The Survival of Micrococcus Radiodurans Irradiated at High LET and the Effect of Acridine Orange

Summary The radiation survival curve of Micrococcus radiodurans consists of a long flat shoulder, followed by an exponential part. When acridine orange is added to the growth medium after irradiation, the shoulder is reduced without changing the final slope. A method is described whereby a measure of repair is obtained from the length of shoulder, and a measure of unrepaired damage from the final slope. With changing LET, these two features of the curve differ. The final slope has a definite optimum LET for maximum sensitivity at a value very near to that for yeast and higher organisms. The length of the shoulder steadily increases as LET increases, suggesting that the target for inactivating the repair mechanism is different from the lethal target.