76 BACTERIOPHAGES 



extensive investigation by Watson (1950, 1952) of the physiologi- 

 cal modifications produced in bacteriophages by both direct and 

 indirect effects of X-irradiation. To study the direct effect, 

 phage preparations were irradiated in broth. It was found that 

 T2 phage particles inactivated by the direct effect are still 

 adsorbed to bacteria as efficiently as active T2 particles. Not 

 all the inactivated phage particles, however, are able to kill the 

 host cells on adsorption, the host-killing property being abolished 

 by X-irradiation at Vs the rate at which plaque-forming ability 

 is lost. X-ray-inactivated T2 particles are also able to interfere 

 with the multiplication of active Tl phages, the interfering power 

 being inactivated by X-rays at the same rate as the host-killing 

 property. X-ray-inactivated T2 phages retain their ability to 

 cause "lysis from without," and are even more effective in this 

 regard than unirradiated phage particles. Cytological obser- 

 vations on bacteria infected with X-ray-inactivated T2 phages 

 show nuclear disintegration and chromatinic change, analogous 

 to those observed with ultraviolet-inactivated T2 phages. The 

 proportion of inactivated phage particles capable of inducing 

 these cytological changes is the same as the proportion capable of 

 killing the host cell, so that host cell death appears to be the 

 result of the phage-induced destruction of the nuclear apparatus. 

 The ability of T2r"'" phage particles to cause "lysis inhibition" is 

 retained after X-irradiation even after the host-killing property 

 has been abolished. Photoreactivation can also be detected in 

 X-ray-inactivated T2 phages, but in a much smaller proportion 

 of the phage particles than after ultraviolet inactivation. Both 

 multiplicity reactivation and cross reactivation of X-ray-inacti- 

 vated T2 and T4 phages exists and will be considered in detail 

 in Chapter XVIII. It is evident that X-irradiation permits one 

 to isolate several physiological properties of the phage particles. 

 It is also evident that the effects of X-rays are quite distinct from 

 those of ultraviolet light, suggesting that the nature of the lesions 

 produced by these two types of radiation are different. This is 

 also indicated by the fact that a population of T2 bacteriophages 

 inactivated by ionizing radiations is no longer able to inject all 



