Production of radiation-resistant E. coli strains by daily X-irradiation.

Exposure of E. coli AB1157 (a K-12 wild-type strain) to very large, daily X-ray doses has produced mutant strains resistant to both X-rays and UV photons. Results reported here are with KS0(160), the most resistant strain isolated thus far. Relative to its parent, KS0(160) is about 2.3 x more resistant to X-rays and about 2.2 x more resistant to ultraviolet photons (ratios of sensitivities in air). Two other characteristics of KS0(160) make its responses to X-rays different from those of AB1157: KS0(160) has an oxygen enhancement ratio of only 1.8 compared with 2.7 for its parent; glycerol reduces the sensitivity of AB1157 by about 75% (in air), but the maximum effect in KS0(160) is only a 49% reduction in response. P1 transduction experiments showed that all the acquired resistance in KS0(160) is lost when SOS repair activity is genetically blocked by an inserted lexAl, indicating that the mutation(s) associated with the acquired resistance in KS0(160) are in wild-type genes involved with induced DNA repair (i.e. SOS repair activity).