Spore radiation resistance was altered by repeated γ-irradiation treatments of surviving spore populations of Bacillus pumilus E601. An increase in resistance from <tex-math>$D_{10}$</tex-math> values of 0.30 to 0.44 Mrad was attained following 14 irradiation treatments. Five morphologically distinct substrains were isolated from the 14th treatment surviving spore population that were shown to have stable <tex-math>$D_{10}$</tex-math> values ranging from 0.34 to 0.51 Mrad. Most other parent strain characteristics were unaltered in the five substrains; however, changes were noted in the nutritional requirements for spore germination and outgrowth with the substrains, requiring an additional two to six amino acids for outgrowth. Spore radiation resistance increases were not passed on to vegetative cell progeny.

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