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PHYSICAL AGENTS 



a-rays) are more effective than the less densely ionizing X-rays and 

 y-rays, but the order of effectiveness is reversed if mutagenesis in A. 

 terreus is the criterion (132). The killing of A. niger spores by «-irra- 

 diation is similarly most effective at high ionization density (161). On 

 the other hand, lethality to vegetative cells of Escherichia coli is great- 

 est with radiation of low ionization density (159). 



Lethal and mutagenic effects of types of radiation other than X-rays 

 have been reported, viz., neutrons (4, 56, 99), a-particles from polonium 

 and radium (9, 10, 11), y-rays from radioactive elements (26, 137), and 

 ^-particles from carbon- 14 or an ion source (84, 150, 151). Earlier 

 studies are reviewed by Smith (127). 



The lethal effect of X-irradiation on spores of fungi and actinomy- 

 cetes can usually be expressed as an exponential or approximately ex- 

 ponential dose-response curve (3, 72, 80, 121). Sigmoid survival curves 

 have only occasionally been reported (45, 90). It seems probable, es- 

 pecially from preliminary work on yeasts, that the form of the curve 

 may be primarily determined by the number of nuclei per cell (116). 

 This consideration cannot apply to the striking response of dry spores 

 of Aspergillus terreus: X-irradiation follows the sigmoidal pattern but 

 survival after irradiation with a-particles is exponential (132, 162). 



Mutation frequency is linear with dose over a range; as shown in 

 Figure 7, some material shows a plateau in the curve at high dosage 

 (54, 80, 108). Presumably this plateau will be reached with any or- 

 ganism provided the dose is high enough. Theoretical models for this 

 saturation effect are considered by Harm and Stein (57). 



100 200 



Dose, kiloroentgens 



300 



Figure 7. Mutation frequency in Streptomyces flaveolns as a function of X-ray 

 dosage. Each curve represents a separate experiment. Redrawn from Reiner (80), 

 by permission of the Williams and Wilkins Company. 



