PEROXIDASES AS FACTORS OF RESISTANCE AND RECOVERY 255 



with appropriate substances after irradiation. This "recovery method" 

 would differ from the "protection method" previously discussed. It 

 may be illustrated by some of the results recently obtained in my 

 laboratory by treatment of ultraviolet-irradiated bacteria with catalase. 

 This study followed an observation by Monod et al. (22) that bacteria 

 {E. coll, strain K12) irradiated with ultraviolet light can be reactivated 

 by addition of catalase to the culture medium. This phenomenon, 

 which recalls Kelner's photoreactivation (11), seems to depend to a 

 certain extent on the presence of visible light. The preliminary results 

 that we have already obtained with two bacteria {E. coli, strains B 

 and K12) can be summarized as follows: 



1. Both bacteria, enriched in catalase content by growing in a medium 

 containing a large amount of this enzyme, show increased resistance to 

 ultraviolet light. Survival rates may be 20 times as high as those of 

 the controls. This effect depends on the dose; weak with low doses, 

 it increases with higher doses. The table shows the results of one 

 experiment : 



Dose, 



grgg / Survival Rates 



mm^ Controls I Catalase II II/I 



800 3 X 10"'' 6.7 X 10"* 2 



1000 3.3 X 10-^ 3.3 X IQ-'' 10 



1200 5 X 10"^ 1 X 10^ 20 



2. This effect does not occur with x-rays, either in B or in K12. 

 This negative result recalls an observation by Barron et al. (2) that 

 catalase does not prevent the inhibiting action of x-rayed water on 

 cellular respiration. With ultraviolet rays, since catalase is present in 

 the cell at the time of irradiation, we may be dealing with either a pro- 

 tection or a recovery effect. 



3. According to Monod's observation, the recovery effect appears 

 when K12 bacteria grown in synthetic medium and sterilized by ultra- 

 violet radiation are plated with catalase and incubated. The extent of 

 the reactivation varies widely, depending on the dose and on the meta- 

 bolic conditions of the cell. We did not succeed in controlling the 

 metabolic factors, and results varied greatly between experiments 

 carried on under apparently identical conditions. So far reactivation 

 has occurred sometimes when catalase is administered without any 

 addition of visible light. But it has been a constant and striking phe- 

 nomenon when, before incubation, the bacteria undergo an exposure to 

 visible light which in itself would produce only a very slight effect. 



