How was the appearance of the disease causing bacteria (S strain) different from the harmless bacteria (R Strain)?

S strain have a capsule (polysaccharides) that protects them from the defense mechanisms of infected animals. In in vitro culture, they form colonies with a smooth appearance. On the other hand, R Strain do not have the capsule present in the S cells and do not cause pneumonia. In in vitro culture, they form colonies with a rough appearance.

Explanation:

In 1928, the microbiologist Fred Griffith, who was researching various strains of pneumococcus, injected mice with the S strain and the R strain of the bacteria. The S strain was harmful, while the rough (R) strain was not. When, inactivated by heat, the S strain was injected, there were no sequelae and the mouse lived. Surprisingly, when combining strain R (non-lethal), with strain S inactivated by heat (non-lethal), the mouse died. In addition, Griffith found live S-strain cells. R Strain apparently became S strain. This finding could not be explained, until in 1944 Avery, Mc Leod, and Mc Carty, cultured S strain and produced cell lysate extract (cell-free extract) and then lipids, proteins and polysaccharides were removed, the streptococcus still retained its ability to replicate its DNA and introduce it into pneumococcus R. Griffith heat inactivation would have left the DNA of the bacteria chromosomes intact, which was the cause of formation of the S gene, and could be released by the destroyed cells and implanted in successive cultures of R Strain.

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