The bactericidal action of rifampicin was compared with that of chloramphenicol in growing and in sporulating cultures of Bacillus subtilis 168. Chloramphenicol kills cells only very slowly, but exposure to rifampicin kills over 95% of cells in a few minutes, causing gross physical damage, which is visible in both phase-contrast and electron microscopy. This is accompanied by a fall in O2 consumption and by lysis. Experiments with synchronized cultures showed that susceptibility to the lethal effect of rifampicin is greater when the cells are dividing. The results suggest that the synthesis of some species of RNA other than mRNA may be necessary for the maintenance of cell integrity, although experiments with actinomycin D do not altogether fit this interpretation. However, we conclude that rifampicin is too toxic to use as an antibiotic for assessing the lifetime of mRNA.

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