33() UAOIATION mOLOOY 



woiglit, t'.f!;., tryptopliane, thiourea, and histidiiie, give complete protec- 

 tion of hacteriophafj;e in \ery low concent rations (Latarjet and Ephrati, 

 1948; Watson, 1952). Ordinary bacteriological media such as beef broth 

 are excellent protecting metlia. 



(Concentrated \'irus suspensions in nonprotective media are inactivated 

 more slowly than dilute suspensions both because of the presence of 

 impurities and because the virus itself may act as a protective agent. In 

 some cases the inactivation rate does not increase with dilution beyond a 

 certain point; the diluting medium itself may contain a small amount of 

 protective substances. In very dilute suspensions there is also the possi- 

 bility that some of the toxic agents produced by the radiation, such as 

 free H and OH radicals, may recombine before having an opportunity to 

 meet a virus particle and react chemically with it (Lea, 194()). 



The rate of inactivation by the indirect effect of ionizing radiation 

 varies from case to case. Deviations from the logarithmic relation Eq. 

 (9-1), with the rate increasing with the dose, could be due to a need for 

 cumulative damage of the virus (true multiple-hit effect), to a progressive 

 accumidation of toxic products in the medium, or to destruction of pro- 

 tective substances by radiation itself. With purified tobacco mosaic 

 virus. Lea et al. (1944) reported logarithmic rates of inactivation by X 

 rays even for dilute suspensions of purified virus, but their data are not 

 very satisfactory. For bacteriophages (Alper, 1948; Watson, 1952) the 

 inactivation rate increases with time of exposure. This was interpreted 

 by Alper (1948) as indicating progressive accumulation of toxic products 

 in the medium. Watson (1952), however, was able to show that the 

 increasing rate of inactivation was due almost exclusively to accumulation 

 of damage in the phage particles. A phage sample in buffer received a 

 small dose of X rays; most inactivation was due to indirect effect. The 

 phage was then diluted in a completely protecting medium and later was 

 again diluted in buffer and exposed to the same dose of X rays. The 

 second exposure was much more effective than the first, as expected from 

 the previously determined survival curve, although the second medium 

 had not been irradiated previously. 



Not all the indirect effect of X rays on viruses is exerted during actual 

 exposure. There are also aftereffects of irradiation, mediated by rela- 

 tively stable toxic products. If either water or buffer solutions are 

 exposed to radiation, and virus is then introduced without appreciable 

 amounts of protective substances, some inactivation will follow. In the 

 case of bacteriophage (Watson, 1952) at least tw^o agents are invoh-ed, one 

 short-lived, detectable only during actual irradiation, and the other a 

 long-lived one, which is quite stable at 5°C and is slowly inactivated at 

 room temperature. Many of the effects of the latter can be duplicated by 

 peroxides. The different nature of the two agents is evident from the 

 different properties of phage inactivated by one or the other of them (see 



