Effect of miracil D on marker frequency ratio and cytotoxicity in Bacillus subtilis.

When Miracil D was added to mid-log phase Bacillus subtilis cells, the rate of growth decreased immediately, and the turbidity of the culture began to decrease within 15 to 20 min after addition of the drug. At this stage, ghostlike cells were observed under phase-contrast microscopy. The viable count also began to decrease rapidly after 15 min in the presence of the drug, and within 60 min there was a 10,000-fold reduction in viability. Incorporation of (3)H-thymidine into deoxyribonucleic acid (DNA) proceeded normally up to 20 min of exposure to the drug, after which incorporation ceased. Cells which were prelabeled with (3)H-thymidine and exposed to the drug released labeled DNA into the medium after 15 to 20 min. Results of transformation analyses with donor DNA from cells grown in the presence of Miracil show a decrease with time in the ratio of origin markers to terminus markers. Electron micrographs of sectioned cells grown for 5, 10, and 15 min in the presence of Miracil D show profound cytotoxic effects. The most striking aspect of such cells is the very condensed appearance of the nucleoid. In those cells about to divide, the nucleoid appears not to be properly separating, being strung out in the region of the developing septum. Finally, the mesosome is very poorly defined or entirely absent in Miracil-treated cells.