Radiation resistance of Bacillus pumilus spores suspended in water in relation to spore concentration

It has been found that the radiation resistance of Bacillus pumilus spores suspended in water changes abruptly in a spore concentration range between 1 x 108 and 1 x 109 spores/ml; the decimal reduction dose (D-value) is about 0.2 Mrad (2 kGy) below this range, whereas more than 0.4 Mrad above this range. This abrupt rise of D-value was considered to be explained in connection with the exhaustion of dissolved oxygen caused by some coexisting organic compounds. This had been suggested from our previous study on the effect of organic compounds on D-value. As the results of measurements of dissolved oxygen in water suspensions of spores of various concentrations after irradiation at various doses, it was confirmed that dissolved oxygen concentration decreased depending strongly on spore concentration. The results of measurements of D-values of spores suspended in dipicolinic scid (DPA) solutions of various concentrations showed that D-value increased abruptly around 5 ppm (w/v). A possible explanation for the above-stated dependency of D-value on spore concentration is that DPA released from killed spores, the amount of which naturally increases with spore concentration, consumes dissolved oxygen under irradiation, and when spore concentration is so high that DPA released from killed spores reduces dissolved oxygen concentration below a critical value, which is at present assumed to be around 2 ppm (w/v), D-value is raised up to 0.4 Mrad or higher. (author)