The development of quantitative methods for studying the effects of ultra-violet radiation on bacteria by such workers as Ward (1893), Bang (1905), Bayne-Jones and yon der Lingen (1923), and more recently by Gates (1929-30) and Wyckoff (1931-32), has only recently been applied to other organisms such as yeast. exposed S. elIipsoideus under a filter which transmitted the band of wave-lengths of the mercury vapor arc spectrum between 2800 and 3800/~.u. and noted that three categories; (a) immediate death, (b) deferred death (two-cell groups), and (c) ceils which, after a retardation in cell division, ultimately recovered their reproductive powers, could be distinguished in the irradiated cells. Wyckoff and Luyet (1931) showed that yeast exposed to high energies at several sharp bands in the mercury vapor spectrum fell into slmiIar categories to those of Lacassagne, with the production of some giant cells. Their results tended to indicate a multiple-quantum-hit relation between survival ratio and time of exposure.It is of interest to extend these results by a determination of the various factors involved in the selective absorption of radiant energy by the organism over a fairly complete range of wave-lengths at carefully measured incident energies.The present investigation 1 was proposed by Professor W. J. Crozier as a search for stimulating and inhibitory effects of ultra-violet energy ' The apparatus and the opportunity for an investigation of the effects of measured monochromatic ultra-violet light on yeast cells
It has been shown previously (1-3) that monochromatic ultra-violet radiation produces inhibitory and lethal effects in the yeast Saccharomyces cerevisiae which are approximately the same at each wavelength tested but require quite different incident energies to effect the same degree of reaction at different regions of the ultra-violet spectrum (2).The present paper deals with these differing energy requirements in the production of a given reaction end-point over the region of the spectrum studied, as an index of the biological or lethal spectrum of yeast.The effectiveness of ultra-violet energy in producing a given effect, whether it be inhibition of normal colony formation, cessation of metabolic processes such as respiration or fermentation, or the stoppage of growth and cell division, must be intimately related to its absorption by the yeast cell or by the medium upon which the cell grows. No demonstrable effect of irradiating the malt agar medium could be observed. Plates of the medium irradiated for 1 and 2 hours supported normal colony formation of yeast seeded immediately after the exposure. Under the conditions of the tests the effect discussed must be directly upon the yeast cell.Measurement of the absorption of ultra-violet energy by intact yeast cells was attempted 1 by the method used by Gates (4) in obtain-1 The opportunity to make these tests and the use of the necessary apparatus
It has been shown previously (1, 2) that the action of monochromatic ultra-violet radiation on the yeast Saccharomyces cerevisiae is not an all or none effect, but is a graded result varying from induction of simple inability to form normal sized colonies to "death" of the ~ell, through different degrees of damage, and involving in some stages the formation of giant cells. The mean "survival" curves for irradiated 24 hour cultures (2) resemble the curve for a first order process, but on this assumption divergences indicate the presence of modifying factors.The present paper deals with several factors which may modify the absorption of energy and the cell processes resulting from irradiation of yeast with ultra-violet light. Age of the CellsSimilar to the effects observed with other organisms (3, 4), the age of the yeast culture has a marked influence on the relative resistance of the cells to lethal irradiation, as measured by the energy required to suppress budding (2). This is illustrated by a comparison of the results obtained on irradiating two cultures of widely different periods of incubation, (A) a 24 hour culture and (B) a 15 day culture of yeast, both incubated at 25°C. and irradiated in a large quartz monochromator at the wave-length 2535 A. u. as previously described (2) after inoculation on malt agar contained in small Petri plates. As shown in Fig. 1, from 20 to 50 per cent more incident energy to produce a given effect is required for the 15 day culture than for the 24 hour cultures. This is best explained on the basis that the greater 243
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