EFFECTS OF RADIATION ON BACTERIA 1137 



into guinea pigs. It was also determined that cultures younger than 

 about four weeks were more resistant to radiation. 



In spite of further studies on lethal effects and on modification of 

 virulence or of pigmentation, the problem remained controversial and 

 quantitative data were few. In 1925, Klovekorn (86) used Staphylococcus 

 pyogenes aureus and Bacillus colt, with radiation from an Apex apparatus 

 provided with an attachment for a Coolidge tube. The intensities are 

 given in terms of SN (1 sn being equivalent to 600 r). The bacterial 

 preparations were arranged to test the effects under several conditions: 

 thus the materials for exposure were (a) growing agar streak cultures; 

 (6) bouillon cultures 24 hr. old; (c) suspensions in physiological salt 

 solution; and (d) plate cultures 25 to 30 days old. In advance it was 

 determined that effects upon the media employed were insufficient to be 

 regarded as a factor in the results. Up to a dosage of 60 sn there was no 

 change in time or character of growth, even in respect to color; while in 

 bouillon and in physiological salt solution changes in cultural behavior 

 began at 80 sn, yet in no case were there definite, lethal effects up to 

 120 SN, the maximum intensity employed. On the other hand, cultures 

 28 to 30 days old, on agar plates, showed diminished growth capacity 

 at 80 SN, and complete killing at 120 sn. Reasonably substantiating 

 results were obtained with B. coli. The findings were also verified by 

 use of the "Tusche" (brushed film) culture technique. Summarizing, it 

 was found that modification of cultural characteristics may be brought 

 about at 60 to 80 sn, w^hile lethal effects at 110 to 120 sn are produced 

 only when the exposed cultures are old (28 to 30 days, under these 

 conditions). Klovekorn's paper should be consulted for brief reviews of 

 other earlier papers not mentioned in this discussion. 



Many of the more recent studies on the effects of X-rays on bacteria, 

 as well as on other organisms, have had the advantage — on the biological 

 side — of improved techniques and conditions, of higher radiation inten- 

 sities, and of a more accurate measurement of dosage in roentgen units. 

 An appreciation of all these developments is important in the later inter- 

 pretations and discussions. In his general review of X-ray effects, 

 Packard (114) has analyzed and systematized the results reported from 

 a variety of biological material, and the reader is referred to that paper 

 for broader orientation. Here it is possible to consider only a few of the 

 many relevant lines of discussion. 



Unfortunately, most of the quantitative X-radiation work on bacteria 

 relates to lethal effects only. From the data now available it is well 

 known that if any population of microorganisms is exposed to radiation 

 inducing lethal effects, one may plot the percentage of survivals against 

 the time of exposure and the results will appear in the form of a logarith- 

 mic or of an S-shaped curve. Biologists are generally agreed that such 

 S-shaped curves are characteristic of a normal variability of a population 



