PHOTOREACTIVATION 471 



5-1. CONDITIONS UNDER WHICH PHOTOREACTIVATION OCCURS 



Photoreactivation occurs when the bacteria are .siLspended in a liquid 

 medium or when they are plated on a solid medium. For quantitative 

 work the first method has been used exclusively. Kelner used bacteria 

 grown for 48 hr with aeration in a glucose-ammonium chloride medium, 

 and then diluted with saline by a factor of 2; Novick and Szilard used 

 resting cells obtained from cultures grown in a lactate-ammonium phos- 

 phate medium to approximately 10* cells/ml, transferred into saline, and 

 incubated in this medium under continuous aeration for 14—18 hr to 

 completely exhaust utilizable reserves; these bacteria were kept in the 

 icebox at 6°C and could be successfully used with reproducible results for 

 about one week. 



5-2. EFFECT OF THE GROWTH STAGE OF BACTERIA 

 ON PHOTOREACTIVATION 



The characteristics of photoreactivation of growing and resting cells of 

 E. coll B are very different. This point will be discussed later. 



Particularly important for the photoreactivability is the growth condi- 

 tion of the bacteria ajter they have been irradiated with ultraviolet. 

 Kelner has found that if the bacteria, prepared as previously specified, are 

 chilled immediately after irradiation, they can be kept chilled for at least 

 8 hr without significant effect on subsequent photoreactivation. On the 

 contrary, if they are kept at 37°C after irradiation, their photoreactiv- 

 ability decreases, the decrease being very strong if the bacteria are in a 

 nutrient medium (a decrease by a factor of 1000 in 2 hr) ; a considerable 

 decrease occurs also in saline, although at a lower rate (Kelner, 1949c; 

 Novick and Szilard, 1949). 



5-3. COMPARISON OF SURVIVAL CURVES AFTER ULTRAVIOLET 

 TREATMENT IN PRESENCE AND ABSENCE OF PHOTOREACTIVATION 



Kelner (1949c) and, independently, Novick and Szilard (1949) have 

 established that these tw^o curves have a simple relation : the ratio between 

 the ultraviolet dose required for obtaining a given survival in the dark and 

 after maximum photoreactivation is constant for any survival. Formally, 

 this phenomenon can be described by saying that the effect of the light is 

 to reduce by a constant factor the ultraviolet dose given to the sample; 

 hence the name of "constant ultraviolet dose reduction" is applied to the 

 observed relation. The demonstration of the principle can be obtained in 

 two different ways (Novick and Szilard, 1949). The first method is to 

 l^lot the dose required for obtaining a given survival after photoreacti- 

 vation (L) versus the dose observed at the same survival in darkness (/)); 

 the slope of this line measures the fraction to which the ultra\'iolet dose is 



