372 u\i)i\ri()N iii()L()(!Y 



li\ciu>.s.s of poloiiuim (I particles liaxc l)i'cii report (id by Zirkle (11)10) who 

 obtained dilTereiit ioni/atioii densities b}- utilizing dilTerent portions of 

 the path lengtii. An increase in elTectivcness with increasing ionization 

 density was obserxed with Aspenjillns nujer spores whereas, for inactiva- 

 tioii of E. coll, an inverse iclation was observed. Little influence of 

 ionization ciensity on sur\ i\al of yeast cells was noted. 



In Table 10-1 are assembled the available data on the relation of ioni- 

 zation density and the bactericidal effect of radiations (see also Fano, 

 Chap. 1, and Zirkle, Chap, (i, \'olume I of this series). The estimates 

 made from Spear (1044) and Zirkle (1040) are only approximate but 

 serve to indicate the general relations. It will be observed that the MLD 

 (37 per cent survival dose) for E. coli increases as the iotiization density of 

 the radiation increases. On the contrary, the MLD for B. mesentericus 

 and for .1. nufcr spores decreases with increasing ionization density. 

 This difference is as yet unexplained, and it may indicate that different 

 mechanisms are involved in the lethal effects of irradiation for the two-cell 

 forms. The increased efficiency of more densely ionizing radiations in 

 killing ,4. terrcus spores is confirmed by Stapleton, Hollaender, and Martin 

 (lOo'i) who add the further interesting observation that more densely 

 ionizing a particles are less efficient than hard X rays in inducing 

 mutations. 



Summarizing, three general conclusions seem warranted: (1) Both 

 exponential and sigmoidal survival curves may be observed, depending on 

 the bacterial strain, the technique of irradiation, the characteristics of the 

 radiation, and the stage of growth of the irradiated cells. (2) The sur\iv- 

 ing fraction for a gi\'en dose of radiation is independent, within limits, of 

 the intensity of the incident radiation or of the fractionation of the dose. 

 (3) For vegetative cells the bactericidal effectiveness of a given dose 

 decreases with increasing ionization density; the opposite seems to be 

 true for spores. 



The interpretation to be placed on these general results is not clear at 

 this time. Lea et at. (1036) and Lea (1947) have discussed at length the 

 various interpretations proposed for the observed survival curv^es. 

 Interpretation in accordance with the target theory seems the most 

 plausible since the first-order kinetics are a natural consequence of the 

 theory. Similaily, the sigmoidal survival curves are also easily accounted 

 for on the basis of either multiple hits re(iuired in one target or single hits 

 in multiple targets. Atwood and Norman (1940) discuss this latter 

 interpretation. Lea (1947) especially has been a strong proponent of the 

 target theory. Il(> interprets the results to indicate that a single ioniza- 

 tion is sufficient to inactivate a bacterial cell. He has developed the 

 hypothesis, first suggested by Rahn (1020, 1030), that the bactericidal 

 effects are due to lethal mutations induced by the radiation. Following 

 elaborate analysis of the results obtained with E. coli exposed to radiations 



