Pretreatment of two strains of bacteria, Shigella flexneri and Escherichia coli B/r, with toxic concentrations of cuprous (Cu I) or cupric (Cu II) solutions resulted in an enhancement of radiosensitivity of the surviving bacteria. Irradiation of the sensitized bacteria in the presence of thiourea (0.3 M) or glycerol (1 M) gave survivals which would be expected with nonsensitized protected bacteria. These results suggested that the modification of the cell by copper treatment increased the sensitivity by producing molecular arrangements which have the same characteristics of energy dissipation as those responsible for radiation sensitivity in the untreated cells. The same strains of bacteria when sensitized to ionizing irradiation by copper pretreatment were not more sensitive to UV light at 2537 A°. However, when survivors from an UV dose sufficient to kill 90% of the population were exposed to toxic solutions of cuprous compounds, they were considerably more sensitive than unirradiated bacteria. This result stands in contrast to that previously reported, where the effect of ionizing radiation was to decrease the sensitivity of the survivors to solutions of cuprous compounds.

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