A previously reported form of microbial persistence whereby large populations of tubercle bacilli can be made to "vanish" uniformly from the tissues of mice has been shown to occur generally throughout each group of animals subjected to the experimental procedure; it does not reflect the eradication of the bacilli in the majority of animals with their persistence and ultimate revival in only a minority. The one demonstrable alteration of the tubercle bacilli while "vanished" is that they are sterile. Thus, they are undetectable by cell-free culture, tissue culture, and blind animal passage, i.e. by any method based on microbial multiplication. Whether they have also undergone alteration in morphology and persist in some unconventional form cannot be stated. Acid-fast forms similar to tubercle bacilli can be detected in small numbers by intensified microscopic search of tissue homogenates but the relationship of these forms to the sterile bacilli that ultimately revive is unclear. Thus, the persisting tubercle bacilli are more correctly designated as being in a "sterile state" than one of true latency. The uniform induction of the sterile state is a specific phenomenon requiring the participation of both the nicotinamide derivative, pyrazinamide, and isoniazid. Once assumed, this sterile state is relatively stable and the time required for revival of the tubercle bacilli in the spleens in one-half the animals is seven months. This process can be speeded up by the administration of large doses of cortisone in the third or fourth month after sterilization but revival is not significantly affected by the administration of cortisone earlier.

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