Bactericidal Effect of Ultraviolet Radiation

Abstract
By the use of a simple integrating meter for measuring the effective bactericidal u.-v. radiation and a unique method of uniformly seeding Petri plates with bacteria, the lethal action of radiation was quantitatively investigated for a number of organisms. The Bunsen-Roscoe Reciprocity Law was proved to hold for Escherichia coli at room temp. over a range of intensities requiring only a few microseconds to several hrs. for producing the same amt. of radiation. The law was found not to hold for low intensities and long times of exposure, in which case greater lethal action was obtained (the bacteria were permitted to proceed to an abnormally sensitive stage of their life cycle and were thus killed by less radiation). These studies and others on the effect of heat, the distribution of resistant and non-resistant organisms in a culture and the retarding effect on the life cycle of sublethal doses of ultraviolet indicate conclusively that the "single photon hit" theory is not applicable to the bactericidal action of u.-v. radiation.

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