Lethality and mutagenesis of E. coli cells irradiated with soft X-rays

Suzaka, Takeshi; Shikazono, Naoya; Fujii, Kentaro; Yokoya, Akinari; Tauchi, Hiroshi*

Soft X-rays have been widely used for the diagnostic of breast cancer (mammography). It has been generally thought that the relative biological effectiveness (RBE) of soft X-ray irradiation to living cells is similar to those of higher energy X-ray or -ray irradiation. On the other hand, Hill indicates that the risk of the soft X-ray exposure to organs may be higher than that of the -ray exposure because the interaction of soft X-ray photons with matter generates low energy secondary electrons. These electrons deposit their energy within a small volume of the scale of micrometers, thus induce dense ionization or excitation in genome DNA, whereas much less is known about the detail of the biological effect of soft X-rays. Recently we have shown that the yields of DNA strand breaks and oxidative base lesions induced in plasmid DNA by the direct effects of soft X-rays (150 kVp) is significantly higher than those by -ray irradiation. Present study investigates the effect of soft X-ray (150 kVp, W-target) irradiation to E. coli cells to highlight cellular responses. For this purpose, we have developed a novel experimental method to examine cell lethality and mutation frequency. A thin layer (100m) of cell-sample was prepared on a membrane so that all cells are uniformly irradiated with soft X-ray photons which have a short range in biological materials. We chose two genes, namely rpsL and mutant lac gene, as indexes of mutagenesis. Since experimental conditions strongly affected the survival of the cells, we have established optimal conditions for the experiment, such as temperature of solutions and cell density during culture. Using this method, we obtained the dose responses of survival fraction and mutation frequency of E. coli cells irradiated with soft X-rays. The correlation between DNA damage and the cellular effect of soft X-rays will be discussed.