EFFECTS OF RADIATION ON BACTERIA 38.) 



Luckiesh, 1946). Ehrismann and Noethling (1932) report a maximum of 

 sensitivity at 2650 A for B. pijocyaneus, Micrococrus candicans, S. aureus, 

 a vibrio, and one species of yeast. For E. coli they report a maximum 

 sensitivity at 2510 A and for Serratia marcescens at 2804 A. Duggar and 

 Hollaender (1934a, h) found a maximum of sensitivity at 2650 A for S. 

 marcescens, and Wyckoff (1932), Hollaender and Claus (1936), and Hol- 

 laender and Duggar (1936) found the maximum sensitivity of E. coh to be 



at 2650 A. 



An improvement in technique was developed by Hollaender and Claus 

 (1936) who studied the inactivation of E. coli cells suspended in a non- 

 absorbing physiological salt solution. This was a modification of the 

 technique used by Duggar and Hollaender (1934a, b). The concentration 

 of bacteria in the suspensions was so great that, except for the small por- 

 tion of light scattered back into the beam, all the energy incident on the 

 suspension was absorbed. This method reduces the amount of nonspe- 

 cific absorption which may occur when cells, seeded on the surface of agar 

 plates, are irradiated with ultraviolet. Furthermore, the energy absorbed 

 per bacterium can be calculated directly without the additional source of 

 error involved in estimating absorption coefficients. The bacteria were 

 grown on agar slants and were either suspended in saUne solution and 

 irradiated or else washed one or more times before irradiation. Washed 

 suspensions gave a lower MLD than unwashed suspensions, indicating 

 either that washing made the cells more sensitive to radiation or that there 

 was a considerable degree of nonspecific absorption by metabohc products 

 and nutrients in the unwashed suspensions, or both. The action spectra 

 obtained by Hollaender and Claus (1936) showed a sharp maximum at 



2650 A. 



The evidence is clear that there is a maximum of bactericidal effective- 

 ness at 2650 A suggesting that absorption of ultraviolet by nucleic acids or 

 by nucleic acid components is the first step in the reactions resulting 

 ultimately in death of the cell. 



Loofbourow (1948) has given the theoretical basis underlying the 

 action-spectrum technique. His analysis showed that the following 

 assumptions are inherent in the action-spectrum method : 



1. The biological effect observed is, on the average, attributable to 

 photochemical change in a given number of molecules of an essential 



substance. 



2. The quantum efficiency of the photochemical process is independent 



of wave length within the region studied. 



3. The attenuation of the intensity of radiation before reaching the 

 sensitive substance is either independent of wave lengths or so small in 

 magnitude as to be ignored. 



4. The relative absorption of suspected sensitive substances hi the cell 

 as a function of wave length either can be estimated with sufficient accu- 



