IONIZING RADIATIONS AND CELL METABOLISM 



237 



The partisans of the "target theory" derive some of their support 

 from experiments on the death of bacteria by irradiation. What is 

 meant by death is really multiplication of bacterial cells after irradiation, 

 which is a more complex process. According to this theory, although 

 many ionizing particles pass through the bacterium before it is killed, 

 its death, when it does occur, is due to one of these particles alone which 

 chances to pass through an especially sensitive region or "target," the 



3 4 



X-ray, r x 10' 



Fig. 17. p]ffect of x-ray irradiation on the multiplication and respiration of Coryne- 

 bacierium creatinovorans. 1, Fraction left of cell multiplication; 2, fraction left of cell 

 respiration; in both cases after x-ray irradiation of bacteria in its own culture me- 

 dium. 



"zone sensible" of the organism. Bacterial cells are very complicated 

 organisms, and within their small volume will be found, among other 

 things, a large number of enzyme systems possessing the same com- 

 plexity as that found in mammalian tissues and with varying degrees 

 of sensitivity towards ionizing radiations. The complexity of the cell 

 renders the "target" theory too simple. 



A more precise and easier method of determining bacterial multipli- 

 cation is that of measuring the turbidity of liquid cultures. Between 

 certain limits, turbidity readings in a colorimeter are proportional to 

 the dry w'eight of bacteria. Simultaneous measurements of turbidity 

 and of respiration of x-irradiated Corynehacterium creatinivorans showed 

 that it was possible to depress respiration without altering cell multi- 

 plication (Fig. 17). On irradiation of "resting," non-growing cells with 

 10,000 r, it was found that, although the residual respiration remained 



