BACTERIOPHAGE INACTIVATION UNDER 

 VARYING CONDITIONS OF IRRADIATION 



TiKVAH AlPER 



Medical Research Council Experimental Radiopathology Research Unit, Hammersmith 



Hospital, London 



Although in recent years bacteriophage has been shown to have some 

 degree of organization i' -, it is probably still the simplest living, or self- 

 reproducing, entity with which we can experiment. In the free state, i.e. 

 when not in contact with bacteria, phage does not metabolize, so far as is 

 known. A stock may retain all its properties for months, or even years. 

 As soon as they are placed in contact with sensitive bacteria, however, 

 active phage particles attach themselves, penetrate and reproduce. Inacti- 

 vation of the phage may be due to an interference with any one of these 

 steps. It is well known, for example, that after exposure to ultrax-iolet 

 radiation, phage particles are inactivated because they do not reproduce, 

 although their ability to adsorb to, and penetrate, bacteria may be unim- 

 paired. On the other hand, phage may be inactive through a failure to 

 adsorb, as is the case with the 'tryptophane-deficient' mutant of phage T4, 

 which will not adsorb to its normal host, coli B, unless tryptophane is 

 present^. In this paper, no attempt will be made to discuss which steps in 

 the reproductive cycle are affected by the various inactivating agents to be 

 described. The 'inactivation' of a phage particle will be defined as a 

 failure to form a plaque on a confluent growth of host cells {Figure 1) 

 (page 40). 



Ionizing radiations may inactivate phage through a variety of 

 mechanisms. Lea and Salaman^ exposed the small dysentery phage S13 

 in the dry state to ionizing radiations of different ion densities, and concluded 

 from their data that a single ionization taking place within a particle could 

 inactivate it. This is usually referred to as the direct effect of radiation, and 

 ultraviolet inactivation appears to be of the same type. When the phage 

 is suspended in an aqueous medium, it may be affected also by the decom- 

 position products of water. This is usually called the indirect effect of 

 radiation. If the suspension is very dilute, the probability of inactivation 

 by direct effect may be so small, compared with that of indirect effect, that 

 it may be neglected. This paper will deal only with such suspensions, and 

 direct effect will therefore not be considered. 



It is possible to show that indirect effect itself may proceed through one 

 of three mechanisms : 



[1] If hydrogen peroxide is formed by radiation, it has an inactivating 

 effect on the phage. 



(2) The phage may be changed by free radicals, so that although it is 

 still able to form plaques if allowed to adsorb to bacteria soon after 

 irradiation, it is rendered hypersensitive to certain inactivating 



39 



