A screening technique which permits the rapid isolation of bacterial mutants sensitive to ionizing radiation is described. The technique utilizes the radiation sensitivity of the mutants as the property used in their detection. Aliquots of the cell suspension to be screened are plated on agar plates and are incubated at 37°C for approximately 10 hours. The plates are then irradiated with a dose of gamma radiation sufficient to reduce the survival of the parent population to between 10% and 50% and are incubated for a further 18 to 24 hours. Visual inspection of the plates at this time reveals, among the normal clones, the presence of a small percentage of minute clones. Any clones composed of radiosensitive cells will be found among these. A second rapid screening permits the true radiation-sensitive mutants to be isolated with ease. The utility of this screening procedure has been demonstrated by the isolation of a spontaneously arising radiation-sensitive mutant from a culture of Escherichia coli K12 AB1157. Some preliminary radiobiological properties of the isolated mutant are presented.

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