Abstract
By means of a compensated thermocouple of extreme sensitivity kindly made for us by Professor Pfund of Johns Hopkins University, direct measurements have been made of the ultraviolet light energy, at various wave lengths, required to kill Staphylococcus aureus. The organisms were exposed on agar plates to the lines of the quartz mercury arc spectrum, isolated by a monochromatic illuminator with quartz lenses and prism. After incubation, counts were made of the colonies produced by surviving organisms in the exposed areas.
Comparison with control counts from adjacent, unexposed, areas shows that the bacteria vary widely in individual susceptibility so that the most resistant may survive an exposure 12 to 20 times that to which the least resistant succumb. The average values of a number of observations at each wave length produce a smooth curve which shows the ratio between the incident energy in ergs per square millimeter and the percentage of organisms killed. The light energy required to kill 50 per cent of the exposed bacteria, at each wave length, has been taken as the mean value of the bactericidal effect of incident ultraviolet light.
Measured thus, as incident energy, wide differences were found in bactericidal power at different wave lengths of the mercury arc spectrum. The lethal energy for 50 per cent of the exposed organisms ranged from 154 ergs per square millimeter at 2378 A.U., through 88 ergs at 2675 A.U, to 3,150 ergs at 3022 A.U. In a single determination the corresponding light energy at 3126 A.U. was 25,000 ergs per square millimeter. Intermediate wave lengths gave intermediate values in an orderly sequence and a line through the observed points produced a curve similar to, but not identical with, that for the absorption of ultraviolet light by S. aureus in the same region of the spectrum.
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