50 XEUTROX EFFECTS OX AXIMALS 



Confirmation of the observation made by Spear (I), that sufficiently 

 hirge doses of fast neutrons are lethal, was secured in an experiment diu'ing 

 which we were able to subject organisms to 3000 n in air. A 48-hour cul- 

 ture was divided into three samples, one of which was unexposed, one 

 subjected to 300 n, and the third to 3000 n jjombardment. The 300 n 

 sample was then found to contain slightly more than the 750 million organ- 

 isms per cc. present in the control, while the 3000 n sample showed a re- 

 duction of the viable count to 40 million. 



Studies on Growth Curves. In the preliminary experiments, observations 

 Avere made only on the population of a given culture immediately after 

 bombardment. If it were true that sublethal dosages of neutrons exert 

 a stimulatory effect on the growth of the organisms, this should be more 

 apparent if the organisms were permitted to multiply for some time after 

 exposure. 



In subsequent experiments, therefore, organisms were exposed to the 

 action of the neutrons, and then planted in fresh nutrient Ijroth. The 

 growth of this subcultiu-e was carefully observed and compared with similar 

 subcultures from samples which originated in the same parent culture and 

 were subjected to identical changes of shape, volume, temperature, and so 

 forth, but not irradiated. As a rule, three 5-cc. samples were removed 

 from 100 cc. of parent culture, placed in sterile tubes, and kept together 

 thereafter except for the period of actual bombardment. One was re- 

 tained as control, one exposed in air and the third to the same number of 

 integron divisions of bombardment in a wax ball. 



The results of a typical experiment are displayed in Fig. 2. The samples 

 were taken from a 48-hour broth culture of E. coli. One sample was ex- 

 posed to 200 integron units in a five-inch wax ball, one to 200 integron units 

 without wax, and the third was not bombarded. Following the bombard- 

 ment of the second specimen, all three were diluted — 1 cc. in 99 cc. of sterile 

 saline — and 1 cc. of this dilution was transferred to 99 cc. of fresh medium. 

 Samples withdrawn from these subcultures at intervals were appropriately 

 diluted and plated out. The graph represents the relation between the 

 population of viable cells and the age of the subculture. 



It will be noted that the initial counts in the three samples were nearly 

 the same, so that no great increase in population of the exposed specimens 

 could have occurred during bombardment. The duration of the lag phase 

 was equal for all three subcultures, and the generation time also seemed 

 to be unaltered by the neutron bombardment. The only observable 

 differences lay in the degree of apparent decrease during the lag phase — 

 a phenomenon which must be interpreted cautiously at best, since agglu- 

 tination would account for it as easily as death of the cells. 



Experiments were undertaken to study the lag phase more carefully. 



