EFFECTS OF RADIATION OX BACTERIA 383 



The optimum temperature for E. coli B/r was 18°C, and for the Texas and 

 Crook strains, the optimum temperatures were 26° and r2°C, respectively. 

 The rate of the recovery has been found to be exponential when plotted 

 against time with the length of the exponential phase varying greatly at 

 the different temperatures. An exponential survival curve is found when 

 the surviving fraction following maximal recovery is plotted against X-ray 

 dose, the slope being less steep than that of the curve for the control cells. 

 Studies of the lag phase before cell division and of the rate of recovery sug- 

 gest that the recovery process is terminated by cell division. 



Appreciable recovery does not occur in buffered inorganic salt solutions 

 or in a glucose synthetic medium which supports growth of the cells but 

 does occur in nutrient broth or yeast extract solutions. This eliminates 

 the theory of simple decay of a toxic product resulting from irradiation 

 and indicates that a metabolic process is involved in the recovery. This 

 process could involve either the enzymatic destruction of a toxic product 

 or the synthesis at low temperature of compounds necessary to overcome 

 the potential damage produced by the radiation. The correspondence of 

 the division time with the time required for twofold recovery plus the 

 failure to observe significant recovery in a synthetic medium with a utiliz- 

 able energy source favor the latter hypothesis. Since it has been shown 

 that irradiation initiates a series of reactions which ultimately are lethal to 

 the cells, Stapleton and coworkers speculate that the optimal temperature 

 of 18°C may be the optimum equilibrium between opposing processes, the 

 recovery process and the unknown processes leading ultimately to inacti- 

 vation, each with a high temperature coefficient. These significant obser- 

 vations form the first well-substantiated case of recovery of X-irradiated 

 bacterial cells. The important cjuestion of the effect of similar recovery 

 on induced mutations is now under investigation. 



ULTRAVIOLET RADL\TION 



Duggar (1936), Ellis et al. (1941), and Loofbourow (1948) have so ade- 

 quately review^ed the early development of the knowledge of ultraviolet 

 effects on bacteria that it is unnecessary to do so here. Few of the experi- 

 ments in the fifty years following the original observations of Downes and 

 Blunt (1877) yielded information of a quantitative nature. In addition 

 to the rather qualitative bacteriological methods of demonstrating the 

 bactericidal properties of ultraviolet radiation, the use of nonmonochro- 

 matic light sources, the failure to measure intensities, the failure to correct 

 adequately or to control absorption of the incident energy in the sus- 

 pending medium, and the lack of information concerning the absorption of 

 ultraviolet radiation by bacterial protoplasm were the main factors 

 responsible for the lack of accurate ciuantitative data. However, 

 research during this period had adequately shown that all bacterial species 

 subjected to ultraN'iolet radiation of appi'opriate wave lengths were inacti- 



