EFFECTS OF RADIATION ON BACTERIA 381 



(1936) observed exponential s\ii\iAal curves when E. coli cells were 

 exposed to /3 and a particles in dried gelatin films, in which any indi- 

 rect effects involving radiodecomposition products of water would be 

 minimized. 



To attempt any explanation of the observed results, one can only turn 

 to the knowledge of the radiodecomposition of pure water. This extrapo- 

 lation is questionable since it is known that slight impurities in the water 

 have a profound influence on the results obtained. Hence, greatly dif- 

 ferent phenomena may occur in the complex chemical milieu within the 

 bacterial cell. However, to attempt any explanation encompassing these 

 considerations seems hopeless in the present state of knowledge (for a brief 

 informative discussion, see Burton, 1951). 



The significant observation would seem to be the large effect of oxygen 

 concentration. Since, under irradiation, oxygen is known to be reduced 

 to peroxide in two steps — as shown in the following equation (Allen, 1948) 



O2 + H = HO2 

 HO2 + H = H0O2, 



the chief effect of reducing the oxygen concentration would be to reduce 

 the concentration of peroxide and HO2 radicals formed. It would seem 

 therefore that the indirect bactericidal effects of X rays are mediated 

 through either peroxide, HO2 radicals, or other unknown radiation prod- 

 ucts, possibly organic peroxides, depending on oxygen for their formation. 

 It seems equally probable that H atoms and OH radicals are not effective 

 in the indirect action since they are formed independently of oxygen, and, 

 indeed, the presence of oxygen would reduce the number of such radicals 

 free to react with the protoplasmic components involved in the indirect 

 mechanism. 



It has been shown that varying the concentration of oxygen has little 

 effect on the frequency of chromosome aberrations produced in Trades- 

 cantia by a particles (Giles, Chap. 10, volume I of this series). No similar 

 studies have yet been made with bacteria. This observation, however, is 

 not incompatible with the hypothesis that HO 2 and/or peroxide are the 

 biologically active w^ater decomposition products, since it is knowai that 

 detectable amounts of peroxide are formed by irradiation of oxygen-free 

 water wdth a particles and negligible quantities are produced by X irradi- 

 ation. Allen (1948) has shown a general relation between the steady- 

 state hydrogen pressure and hydrogen peroxide concentrations produced 

 in pure water by radiations of different ionization densities, higher concen- 

 trations being observed as the ionization density increases. 



In summary, the indications are that the indirect effects of X rays are 

 mediated in part by either HO2 radicals and/or hydrogen peroxide and 

 that the protective compounds exert their effect either by reducing the 

 oxygen concentration within or immediately surrounding the cells or by 



