Modification of Sensitivity to X-Irradiation by Water in Bacillus megaterium

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Abstract
Dry spores of B. megaterium were equilibrated at 25[degree] C with partial pressures of H2O ranging from 2.5 X 10-5 mm to 22 mm of Hg (= toor). After exposure to X-rays, the spores with different H2O content were treated to permit recognition of the influence of varying amounts of H2O in the spore. Probability of damage from O2-independent causes (class-I) is unchanged from about 10-1 torr to about 10-1 torr but may rise by about 20% as the vapor pressure is raised to saturation (22 mm at 25[degree]C). Probability of damage from radiation-induced radicals that must combine with O2 after formation (class-III) decreases from the very dry (10-5 torr) spore to about 1/3 at 1 torr, remains constant to about 10 torr and then decreases to 0 at 22 torr. Water protects the spore from damage by these radicals. Probability of damage observed only when O2 is present at the time of irradiation (Class-II) decreases from a high value at 10-5 torr to about 1/5 that value at 1 torr, remains constant to about 10 torr and then increases as vapor pressure of H2O increases to saturation. Water has a very large over-all protective effect; it sensitizes in 2 ways, one O2-independent and one O2-dependent, as water content approaches maximal values.