290 FACTORS MODIFYING THE SENSITIVITY OF CELLS 



SENSITIVITY OF AEKOBic VERSUS ANAEROBIC Escherichia coli (fi/r) 



Since it is well established that E. coli is a facultative anaerobe, it was 

 decided to compare the relative sensitivity of this organism grown an- 

 aerobically before irradiation with that of the same strain grown under 

 strictly aerobic conditions. The bacteria grown anaerobically before ir- 

 radiation and irradiated anaerobicallj^ were found to be extremely radio- 

 resistant. 



The ratio of survival for the cells grown anaerobically before radiation 

 is 



nitrogen/oxygen =10^ 



The ratio of sensitivity between oxygen- and nitrogen-treated anaero- 

 bic cells is again, perhaps fortuitously, very close to 3. 



If the sensitivity of the extreme cases is compared, that is, cells grown 

 aerobically before irradiation and irradiated in the presence of oxygen, 

 and those grown anaerobically before irradiation and irradiated in the 

 absence of oxygen, a factor of 10 is found. 



USE OF ORGANIC COMPOUNDS (aMINO ACIDs) AS PROTECTIVE AGENTS 



It was noticed that bacteria {E. coli, B/r) were more sensitive to 

 x-rays when exposed in phosphate buffer than in nutrient broth (Difco), 

 8 gm per liter. Since broth contains a wdde variety of amino acids, it 

 was decided to test the protective action of amino acids, first in groups 

 and then individually, if the group tests appeared promising. 



The effect of amino acid solutions as protective agents was studied 

 from two points of view: (a) the effect of amino acid concentration on 

 bacterial survival; (h) the protective action of optimum concentration 

 of amino acids as obtained from survival (a) as a function of x-ray 

 dose. 



The results indicate that only glutamic acid and cysteine afford in- 

 creased protection over the range of concentrations used in these ex- 

 periments (Fig. 3). 



It appears from the data presented here that there are two different 

 effects: (1) radiation produces changes in the medium which can be 

 reduced by lowering the oxygen tension of the medium; (2) the studies 

 with anaerobic cells, on the other hand, indicate that essentially com- 

 plete removal of oxygen from the cells also results in lowering their 

 x-ray sensitivity. The combination of two protective systems results 

 in extreme resistance of the organisms to radiation. 



The possibility that respiration is tied up with radiation sensitivity 

 seems to be indicated by these tests. 



