Ultraviolet Damage to Bacteria and Bacteriophage at Low Temperatures

The survival of Escherichia coliB/r WP2 (tryptophan-requiring) from ultraviolet irradiation when suspended in 0.067Mphosphate buffer (pH 7) has been studied over the temperature range 22° to -269°C. In unfrozen suspensions there was no appreciable change in sensitivity between 22° and -10°C. The sensitivity in the presence of ice progressively increased by a factor of 7 when the temperature was lowered to -79°C. Between -79° and -196°C the sensitivity decreased to less than four times the sensitivity at 22°C and was not appreciably different at -269°C. Evidence from experiments with bacteriophage T1 and E. coliWP2 HCR-(a strain unable to excise thymine dimers) indicates that a new, qualitatively different lesion, less amenable to repair, may replace the thymine dimer in E. coliirradiated at -79°C.