284 Raymond Latarjet 



respectively more radioresistant than phage T2 and its host B. 

 However, the K12S-X complex is as radiosensitive as the B-T2 

 complex. This is due to the fact that the "capacity" of K12S 

 to grow X is far more sensitive than the capacity of B to grow 

 T2. As a matter of fact, in both X- and u.v.-irradiation, if one 

 adds the cross-sections of X and of the capacity of Kl2S, one 

 finds roughly the cross-section of T2, the cross-section of the 

 capacity of B being negligible (Latarjet, unpublished). 



A situation similar in all respects has been found with 

 cumene peroxide: Kl2S and X are respectively more resis- 

 tant to peroxide than B and T2. However, the infectivi- 

 ties of K12S-X and B-T2 complexes have about the same 

 sensitivities. 



Such parallelism, concerning a very specific biological 

 situation, appears to be very significant for the radiomimetic 

 character of peroxide. 



Posteffect after treatment with peroxide 



A posteffect has been observed by Alper (1954) after X-ray 

 treatment of bacteriophage. In my laboratory, Miletic (1955) 

 found that under strong illumination, nutrient broth becomes 

 toxic for B-T2 complexes, and that this toxicity proceeds for a 

 certain time after the treated complexes have been washed 

 and resuspended in an inert medium. Catalase suppresses this 

 posteffect. A similar phenomenon has been obtained after 

 peroxide treatment of B-T2 complexes. When the treated 

 complexes are diluted and plated on a minimal synthetic 

 medium, the toxic effect initiated by the peroxide proceeds in 

 such a way that the same final number of survivors is obtained 

 independently of the time of plating. This number is the 

 result of the immediate effect plus the posteffect. On the other 

 hand, when the complexes are plated on broth agar supple- 

 mented with catalase, the toxic effect is stopped immediately; 

 the final number of survivors shows the actual figure at the 

 time of plating. 



The following experiment, carried out in my laboratory by 



