EVIDENCE FOR THE INDUCED NATURE OF THE CHANGE FROM NONTOXIGENICITY TO TOXIGENICITY IN CORYNEBACTERIUM DIPHTHERIAE AS A RESULT OF EXPOSURE TO SPECIFIC BACTERIOPHAGE

Abstract

The origin of toxigenic C. diphtheriae resulting from the action of diphtheria phage 444V/A on nontoxigenic C. diphtheriae. strain 444A, was investigated. An in vitro toxigenicity testing medium supplemented with an appropriate concn. of antitoxin was used to differentiate between toxin-producing and non-toxin-producing colonies. The former are recognized by the presence of a halo of toxin-antitoxin precipitate around the colony. Changes in the bacterial population occurring after mixing the phage and bacteria in broth were followed. The observed rate of development of the toxigenic population cannot be reasonably explained by a mutation-selection hypothesis. The high concn. of toxigenic mutants required by this hypothesis to be present in the original inoculm (0.07%-0.47%) was not revealed in suitable control expts. In short term expts., 20 to 30-min. exposures of the nontoxigenic strain to bacteriophage followed by immediate plating of aliquots resulted in the development of from 0.1%-7.7% of the phage-infected cells as toxigenic colonies. Controls did not reveal any toxigenic cells in an unexposed aliquot. All toxigenic clones in both types of expts. were found to be lysogenic. On the basis of this evidence it is proposed that the mutation-selection hypothesis be discarded, that the change to toxigenicity is induced, and that the extablishment of the lysogenic state is intimately concerned with this change.