380 RADIATION BIOLOGY 



xiolet inucliation of hydroj^cii-saturated sii.spoii.sions of Azolohaclrr colls, 

 contaiiiiiig an actixe hydrojicnase, results in less killing and fewer muta- 

 tions than irradiation of nitrof^en- or methane-sat uraled suspensi<jns. 

 Similar (■()mj)arisons with Ji. (utlhraris cells which contain no hydroj^enase 

 did not show an effect of hydi'o}i;en. Oxygen accentuated the radiation 

 efTects with both organisms. It is postulated thai p>ru\ate may exert its 

 elTeet by reacting with hydrogen peroxide and tiiat the hydrogenase may 

 permit the organisms to destroy oxidizing radicals and peroxide formed by 

 the radiation. 



Although speculation al)out the mechanism of the protective elfect of 

 X rays on bacteria may be unprofitable since so little is known concerning 

 this phenomenon, there are a few considerations which may be worth 

 pointing out. First, it seems clear, from the influence of concentration of 

 bacterial cells in li(}uid suspensions on the inactivation rate and the pro- 

 tective effect exerted bj' the constituents of nutrient broth and other 

 compounds, that at least part of the inactivation of bacteria by X rays is 

 by an indirect mechanism. Such indirect action of X rays has been shown 

 for bacteriophages by Luria and Exner (1941), for tobacco mosaic virus 

 by Lea et al. (1944), and for rabbit papilloma virus by Friedenwald and 

 Anderson (1941). Latarjet and Ephrati (1948) studied the protective 

 effect for bacteriophages of certain amino acids and physiological reducing 

 compounds. In discussing the indirect effects on virus inactivation, 

 Lea (1947) showed that the inactivation dose (37 per cent survival) 

 increases as the nonvirus protein content of the irradiated suspension 

 increases until, at sufficiently high concentrations of nonvirus protein, 

 the inactivation dose essentially ecjuals that for the presumed direct 

 effect as determined by irradiation of dried purified virus protein. He 

 attributed the influence of nonvirus protein to the competition for the 

 active decomposition products formed in water by the X rays. Lea 

 showed that exponential survival curves are obtained by either the direct 

 or indirect action of X rays. 



It therefore seems logical to assume that decrease of the oxygen ten- 

 sion or addition of a protective compound is influencing the indirect bac- 

 tericidal effects of X rays and that the residual killing in the absence of 

 oxygen may result from a direct effect on vital elements of the bacterial 

 cell. Certain complications in the kinetics observed arise on this assump- 

 tion. Thus, if the presumed direct effect is trul}' sigmoidal as shown in 

 Fig. 10-3, then the indirect bactericidal effects, when plotted semilogarith- 

 mically, must show a very steep negative slope initially which increases 

 as the direct-effect slope decreases and becomes constant w^hen the direct- 

 effect curve becomes semilogarithmically linear. It is difficult to explain 

 such a curve for the indirect effects. If, however, the residual or direct 

 survival curve is exponential as shown l)y Burnett ct al. (1951), no diffi- 

 culty arises. In this connection, it should be pointed out that Lea rial. 



