3(1S It \i)i A iio.v iii()L(jr.Y 



vival curves \vh(Mi icpcMliiifj; this work with the same strain and wave 

 Iciijitlis. 



Wyckort" (lU:i()a. 1)) and WyckolT and Rivers (llKiO), in a series of care- 

 tul stiuHes, more firmly estal)Msiied the occui'rence of exponential survival 

 curves followiufi; exposure to ionizing radiations. Wyckoff and Rivers 

 ( 1930) studied the bactericidal effects of 15o-kv (8 rays on E. coli, Salmo- 

 nella h/pliimurium, and Staphylococcus aureus cells seeded on the surface 

 of agar plates. Exponential survival curves were obtained for all except 

 S. aureus. By allowing E. coli to divide before irradiation, they showed 

 that clumping of the cells w'as probably the reason that exponential sur- 

 vival curves were not obtained for S. aureus. They conclude that a single 

 electron is sufficient to inactivate a cell of these species. Similar results 

 (Wyckoff, 1930a) wxre obtained in studies with E. coli and *S. tijphimurium 

 using copper-K X rays and the soft general radiation from a tungsten 

 tube operated at 12 kv. Later, Wyckoff (l!)30b) studied the killing of 

 E. coli with X rays of wave lengths varying from 0.5 to 4 A. Exponential 

 killing was observed at all wave lengths. Wyckoff interpreted his results 

 to indicate that a single quantum of X rays was sufficient to kill the bac- 

 teria. His estimated values of the sensitive volume of the organisms 

 decreased with increasing wave lengths as a result of the greater incident 

 energy required for inactivation at the longer w^ave lengths. 



Similar results have been obtained by other workers with various bac- 

 terial species subjected to various ionizing radiations. Hercik (1933, 

 1934b) observed exponential sur^•ival curves wuth Serratia marcescens 

 irradiated with a particles emitted by polonium. Pugsley et al. (1935) 

 observed exponential killing for E. coli irradiated wdth 40-kvp X rays but 

 obtained sigmoidal curves for Sarcina lutea. A correction applied for the 

 degree of clumping as determined by microscopic examination of the 

 irradiated suspension resulted in an exponential survival curve. Lorenz 

 and Henshaw (1941) made extensive tests of the bactericidal effects of 

 200-kvp X rays on Achromobacter fischeri. Statistical analj'sis showed no 

 systematic deviation from an exponential survival curve. During the 

 past decade, in which there has been almost a routine use of radiation 

 for the induction of mutations in microorganisms, numerous investigators 

 have observed exponential survival curves (e.g., Lincoln and Gowen, 

 1942; Demerec and Latarjet, 1946; Witkin, 1947; Roepke and Mercer, 

 1947; Anderson, 1951a). Frara et al. (1950) report exponential survival 

 curves for six species following irradiation with 50-kvp X rays. A typical 

 exponential survival curve with 5 per cent confidence limits of the plotted 

 points is shown in Fig. 10-1 (Stapleton, unpublished data). 



Not all investigators have observed exponential survival curves, how- 

 ever. Observation by Holweck (1929) and Lacassagne (1929) of sig- 

 moidal survival curves with 8.3 A X rays has already been mentioned. 

 Glaus (1933) observed sigmoidal sur\i\al curves for E. coli following 



