide capsule that protected itself from the host's immune system, resulting in the death of the host, while the II - R strain did not have that protective capsule and was defeated by the host's immune system . A German bacteriologist, Fred Neufeld, had discovered the three pneumococcal types (Types I, II, and III) and discovered the Quellung reaction to identify them in vitro . Until Griffith's experiment, bacteriologists believed that the types were fixed and unchangeable, from one generation to another . </P> <P> 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 alone harmed the mice, the combination was able to kill its host . Griffith was also able to isolate both live II - R and live III - S strains of pneumococcus from the blood of these dead mice . Griffith 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 . </P> <P> Today, we know that the "transforming principle" Griffith observed 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 protective polysaccharide capsule . Equipped with this gene, the former II - R strain bacteria were now protected from the host's immune system and could kill the host . The exact nature of the transforming principle (DNA) was verified in the experiments done by Avery, McLeod and McCarty and by Hershey and Chase . </P>

Why were griffiths s strains able to produce a polysaccharide capsule while the r strains could not
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