THE RELATION OF GROWTH TO THE LETHAL DAMAGE INDUCED BY ULTRAVIOLET IRRADIATION IN ESCHERICHIA COLI

Abstract

Strain 15 of E. coli undergoes a temporary restoration of viability when incubated at 37[degree] immediately after subjection to a light dose of UV-irradiation which kills 90% of the cells. The viable count of irradiated cultures increases 3-5 fold within 20 minutes. During the next 25 minutes the number of viable cells decreases sharply. Fifty-five minutes from the start of incubation all restored cells have died and cell number begins increasing with a generation time of 50 minutes as in the control. This represents growth of UV survivors. Death of restored cells was prevented by suspending metabolism; i.e., by the exhaustion of glucose and N or by the use of 5-methyl tryptophan. Restoration was unaffected by these changes. Restoration is markedly sensitive to temperature changes but death of all restored cells occurs independently of temperature. These phenomena were observed with both the parent strain 15 and 15T-, a thymine-requiring mutant. The rate of death of restored cells of 15t- is indistinguishable from thymineless death which unirradiated cells undergo when deprived of thymine. The results are discussed in relation to the unbalanced growth which occurs when inhibitory agents specifically attack deoxyribosenucleic acid synthesis without preventing cytoplasmic syntheses.