SUMMARY: Sublethal doses of rifampin (0.005 μg/ml), added to vegetatively growing cultures of a sporogenic mutant of at inoculation time or after 4 h, resulted in a decrease of growth and in blockage of spore formation. But when rifampin was added 6 to 24 h after inoculation, normal growth and sporulation occurred, indicating that the time of addition was critical and that rifampin was most effective on rapidly dividing, exponential-phase cells. Ultrastructural studies showed that when rifampin was added at the time of inoculation, endospore development was blocked at stage III. During subsequent incubation (> 10 h) the cells lost their rigidity, and lysis of the mother cell was followed by that of the forespore. When the cultures were treated with rifampin at 4 h, about 40 % of the cells were blocked at stage III and about 60 % reached stages IV and V. Some showed excessive elongation and contained developing spores at each pole. They appeared to be derived from two daughter cells unable to form a division septum because of a specific inhibitory effect of rifampin on division. It would seem, therefore, that two daughter cells which are genetically coded to form endospores will do so irrespective of the development of a division septum, and the spores are formed at the ‘old’ polar regions.


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