EFFECT OF RADIATION ON PHAGE PARTICLES 75 



the radiation-activated solvent molecules — free radicals and 

 peroxides^in competition with the material under study. 

 Since target volumes are calculated on the assumption that the 

 lethal action has resulted from energy absorbed within the 

 physical domain of the biological structure whose death is being 

 followed, it is important to remember that target-size estimates 

 of bacteriophages can be based only on experiments in which 

 exclusively direct effects come into play. Indirect effects must 

 be excluded by the addition of protective substances. 



Luria and Exner (1941) found, while studying the inactiva- 

 tion of phages with X-rays, that the inactivation rate in distilled 

 water or buffers is very much greater than the rate in nutrient 

 broth. The addition of gelatin to water, however, gives maxi- 

 mum protection, so that the rate of X-ray inactivation of phages 

 suspended in this medium is no greater than that in broth. Egg 

 albumin and serum albumin also protect against this indirect 

 effect, while sugars do not. Alper (1948) made similar obser- 

 vations of the protective effect of proteins and attributed the 

 indirect effect to the lethal action of hydrogen peroxide. Latar- 

 jet (1942) had considered this possibility but his experiments 

 indicated that not enough peroxide is produced to account for 

 the indirect effect on phage CI 6. It seems to be generally 

 agreed that although some peroxides are produced by ionizing 

 radiations, the major portion of the indirect effect is due to short- 

 lived radicals generated in the water (Alper, 1954). Latarjet 

 and Ephrati (1948) investigated a number of simple chemical 

 substances for protective action against the indirect effect. 

 They found that glucose, sucrose, alanine, and histidine confer 

 little or no protection, while thioglycolic acid, tryptophan, 

 cysteine and cystine, glutathione, ascorbic acid, and phenylal- 

 anine possess considerable protective power. The presence or 

 absence of oxygen has no effect on the inactivation rate either in 

 the presence or absence of protective substances. 



The properties of X-ray-inactivated phages may be compared 

 with those of ultraviolet-inactivated phages discussed in the 

 previous section of this chapter, principally on the basis of an 



