874 UADIATION ItlOI.OOY 



of (lilTcrcMit ionization (ioiisitics, Leu (11)47) coMcludos ih.il the hiictcricidal 

 cITccts can be accounted for on the hypothesis of lethal niiitalions induced 

 aiuoiifi; 2')() ^enes liavinj; an avera}j;e diameter of 12 ni^u. 



An iucreasinii; amount of evidence has l)een accumuhiting to indicate; 

 that a major porti(in of tlie effects of ionizing radiations may he iiuhrect. 

 Conse(iuently there is consi(hMal)le doubt that the target theory, without 

 modification, may Ix' Nalidly ajjplied to interpretation of those bac- 

 tericidal phenomena in which indirect actions aic known to be involved. 

 Further discussion is gix en in a later section. 



FACTORS INFLUENCING SENSITIVITY TO IONIZING RADIATIONS 



Few comparisons have been made of the X-ray sensitivity of spores and 

 the parent vegetative forms. In general, spores have been found to l^e 

 more resistant (Green, 1904; Chambers and Russ, 1912; Baker, 1935; 

 Lea et al., 193G, 1941). The greater resistance to radiation of spores may 

 be partially due to the lower water content (Stapleton and Hollaender, 

 1952). Of equal interest is the generality of the inverse relation of sensi- 

 tivity of spores and vegetative cells to radiations of different ionization 

 densities (Table 10-1). 



The relative sensitivity of E. coli B/r cells to 250-kvp X rays at different 

 stages of the growth cycle has been studied by Stapleton (1952), as dis- 

 cussed earlier. There is a sharp decrease in sensitivity during the lag phase, 

 followed by a marked increase in sensitivity during the logarithmic phase. 

 The maximum sensitivity is reached at the end of the logarithmic phase, 

 and as the stationary phase progresses, the sensitivity gradually declines 

 to the initial level. 



Table 10-2. Mean Lethal Doses" of Escherichia coli Strains Irradiated 



DURixr, the Stationary Phase in Air (Incubatkd at 37°C and 



Irradiated with 250-kvp X Rays) 



(PVoin Stapleton, pensonal (.'ommunication.) 



Strain .MLD, (r) X 10' 



B 3.5 



B/r 6.2 



Tennessee 6.0 



86G 6.2 



Gratia 6.5 



n-52 6.5 



• Crook 8.0 



Texas 10.0 



"• MLD, 37 per cent survivaL 



The relation of genetic constitution to radiation resistance wall be dis- 

 cussed later. That strains within a species maj^ vary widely in sensitivity 

 to radiation is showai in Table 10-2, in which the MLD \alues for eight 

 E. coli strains exposed to 250-kvp X rays are showai (Stapleton, personal 



