EFFECTS OF RADIATION ON BACTERIA 387 



Sharp (1939) presents survival curves for ten species irradiated on agar 

 surfaces with 2537 A ultraviolet. Although much scatter is evident in the 

 points plotted, in seven of the species there is no systematic deviation 

 from an exponential. The three exceptions were S. aureus, S. albus, and 

 B. anthracis, and clumps or chains of cells could be at least partially 

 responsible. 



Lea and Haines (1940), in very careful studies, observed exponential 

 survival curves for S. marcescens, E. coli, and B. mesentcricus spores when 

 aqueous suspensions were exposed to 2537 A ultraviolet. 



Witkin (1946, 1947) observed exponential survival curves with E. coli B 

 when exposed to 2537 A ultraviolet but sigmoidal curves with strain B/r. 

 Numerous other workers have made similar observations with these 

 strains. Hence in this case, a bona fide sigmoidal survival curve has been 

 observed since the same bacteriological and physical techniques yield an 

 exponential curve for strain B. Both strains B and B/r are inactivated 

 exponentially by X rays. The explanation for these results is as yet 

 luiknown. 



In conclusion, it appears that, as with high-energy radiation, exponen- 

 tial or sigmoidal survival curves may be obtained following ultraviolet 

 irradiation, depending on the strain of bacteria and the technique of irra- 

 diation including precautions against clumping in the preparation of the 

 bacteria to be irradiated. It should be remembered that it is difficult to 

 determine the precise shape of the killing curve since the low dose range 

 where killing is slight is the most important (see Fano, Chap. 1, volume I 

 of this series). 



A number of workers, e.g.. Gates (1929a) and Rentschler et al. (1941), 

 have attempted to account for the observed survival curves on the basis of 

 variable resistance among the cells making up the population. Others, 

 notably Wyckoff (1932), HoUaender and Claus (1936), and Lea and 

 Haines (1940), interpret the exponential survival curves as indicating that 

 a single quantum is sufficient to kill, sigmoidal survival curves from this 

 point of view being accounted for by the multihit or multitarget theory. 

 Most investigators favor the Cfuantum-hit interpretation since the expo- 

 nential distribution of resistance necessary to explain cases of exponential 

 survival curves is highly improbable. 



EFFECT OF INTENSITY 



The Bunsen-Roscoe reciprocity law states that the effect of exposure to 

 radiation is a function of the total energy and is independent of intensity 

 and time. Loofbourow (1948) points out that the reciprocity law is 

 meaningful for photobiological phenomena only when it is restricted to 

 periods of time so short that other kinetic and metabolic activities do not 

 influence the reaction. 



Numerous workers have tested the applical)ility of this law with some- 



