;^()() UADIATION lilOl.OGY 



(Dufijrar. \\yM\\ (liese, 1945, H)47. lUoO; K\\\s rl ai. 1041; Latarjet, 1046; 

 Lea, 1017; Loofbourow, 1048; Mitchell, 1051; Rahn, 1045). 



BACTERICIDAL EFFECTS OF RADIATION 



When bacteria are exposed to radiation, in either the high-ener{j;y or the 

 ultraviolet range, the most prominent clTcct is the apparent killing of a 

 percentage of the cells, the fraction killed being a function of the absorbed 

 energy. The usual criterion of survival is the ability of the bacteria to 

 form a colony visible to the eye when incubated following plating on ordi- 

 nary culture media. This arbitrarily adopted measure of the bactericidal 

 effects of radiation, although convenient for (|uantitative studies, is influ- 

 enced by a variety of experimental conditions. Ilollaender (1043) 

 showed that prolonged exposure to saline, following long-ultraviolet or 

 short-visible irradiation, reduced the fraction of colony-forming organisms. 

 Roberts and Aldous (1940) made careful studies of various experimental 

 conditions both before and after ultraviolet irradiation which affect the 

 survival of Escherichia coli, strain B. TIkmt results will be considered in 

 more detail later, but for this strain, a hundredfold variation in survival 

 could be produced by changing the postirradiation treatment. Further- 

 more, the shape and slope of the survival curve were markedly influenced 

 by different conditions. Strain B/r, a radiation-resistant mutant deri\'ed 

 from strain B (Witkin, 1946, 1947), did not show similar variation in 

 survival when subjected to the same experimental treatments. Kelner 

 (1949a, b) observed that exposing bacteria to visible light following 

 exposure to ultraviolet significantly increased sur\'ival (photoreacti- 

 vation). Anderson (1949, 1951b) and Stein and Meutzner (1950) have 

 shown that increasing the temperature of incubation increases the sur- 

 vival of ultraviolet-irradiated E. coli B. Survival following X irradiation 

 was shown by Hollaender, Stapleton, and Martin (1951) to be influenced 

 by the oxygen concentration of the medium at the time of irradiation. 

 Stapleton et al. (1953) have discovered that incubation at suboptimal 

 temperatures markedly increases the survival of bacteria exposed to X 

 rays, and Stapleton (1952) has found marked differences in the radio- 

 sensitivity of E. coli cells at different stages of the growth cycle. 



These findings are mentioned to emphasize the multiplicity of factors 

 which influence the quantitative results obtained in studies of the bac- 

 tericidal effects of radiation. Consequently, in order to obtain reproduc- 

 il)le results, it is necessary that these variables be adequately controlled. 

 Furthermore, it is impossible to determine how much of the variation in 

 the results obtained bj' difl'erent investigators is attributable to differences 

 in their experimental techniques. This is especially true in regard to 

 many of the earlier studies. 



Bactericidal effect, lethal effect, killing, and inactivation are used 



