EFFECTS OF RADIATION ON BACTERIA 413 



1947). Evidence was presented (Stone et al, 1948) that selective growth 

 of spontaneous mutations dready present in the inoculum could not 

 account for the observed results. 



More critical evidence that irradiated substrate or peroxide-treated 

 substrates could induce mutations was presented by Wagner et al. (1950) 

 who observed increases in mutations in Neurospora conidia treated with 

 irradiated broth. An increase in mutation rate when inhibitors of cata- 

 lase and the cytochrome system were incorporated into the medium, thus 

 permitting an accumulation of hydrogen peroxide within the cell, was 

 reported by Wyss et al. (1948) and Wagner et al. (1950). 



Haas et al. (1950) summarize these results and, in addition, report that 

 the most effective wave lengths in producing mutagenic substrates are 

 those below 2000 A. Increasing the temperature of the substrate during 

 the preinoculation irradiation from 0° to 60°C results in a significant 

 increase in the mutagenic properties. They report that the bactericidal 

 and mutagenic effects of irradiated medium were partially reversed by 

 subsequent exposure to visible light. 



Wyss et al. (1948) failed to demonstrate an increase in mutation rate by 

 direct treatment of bacteria with hydrogen peroxide, although positive 

 results were obtained by Wagner et al. (1950) with Neurospora. They 

 conclude that formation of organic peroxides is responsible for the muta- 

 genic effects of peroxide treatment of the broth and probably also of ultra- 

 violet irradiation. This conclusion is strengthened by the work of Dickey 

 et al. (1949), who showed that various organic peroxides are effective in 

 inducing reverse mutation in an adenine-reciuiring strain of Neurospora. 

 In their experiments the efficiency of the mutagenic peroxides was con- 

 siderably less than that of direct ultraviolet irradiation. 



Summarizing, it seems established that the mutation rate is increased 

 when the cells are grown in ultraviolet-irradiated or hydrogen peroxide- 

 treated substrates. It is difficult, however, to assess properly the signifi- 

 cance of these results as they relate to the induction of mutations by direct 

 irradiation of the cells with ultraviolet. In general, the mutagenic effi- 

 ciency of the various substrate treatments is low; where comparisons were 

 made, it is lower than that of direct ultraviolet radiation. Furthermore, 

 there is a complete lack of quantitative data as to the amounts of radia- 

 tion absorbed by the substrates. It is certain, however, that doses far 

 greater than those employed in direct irradiation of cells are employed hi 

 the substrate treatments. Furthermore, the wave lengths effective in 

 producing mutagenic compounds in irradiated substrates lie mainly below 

 2000 A and are not present in the spectrum of the widely used germicidal 

 lamps with glass envelopes or in monochromatic beams of ultraviolet 

 radiation of the wave lengths most efficient in producing mutations by 

 direct irradiation. Loofbourow (1948) points out that, although the 

 formation of hydrogen peroxide by ultraviolet radiation is essentially con- 



