On the sensitivity of frozen micro-organisms to ultraviolet radiation

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Abstract
The sensitivity of a variety of microorganisms to the lethal effects of UV irradiation has been studied at low temperatures in the frozen state. In the majority of instances microorganisms were found to be supersensitive to UV irradiation at low temperatures. This increase in sensitivity only occurred when microorganisms were irradiated while frozen and was independent of the effects of freezing and thawing. Supercooled organisms were not supersensitive. Mutation induction by UV irradiation at low temperatures was also enhanced in Escherichia coli and the observed pattern of supersensitivity was similar to that seen for killing in that there was a progressive increase in the effectiveness of UL irradiation until a maximum effect was observed at -79[degree]C. At -196[degree]C there was a decrease in the observed supersensitivy both to the lethal and mutagenic effects of exposure to UV. The enhancement factors for the lethal effects of UV irradiation at -79[degree]C compared with irradiation at 22 UV for various microorganisms are as follows: E. coli B/r, 7.0; Staphylococcus aurens, 5.2; Aerobacter aerogenes (e), 5.0; A. aerogenes (s), 5.5; Bacillus subtilis, 5.0; B. subtilis (spores). 2.5; Pseudomonas sp., 7.0; Saccharomyces cerevisiae (diploid), 3.5; Serratia marcescens. S. bacteriophage T1, 2.5; Micrococcus radiodurans and the spores of the fungus Aspergillus nidulans were not supersensitive to UV irradiation at -79[degree]C. Purified DNA (transforming principle, B. subtilis) was not supersensitive to irradiation at low temperatures. The probability that the damage induced by UV irradiation at low temperatures does not involve the formation of pyrimidine cyclobutane dimers is discussed.