EFFECTS OF RADIATION ON BACTERIA 1135 



O 



85 per cent, the maximum killing of spores was at X2650 A, where the 

 energy requirement was approximately 18.2 X 10^ erg/cc; whereas with 

 the vegetative stage the same wave-length was most efficient, but the 

 energy requirement was 16.5 X 10' erg/cc. It should be noted, however, 

 that at high survival percentages the energy requirements for killing the 

 vegetative and the spore stages are more nearly equal, becoming farther 

 apart as the survival value diminishes. Accordingly, qualitative 

 experiments, or those giving comparative values for practically complete 

 killing, do not adequately present this relation. Nevertheless, it seems 

 clear that spore stages are more resistant than vegetative cells, and no 

 doubt the extent of this difference is dependent somewhat upon the age 

 of the vegetative cells with which the comparison is made and upon other 

 conditions as well. 



Such results as those just referred to are a striking indication of the 

 fact that there is very little relation between heat resistance and light 

 resistance. It is to be noted, for example, that the spores of B. anthracis 

 are almost as readily killed by light as are vegetative stages, whereas the 

 spore stage is strongly resistant to high temperatures. In the case of 

 B. subtilis, many spores withstand boiling for 15 min., yet the vegetative 

 stage will scarcely withstand an exposure of 15 min. at 65°C. The 

 papers of Pothoff, Ehrismann, and Duggar and Hollaender should be 

 consulted for the technique of securing spores and vegetative stages of 

 suitable purity. 



Reports of the effects of age of the bacterial cell on resistance to 

 irradiation are not consistent. Bang (8) reported increasing resistance 

 with age, up to 62 hours, and Gates, working with Staph, aureus, found 

 "the recently divided and genetically and metabolically active bacteria 

 in the four-hour culture were appreciably less resistant to the ultra-violet 

 energy" than cultures 28 or 52 hr. old. On the other hand, Stenstrom 

 and Gaida (144), among others, have obtained results indicating greater 

 resistance in B. coli as cultures 24 hr. old than as cultures from 1 to 6 weeks 

 old. Apparently the problem of the age relation is not so simple as has 

 been assumed. In the case of organisms adhering in twos or in chains 

 while the cells are young, accurate comparative counts are difficult; while 

 with old cultures many of the cells are dead previous to exposure, and if 

 intensity measurements are made, there is obviously much ineffective 

 absorption. 



X-RAYS 



The earlier work on the effects of X-rays on bacteria was very largely 

 negative, as might be expected when viewed in retrospect, if due con- 

 sideration is given to the probable intensities of the radiation and to other 

 conditions of the experimental work. Of this earlier work only the brief- 

 est indications need be furnished. Stimulated by the work of Buchner on 



