Dark reactivation in ultraviolet irradiated Escherichia coli.

Reversal of the lethal effect of ultraviolet radiation on microorganisms by a variety of treatments has been demonstrated with two essentially different plating techniques. Cells may be plated immediately after irradiation, in which case reactivation occurs on the agar plate. By employing such a plating procedure, reactivation has been accomplished through the use of heat (Anderson, 1951), acetate (Ellison, et al., 1955), and a recovery factor isolated from irradiated cells (Whitehead, 1955). The second technique requires that the recovery treatment occur prior to plating. In this manner Kelner (1949a), with visible light, and Heinmets (1953) and Heinmets et al. (1954), with metabolic intermediates, were able to reactivate injured cells. Although Heinmets (1953) also was able to demonstrate limited reactivation in buffer alone, he attributed this recovery to "heat reactivation and possible limited multiplication." Recently Wainwright and Nevill (1955a, b, c, d) have been able to reactivate spore suspensions of Streptomyces by incubating the ultraviolet irradiated spores in distilled water alone or with various metabolic inhibitors prior to plating. The data presented in this paper indicate that it is possible to reverse the effect of ultraviolet irradiation to an extent that equals photoreactivation when irradiated cells are incubated in only a saline solution, prior to plating.