When pneumococci are grown repeatedly in broth containing immune serum or bile, they become less virulent and less type-specific. These changes are apparently due to the fact that under the conditions mentioned a number of organisms appear which have lost certain properties. These variant organisms form colonies differing in appearance from the colonies of the typical organisms, and cultures made from the atypical colonies are avirulent and are not type-specific, The degree of modification of the original culture is directly related to the relative number of organisms that have undergone variation. After prolonged growth in bile or homologous immune serum variation may have become manifest in all the bacteria of the cultures and the change is then irreversible. The significance of these findings as regards the epidemiology of pneumococcus infections is noted.

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