EFFECTS OF RADIATION ON BACTERIA 



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The contribution made by Gates (59) to the effects of monochromatic 

 radiation on bacteria involved nothing new in the way of apparatus or 

 procedure, but rather a selection of efficient and practicable methods by 

 means of which consistent quantitative results were obtained. A large 

 quartz-monochromator and quartz-lens system was used for the separa- 

 tion and focusing of the specific wave-lengths required, a thermopile and 

 galvanometer for energy measurement, and a quartz-mercury-vapor 

 lamp as a source of radiation. The work was done primarily with 

 Staphylococcus aureus, the bacteria for exposure being washed over a 

 hardened agar surface; and after exposure the surface was covered with 

 a thin layer of agar, with the 

 idea of reducing colony spread 

 and promoting accuracy of 

 colony counting. 



The wave-lengths chosen 

 were wholly in the ultra- 

 violet, chiefly 2378 to 3126 A. 

 At all wave-lengths studied 

 the reactions of this organism 

 followed essentially similar 

 curves, each at a different 

 energy level. At X2600 to 



o 



2700 A the incident energy 

 (see Fig. 1) required to kill is 

 less than in any other region 

 of the spectrum examined. 

 The data suggest a second maximum effect below X2300 A. Using 50 

 per cent survival as^a basis, the amount of energy required to kill was 88 

 erg/mm. 2^at 2675 A, 3150 erg/mm. ^ at 3020 A, and 25,000 erg/mm. ^ 

 at 3126 A. The curves exhibiting the reactions of S. aureus show a 

 general similarity to those for monomolecular reactions, but with some 

 evidence of the effect of age and metabolic activity on the form of the 

 curve, the author concludes that "the typical curve seems to be best 

 interpreted as one of probability." 



A well-rounded discussion of the lethal action of ultra-violet on 

 bacteria has been presented by Ehrismann and Noethling (45), but it is 

 not possible to compare very accurately the values obtained by these 

 authors with those of Gates (59), since the former authors give the lethal 

 effects only in terms of 1 to 10 per cent and 90 to 100 per cent kiUing, 

 respectively. The procedure employed seems to have been the best 

 available, and the techniques have been well chosen. The apparatus 

 set-up included a double monochromator with quartz optics, a K photo- 

 cell, and a bolometer. The agar-plate, surface-exposure method was 

 employed in some phases of the work. The authors used one species 



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Wave Lengths in yu 



A B 



Fig. 1. — A, curve of incident energies involved 

 in the destruction of 50 per cent of B. coli; B, curve 

 of the reciprocals of A. {From Gates, 60.) 



