SOfi RADIATION HIOLOGY 



the liig pluuso follow iiif^ ultraviolet irradiation is photoreversible by visible 

 light. Latarjet (1951) has shown that the induction of active bacterio- 

 phage in lysogenic cultures l)y ultraviolet and also by X rays is similarly 

 photoreversible. 



Except for Latarjet's observation (19ol), no similar visible-light- 

 induced recovery from the effect of ionizing radiations has been reported, 

 any recovery which may occur being so small in magnitude as to be diffi- 

 cult to demonstrate. This would indicate again that quite different 

 mechanisms are involved in damage by ultraviolet and by ionizing radia- 

 tions. Furthermore, not all the ultraviolet effects can be reversed by 

 visible light since, in every instance thus far studied, recovery has not 

 been complete. This seems to indicate that more than one mechanism 

 exists by which ultraviolet radiation produces lethal or other effects 

 in cells. 



Other Types of Reactivation. Working with K. coli K12, Monod et at. 

 (1949) observed that treatment of the irradiated cells with catalase 

 increased the survival ratio following exposure to 2537 A ultraviolet. 

 Latarjet and Caldas (1952) have studied catalase restoration in greater 

 detail. Catalase restoration is enhanced by small doses of visible light. 

 The greatest degree of catalase restoration has been observed with E. coli 

 K12 and with B. megatherium 899, both of which are lysogenic. A non- 

 lysogenic B. megatherium strain and E. coli B/r show only a slight degree 

 of catalase restoration, whereas E. coli B shows none. Neither strains B 

 nor B/r are known to be lysogenic. Catalase restoration requires only 

 small amounts of catalase in contact with the cells for a short period; 

 5 min is sufficient, and maximum catalase restoration is observed only 

 with rather heavy doses of ultraviolet. The catalase restorability of cells 

 persists for about 2 hr after ultraviolet irradiation and then drops 

 rapidly. No catalase restoration is observed following exposure to X 

 rays. 



Lembke ei al. (1951), in a preliminary note, reported partial reversal of 

 the effects of ultraviolet by treatment of the irradiated cells with certain 

 chemicals. Phenol, glycine, and hydrogen sulfide were effective, whereas 

 chloroform resulted in no reversal. 



EXTREME I'LTRA VIOLET RADIATION 



Very few data are available concerning the bactericidal or other effects 

 of the extreme ultraviolet (Schumann region) on bacteria. The tech- 

 nical difficulties involved in bactericidal studies in this region are consider- 

 able, owing to the absorption of air below 1850 A. However, Bovie 

 (1910) and Blank and Arnold (1935) have demonstrated the lethal effects 

 of radiation between 1850 and 1100 A. 



Curran and Evans (1938) found that ultraviolet radiations of 2537 A 

 wave length and in the Schumann region were bactericidal and also that 



