The Effect of Serum upon Dissociation in Brucella abortus : A Demonstration of the Role of Selective Environments in Bacterial Variation

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Abstract
Serum or plasma of normal cows, rabbits, hogs, and goats contains factors which suppress the establishment of rough and brown variants of B. abortus. When serum or plasma, in concs. as low as 2%, was added to smooth (beef extract broth) cultures, which normally would show considerable dissociation after 10 days, no dissociation occurred. It was proved that this suppression of the establishment of dissociated types by serum or plasma is not due to factors which are responsible for the general bactericidal action of sera or plasma.[long dash]These results form the basis for a discussion of the selective role of environments in bacterial variation, the appearance of apparently successive orderly changes during dissociation, the difficulties encountered in reversing the direction of dissociative changes, and the apparently specific adaptation of bacteria under natural conditions.