Killing and mutagenic action of sunlight upon Bacillus subtilis spores: a dosimetric system

A method to monitor killing and mutagenic activity of sunlight was established by using wild-type spores (UVR) of Bacillus subtilis and mutant spores (UVS) sensitive to UV radiation. Samples exposed to radiations consisted of the spores spotted and dried on membrane filter. After the exposure, they were recovered as suspensions in water and assayed for colony-forming survival and frequency of reversion of an auxotrophic marker (hisB101). In this system, the UVS spores were inactivated exponentially, and the 37% survival was attained with 2.0 Jm-2 of 254 nm or 2.5 X 10(3) Jm-2 of 313 nm radiation, and with 7 min (August) or 63 min (December) exposure to noon-time sunlight under a clear sky at Tsukiji (latitude 35 degrees 40' N) at sea level in Tokyo. The doubling of the spontaneous mutation frequency of the UVR spores was attained with 3.0 Jm-2 of 254 nm or 2.2 X 10(3) Jm-2 of 313 nm radiation, and with 32 min (August) or 136 min (December) of solar exposure. The results encourage the use of this B. subtilis spore system to determine the gene-damaging activity of the solar-UV radiation under a variety of environmental conditions.