INCREASED INACTIVATION OF SACCHAROMYCES-CEREVISIAE BY PROTRACTION OF UV IRRADIATION

The principle of equi-effectivity of the product of intensity and expo sure time (principle of Bunsen-Roscoe) of UV irradiation has been assu med to be valid for the inactivation of microorganisms in general, Ear lier studies claimed higher survival of Escherichia coli Bir with frac tionated irradiation compared with single-exposure survival, However, data on the inactivation effect of protraction of UV irradiation are n ot available, By means of a specially designed UV irradiation apparatu s which secured absolute UV dose measurements throughout the experimen ts, the effects of variations of UV irradiation intensities (253.7 nm) and exposure times were tested on the inactivation of a bacterial vir us (Staphylococcus aureus phage A994), a vegetative bacterial strain ( E. coli ATCC 25922), and bacterial spores (Bacillus subtilis ATCC 6633 ) as well as three haploid laboratory strains (RC43a, YNN281, and YNN2 82) and two diploid strains (commercial bakery yeast strain and labora tory strain YNN281xYNN282) of yeast (Saccharomyces cerevisiae) and spo res of the latter diploid yeast strain, Each test organism was exposed to three UV intensities (0.02, 0.2, and 2 W/m(2)), with corresponding exposure times resulting in three dose levels for each intensity, Dif ferences in inactivation rates were tested by analyses of variance and Newman-Keuls tests, Virus and bacteria showed no differences in inact ivation rates by variation of intensities and exposure times within se lected UV doses; hence, the principle of Bunsen-Roscoe could not be re jected for these strains, However, in the eukaryotic test strains of S . cerevisiae longer exposure times with lower intensities led to enhan ced inactivation in both haploid and diploid strains, with a more pron ounced effect in the diploid yeast strains, whereas in yeast spores th is dose rate effect could not be observed.