EFFECTS OF RADIATION ON BACTERIA 395 



extended his observations to E. coli B/r, Pcnicillium notatum, and Saccha- 

 romyces cerevisiae. By employing a standard photoreactivation treat- 

 ment which gave maximum recovery, Kehier showed that the effect of 

 photoreactivation was, essentially, to decrease the inactivation rate per 

 unit dose of ultraviolet. Defining the dose-reduction ratio as the ratio of 

 the ultraviolet dose followed by maximum photoreactivation to the ultra- 

 violet dose with no photoreactivating light for the same inactivation, 

 Kelner observed a constant dose-reduction ratio for E. coli of about 2.5, 

 which was independent of the survival ratio of the ultraviolet-irradiated 

 suspension. Similar conclusions were reached by Novick and Szilard 

 (1949) who demonstrated that a simple linear relation existed between the 

 survival curves of photoreactivated and nonphotoreactivated ultraviolet- 

 irradiated bacteria. Johnson et al. (1950) found that photoreactivation 

 of E. coli B following exposure to ultraviolet radiation was independent of 

 the presence of oxygen and that vegetative cells of Bacillus cereus showed 

 but little photoreactivation. 



The action spectra for photoreactivation of E. coli cells and S. griseus 

 conidia were investigated by Kelner (1951). For reactivation of S. 

 griseus conidia, the effective spectral region extended from 3650 to about 

 5000 A with the most effective wave lengths lying near 4360 A. For E. 

 coli, however, the effective wave lengths extended from 3650 to 4700 A 

 with the most active wave length lying near 3750 A. Kelner suggests the 

 sharp peak for *S'. griseus conidia at 4360 A may indicate that porphyrins 

 are involved in photoreactivation. 



Heinmets and Taylor (1951) have shown that cells inactivated by ultra- 

 violet at temperatures as low as — 70°C can be photoreactivated when in 

 the liquid state but not in the frozen state. They further showed that 

 cells which have been inactivated by wave lengths of 3000-4000 A while in 

 the frozen state do not exhibit photoreactivation when in the liquid state, 

 suggesting a qualitatively different mechanism for inactivation by this 

 spectral region. 



Photoreactivation seems to be a rather general phenomenon, having 

 been reported for several species of bacteria and bacteriophage (Dulbecco, 

 1949, 1950), Paramecium aurelia (Kimball and Gaither, 1950), Amhlij- 

 stoma larvae (Blum and Mathews, 1950), and gametes of the sea urchins, 

 Arbacia punctulata (Marshak, 1949; Blum et al., 1950), and Strong ylocen- 

 trotus purpuratus (Wells and Giese, 1950). Bawden and Kleczkowski 

 (1952) observed similar visible-light-induced recovery of ultraviolet- 

 irradiated tobacco necrosis virus inoculated onto French bean leaves and 

 of tomato bushy stvmt \'irus inoculated onto Nicofiana glutinosa. No 

 recovery was noted for tobacco mosaic virus inoculated onto A'', glutinosa. 

 The same workers observed that visible light prevented the ultraviolet- 

 induced necrosis of epidermal cells of Phaseoleus vulgaris leaves. 



Beckhorn (1952) and Kelner (1952) have shown that the extension of 



