KAIMATION AND VIRUSES S.S.') 



the recognition that ba('teriophag;o inactivated l)y radiation can be reac- 

 tivated in its host (Luria, 1947; Dulhecco, IQoOj; siniihir ])henomena may 

 occur for other viruses. In the case of bacterioj^hases, because of the 

 possil)iht3" of reactivation, an inactive virus particle must be defined as 

 one that is unable to parasitize a host cell or that, upon parasitizing a 

 bacterial cell under conditions where no reactivation occurs, fails to give 

 rise to the production of active bacteriophage (see Sect. 3-2). 



A comparison of the titers of control and irradiated virus suspensions 

 generally provides reproducible inactivation curves. In many cases the 

 rate of inactivation of viruses by irradiation has been found to follow a 

 simple exponential relation according to the equation 



N - iVoe-^-«, (9-1) 



where A^o = the titer in the unirradiated control, 



N = the titer in an irradiated sample, 



D = the dose of radiation, and 



k = Si constant, characteristic for a given virus, a given radiation, 

 and sometimes for a given mode of titration. 

 The dose for which N = A^oe~^ is often called the "inactivation dose" 

 and designated as Di/e (Lea, 1946). 



2-1. IONIZING RADIATIONS 



X rays, y rays, a rays, electrons, neutrons, and deuterons have all been 

 used to inactivate virus particles. It is now recognized that the inacti- 

 vation of viruses by ionizing radiations may result from two major cate- 

 gories of effects: indirect and direct. 



2- la. Indirect Effects. These effects are mediated by toxic substances 

 produced by radiation in dilute aqueous solutions. The toxic products 

 are responsible for most of the inactivation observed, provided that suffi- 

 cient amounts of protective substances which compete for the toxic 

 products are not present. The current theories on the nature and 

 chemical properties of the toxic substances produced by ionizing radia- 

 tions in water and on the mode of their action are discussed by Dale and 

 by Barron (in Vol. I of this series). As far as viruses are concerned, the 

 indirect effect of X rays was first recognized on papilloma virus (Friede- 

 wald and Anderson, 1940, 1941), then on phage (Luria and Exner, 1941), 

 and on plant viruses (Lea et ai, 1944). The effect manifests itself by a 

 higher inactivation rate of viruses exposed to radiation in water or in 

 saline solutions than when exposed either in crude suspensions containing 

 large amounts of foreign substances or in the dry state. A variety' of 

 substances can act as protective agents. Proteins such as gelatin and egg 

 or serum albumin are effective. Several substances of small molecular 



