INFLUENCE OF OXYGEN ON DAMAGE TO 



MICRO-ORGANISMS BY IONIZING 



IRRADIATION 



H. Laser 



Molteno Institute, University of Cambridge 



A NUMBER of bacteria representing strict and facultative aerobes and 

 anaerobes, as well as some yeasts, were exposed to X-rays (190kvp, no 

 added filtration, dose rate 6500r/inin.) at doses from 6-5 to 65xl0^r. 

 Known cell concentrations of washed cells were irradiated in phosphate 

 buffer -f glucose in absence of any added growth-promoting substance. 

 During irradiation the liquid phase was either free from dissolved Og (the 

 gas space containing N^ or Hg) or it was in equilibrium with Og of decreasing 

 percentage (100 per cent Og ; 20 per cent or 5 per cent Og/Ng). In some 

 experiments with 20 per cent and 5 per cent O2 the nitrogen was replaced by 

 carbon monoxide. 



Immediately after irradiation a known number of cells were transferred 

 either into fresh buffer + glucose or into a nutrient medium in manometer 

 flasks and the rate of some metabolic processes measured for up to 10 houis. 

 These included : Og uptake, COg production, aerobic and anaerobic fermen- 

 tation or acid production and utilization of Hg ( Vibrio desulphuricans) . Under 

 these conditions the increase, for example, in Og uptake or anaerobic fermen- 

 tation with time is proportional to the increase of dry weight of bacteria, 

 i.e. it is a true reflection of growth. 



The main results are as follows : 



(i) Og uptake, COg production and anaerobic fermentation or acid pro- 

 duction were not significantly affected, when measured in washed non- 

 growing cells. Aerobic fermentation of baker's yeast was slightly inhibited. 



[2) Growth inhibition became apparent only after a lag period which is 

 the shorter, the higher the X-ray dose. 



[3) All growing cells showed an ' Og-effect ' on irradiation. The relative 

 sensitivity for Oa-treated as compared to Ng-treated cells was approximately 

 three-fold for some cells {Staphylococcus albus, Lactobacillus Delbriickii) . On the 

 other hand a strictly anaerobic organism ( Vibrio desulphuricans) was unaffected 

 by 6-5 X 10"^ r in Ng but almost completely inhibited by the same dose in 

 air ; and the growth of vegetative forms from spores of B. subtilis was only 

 inhibited after irradiation (2-6 X lO^r) in presence of O2 but not in Ng. 



[4) In order to study whether the ' Og-effect ' is dependent on a particular 

 metabolic state of the cell or a particular equilibrium of an enzymic system 

 involved in respiration, cells were irradiated in presence of Og while their 

 respiration was inhibited by respiratory poisons. Thus, in the case of 

 Sarcina lutea, it has been possible with an X-ray dose of 26,000 r to suppress 

 almost completely the Og-effect, i.e. the cells, after removal of the poison, 

 behaved as if they had been irradiated in Ng. The effective inhibitors were 



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