抄録
In connection with food irradiation in the frozen state, radiation sensitivity of Micrococcus radiodurans, an extremely radioresistant vegetative bacterium, was determined under different conditions of irradiation. Stationary- phase cells suspended in 0.067M phosphate buffer were irradiated at liquid nitrogen temperature (-196°C) and also at room temperature. The induction dose and D10 value were 6.12 Mrads and 0.53 Mrads at liquid nitrogen temperature and 0.87 Mrads and 0.11 Mrads at room temperature, respectively. The protection factors afforded by freezing were 7.0 for the induction dose and 4.8 for the D10. In comparison with the protection factors obtained with other species of vegetative bacteria which have been previously reported (2.5-8.5), the factor of 4.8 for the D10 is not unusual. However, the factor of 7.0 for the induction dose in M. radiodurans is comparatively large. It is noteworthy to mention that the induction dose of this radioresistant bacterium, about 6 Mrads, in frozen state irradiation is larger than the sterilization dose in radappertization, 4.5 Mrads, which corresponds to the 12D of Clostridium botulinum spore (type A and type B). Thereby, it would be worthwhile to calculate the numbers of hits in DNA per cell. If one could postulate that an average ion cluster contains about 2.3 primary ion pairs and subsequently the mean energy dissipated in one effective event is 78 eV (=34eV× 2.3). On this basis, the direct hits by effective primary ion clusters in DNA at a given dose, D, are calculated as follows: hits in DNA=8× 1011× gmDNA (rads). The survivors of M. radiodurans were about 90% by 3 Mrads at liquid nitrogen temperature and by 0.3 Mrads at room temperature. Hits in DNA per cell are calculated as 1.2× 105 for 3 Mrads and 1.2× 104 for 0.3 Mrads. To elucidate the very high radioresistance of this bacterium, further investigations are needed. Those should include the study on the non-DNA damage caused by radiations as well as the repair mechanism for DNA damages. The D10 values at the above two temperatures of five strains of E. coli, which were obtained directly or indirectly from strain B, were also determined. As the result, the D10 ratio defined as the D10 for liquid nitrogen temperature divided by the D10 for room temperature was found higher with the more radioresistant strain. The possible interpretation was briefly discussed.
