160 Applied Biophysics 



was appropriate for phage S-13, and the plant viruses, but as a 

 single-celled organism with many genes. 



Shortly after this suggestion was made, electron micrographs 

 were published,'^ showing internal structures in the particles of 

 vaccinia virus, and making it difficult to doubt that the particle 

 of vaccinia is a single-celled organism rather than a macro- 

 molecule. 



It appears from these examples that radiation experiments 

 may be of value in elucidating the nature of viruses. Some 

 recent experiments ^^ on bacteriophages somewhat larger than 

 S-13 suggest that these are very primitive organisms with only 

 10 or 20 genes. 



Lethal Mutation in Bacteria 



Effects of radiation upon bacteria which have been investigated 

 are, the production of mutations, i.e., permanent changes in form 

 or color of colony, the reduction of motility, a temporary inhibi- 

 tion of division, and the lethal action, the great majority of 

 investigations being concerned with the last mentioned effect. 



What is described as a lethal action in these investigations 

 is the inability of a bacterium after irradiation to give rise to a 

 colony visible to the naked eye when inoculated on a nutrient 

 medium. There are, however, distinct differences between the 

 "killing" of a bacterium by radiation, and killing by other agents, 

 e.g., heat or chemical disinfectants. Thus, after irradiation, the 

 bacterium which is rendered incapable of giving rise to a colony 

 may still be motile,-^ may still be capable of respiration,- and 

 may, when cultured and examined microscopically, show some 

 growth. ^"^ In view of these facts, it is probable that one is dealing 

 with lethal mutation. 



The internal evidence of the radiation experiments supports 

 this interpretation. It appears ^' ^ that a single ionization is able 

 to "kill" a bacterium, but that, as with the large viruses, it does 

 not suffice for it to be produced anywhere in the bacterium. It 

 must be produced in a radiosensitive part which constitutes only 



