288 FACTORS MODIFYING THE SENSITIVITY OF CELLS 



Water Content 



I am sure that you are acquainted with the work on the sensitivity 

 of plant viruses to x-rays, depending on the water content of the virus 

 crystals. Similar work has been done by Stapleton. in our laboratory 

 in regard to irradiated Aspergillus terreus spores, (a) suspended in water, 

 (b) freshly removed from an agar slant culture containing about 40-50 

 per cent water, and (c) dried and desiccated for 3 days, and containing 

 only about 20-25 per cent water. There is a striking increase of resist- 

 ance to x-rays of very dry spores. Water apparently sensitizes the 

 spores to x-rays by bringing in direct contact some substances formed 

 in the water by x-rays. However, what we call "dry" spores in our 

 experiments still have a water content which should be an important 

 factor in the sensitivity. 



In connection with the irradiation in water, we should discuss the 

 work of the Te cas group [Wyss, Stone, and Clark (21)], who found that, 

 when the medium was irradiated with wave lengths shorter than 2537 

 A, bacteria and fungi grown in this irradiated medium showed a some- 

 what increased mutation rate. Hydrogen peroxide produced the same 

 effect. Certain organic peroxides have been found to increase mutation 

 rates, as reported by a California Institute of Technology group [Dickey, 

 Cleland, and Lotz (6)]. 



These findings point to the possibility that hydrogen peroxide, or 

 certain organic peroxides which may be produced by radiation in the 

 medium or inside living cells, may be an important factor in regard to 

 radiation sensitivity. 



Further evidence is also brought out in the following discussion. 



Oxygen Tension 



The effect of oxygen tension on the sensitivity of chromosomes to 



x-radiation has been reported by Giles. R. S. Anderson reported in 1941 



that yeast irradiated in the presence of oxygen was much more sensitive 



to x-rays than yeast irradiated in the absence of oxygen. We have carried 



out similar studies in regard to Escherichia coli, the experimental details 



of which, although important, will be omitted here for the sake of 



brevity. All data presented are based on plate counts of bacteria grown 



aerobically after irradiation. Figure 2 illustrates results obtained with 



aerobically grown bacteria irradiated in oxygen, air, and nitrogen. This 



graph shows that the ratio of survivors at 60,000 r under two different 



gases is 



nitrogen/oxygen = > 1000 



