412 RADIATION HIOIiOGY 



are actu;ill>' jfeiie niutatioii.s (scems dou^ttul since up to lOU per eeiit ot 

 such mutations can l)e induced at liifi;li doses. 



Ivaplan (1952) was the first to puhHsii an action spectrum tor mutation 

 induction in bacteria. He studied the muta}i;enic efficiency of (i\(' wave 

 lengtlis ranging from 2480 to 'HYM) A. A maximum at 2()50 A was 

 observed with eacli of tlie three types of mutations in Serratia marcescens 

 studied. Simihir results have been obtained by Zelle and Ilollaender 

 (unpublished data), who used six wave lengths from 2378 to 29()7 A. 

 Maximum efficiency of mutation induction at 2V)i)0 A was observed for 

 both the streptomycin dependence to independence and the purineless 

 reverse mutations in E. coli B/r. These are the same mutation systems 

 that were employed by Anderson (19511)) in his study of the relation of 

 oxygen concentration to mutation production by X rays. Absolute 

 increases in the number of mutant cells were observed by Kaplan and by 

 Zelle and Ilollaender, thus eliminating differential survival of spontaneous 

 mutants as the reason for the increased mutation rate. The observation 

 of a maximum efficiency at 2(350 A in bacteria, as in sexually reproducing 

 forms, is additional supporting evidence of the genie nature of bacterial 

 variation. 



INDUCTION OF MUTATIONS BY IRRADIATED MEDIA 



In many of the earlier studies of bactericidal effects, the bacteria were 

 irradiated on the surface of agar plates. To test the possibility of an 

 indirect action through the photochemical production of toxic substances 

 in the medium, several workers irradiated media before inoculation. In 

 general, it was possible to demonstrate toxic properties of the irradiated 

 media, but the doses required to produce an appreciable degree of toxicity 

 were far beyond those utilized in the bactericidal tests by direct irradia- 

 tion of the organism (Loofbourow, 1948). 



The discovery of photoreactivation and of the influence of oxygen 

 concentration on the bactericidal and mutagenic effects of ultra- 

 violet and X rays, respectively, indicate clearly that at least part of 

 the bactericidal and mutagenic effects of radiation are indirect and 

 involve largely unknown reactions which, initiated by the radiation, 

 ultimately alter a vital structure of the cell and cause either mutation or 

 inactivation. 



Interest in the indirect effects of the irradiation of substrates has been 

 revived by a series of publications by a group at the University of Texas. 

 Mutations to resistance to antibiotics in S. aureus were usually studied. 

 Thus Stone et al. (1947) demonstrated that irradiation of the substrate 

 with the entire spectrum of a (luartz mercury arc lamp, prior to inocu- 

 lation with bacteria, caused an increase in the mutation rate. Chemical 

 treatment of the substrate or of amino acids added to the substrate with 

 hydrogen peroxide caused a similar increase in mutations (Wyss et al., 



