EFFECTS OF RADIATION' ON BACTEIUA 415 



hypothesis on several grounds. It is difficult to account for the kinetics 

 observed without making assumptions which are somewhat contradictory 

 to the behavior of chemical disinfectants in bacteria. Furthermore, the 

 independence of bactericidal effect and intensity and the lack of a large 

 temperature coefficient characteristic of chemical disinfectants are like- 

 wise difficult to explain unless it is assumed that a cell poison produced 

 internallj^ by radiation behaves quite differently from a chemical disin- 

 fectant applied externally. 



Postulating that a cell poison is produced by radiation really does little 

 to explain the mechanism of radiation damage except to infer that an 

 indirect mechanism exists between the initial chemical change produced 

 by the radiation and the lethal end result. To be meaningful, the 

 hypothesis must be elaborated to indicate the nature of the lethal effect 

 produced and to take into account the kinetics observed. This has been 

 done by Roberts and Aldous (1949) to explain their results on recovery of 

 E. coll B from the effects of ultraviolet irradiation. They postulate that a 

 cell poison is produced by the radiation by photochemical reaction and 

 that this cell poison selectively affects the division mechanism. To 

 explain the kinetics involved in the recovery, they further postulate that 

 one molecule of this poison is effective in inhibiting division and, further- 

 more, that the poison exponentially disappears independently of ultra- 

 violet irradiation. 



Similarly, although a target-theory interpretation can adequately 

 account for the kinetics observed, it has little meaning unless it is 

 expanded to indicate the nature of the target and the change induced in 

 the target by the radiation. Lea and his associates (1941; Lea, 1947) 

 have done this and postulate the nucleus as the target and the induction 

 of lethal mutations as the damage causing inactivation. On this hypoth- 

 esis, Lea has estimated that E. colt possesses 250 genes capable of lethal 

 mutation, each 12 m^t in diameter. 



In view of the developments since 1947. however, a reexamination of 

 the lethal-mutation interpretation seems indicated. 



The growing body of evidence indicates that a major proportion of the 

 effects of both ultraviolet and ionizing radiation on bacteria are indirect 

 and involve a largely unknown chain of reactions occurring betw^een the 

 initial ionization, or quantum absorption, and the final lethal or muta- 

 genic change. The assumption that the residual effect of radiations, i.e., 

 the nonphotoreactivable portion of ultraviolet effects or the residual 

 bactericidal effects of ionizing radiations in the absence of oxygen, is due 

 to direct effects of the radiation on vital cell structures is by no means 

 proved; the "residual effect" may actually involve still other indirect 

 mechanisms. 



The existence of an indirect mechanism, however, does not necessitate 

 abandoning the assumption that the final decisive end product of the 



