EFFECTS OF RADIATION ON BACTERIA 



369 



100 



X irradiation in the presence of heavy metal ions. The sigmoidal curves 

 may have been due to the short wave lengths of the secondary radiations 

 which were considered to be the main cause of inactivation. Luria 

 (1939), employing the same bacteriological techniques in both cases, 

 observed exponential survival curves with polonium a particles and a 

 two-hit sigmoidal curve with 0.7 A X rays. Microscopic examination of 

 the irradiated cells of E. colt revealed that, although some of the cells were 

 killed immediately and did not divide or grow, others continued to grow 

 without dividing and ultimately 

 developed into long filamentous 

 forms. These filamentous forms 

 either divided a few times and 

 then died or else recovered and 

 proceeded to develop normal col- 

 onies. Exposure to both a and 

 X radiation caused filamentous 

 forms, but the proportion was 

 higher with X rays. Luria points 

 out that death by several mecha- 

 nisms is incompatible with the 

 simple mathematical formulation 

 of the target theory. 



Similar results with Aspergillus 

 terreus spores have been reported 

 by Stapleton, Hollaender, and 

 Martin (1952) and Zirkle et al. 

 (1952) who report sigmoidal sur- 

 \ival curves with hard X rays and 

 exponential curves with densely 

 ionizing protons and a particles. 

 Since in both cases air-dried 

 spores were irradiated, there is no 

 possibility that prodviction of a 

 toxic substance in the irradiated suspending medium could be responsible 

 for the sigmoidal curves. 



Stapleton (1952) made the important observation that the form of the 

 survival curve obtained after X irradiation (250 kvp) of E. coli B/r cells 

 depends on the stage of the growth cycle of the culture. Cells from fully 

 grown cultures in the stationary phase yield exponential survival curves 

 (Fig. 10-1). However, when cells in the lag phase were exposed to X rays, 

 sigmoidal survival curves were obtained, the deviation from exponential 

 killing increasing to a maximum at the end of the lag phase. Interpreta- 

 tion of the sigmoidal cur\es on the multitarget theory (Atwood and Nor- 

 man, 1949) shows an increase from one to about eight targets during the 



001 



40 



DOSE, kr 



Fig. 10-1. Exponential survival curve with 

 5 per cent confidence limits for each point; 

 E. coli B/r resting cells irradiated in air- 

 saturated buffer with 250-kvp X rays. 

 (Stapleton, unpuhlished data.) 



