Inherited Differences in Sensitivity to Radiation in Escherichia Coli

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Abstract
Strain B of E. coli yields variants which are resistant to both u.-v. and x-rays and which can be detected by the selective action of these radiations. This resistance is a stable heritable character. The no. of resistant bacteria in samples from normal cultures can be detd. by irradiation with 50 ergs per mm., which permits 100% survival of resistant bacteria, incubation for 3 hrs. and reirradiation with 700 ergs per mm. This allows one visible colony for each originally present resistant bacterium but eliminates all sensitive survivors of the 1st treatment. The change from sensitivity to resistance is a spontaneous mutation for in independent cultures the no. of resistants varies widely. Mutation occurs in normal cultures at a rate of about 1 X 10-5. Doses of u.-v. that inhibit cell division in sensitive bacteria for several hrs. (resulting in the production of elongated cells) do not appreciably inhibit division in resistant bacteria.