EFFECTS OF RADIATION OX BACTKHIA 367 



synonymously to indicate the failure of the cells to form a colony visible 

 to the naked eye when plated under the particular conditions of the 

 experiment under discussion. 



HIGH-ENERGY RADIATIONS 



GENERAL RESULTS OF QUANTITATIVE INVESTIGATIONS 



Minch in 1896 was apparently the first to attempt to study the bac- 

 tericidal effects of X rays. His results were essentially negative as were 

 those of man}' other investigators during the next thirty years. Low 

 intensities of X rays and rather insensitive bacteriological techniques 

 seem to be the chief reasons for the conflicting and often negative results 

 of different investigators during this period. Duggar (1936) briefly dis- 

 cusses these early X-ray results, and the bibliographies given by him and 

 by Pugsley et al. (1935) form a helpful guide to the early literature. 



Diu'ing this period, however, it was convincingly demonstrated by a 

 number of investigators that ionizing radiations do exert a marked bac- 

 tericidal effect. Green (1904)^ employing semiquantitative bacterio- 

 logical techniques, studied the bactericidal effects of radium /3 rays on 23 

 species of bacteria including five spore formers. In his experiments all 

 species were killed by the /3 rays, and the spore formers were found to be 

 considerably more resistant than the vegetative forms. One of the out- 

 standing contributions during this period was that of Chambers and Russ 

 (1912). These workers studied the effects of radium emanation, pri- 

 marily /3 rays, on distilled water suspensions of Staphylococcus aureus, 

 Escherichia coli, Bacillus pyocyaneus, and Bacillus anthracis. A pro- 

 nounced bactericidal action was observed with all species; anthrax spores 

 were observed to be the most resistant to radiation. Quantitative esti- 

 mates of surviving organisms made by plate counts of an irradiated S. 

 aureus suspension, when plotted semilogarithmically, gave rise to a 

 straight line. This is the first exponential survival curve reported for 

 bacteria subjected to radiation. Furthermore, these workers observed 

 motile cells of B. pyocyaneus in irradiated suspensions in which no colony- 

 forming organisms were present. Similar observations of inactivated but 

 motile cells have been made by Bruynoghe and Mund (1925). 



Following application of the target theory by Crowther (1924, 1926) 

 to inhibition of mitosis in tissue culture cells observed by Strangeways and 

 Oakley (1923) and to his own data on killing of Colpidium colpoda, a num- 

 ber of investigators applied similar analyses to the bactericidal effects of 

 ionizing radiations. Holweck (1929) and Lacassagne (1929) irradiated 

 "pyocyanique S" with soft X rays of 4 and 8.3 A wave lengths. They 

 observed exponential killing with 4 A X rays, but a multihit or sigmoidal 

 type of survival vxivxe was obtained with 8.3 A. However, Lea, Haines, 

 and Coulson (unpublished, see Lea, 1947) observed exponential sur- 



