On the sensitivity of frozen micro-organisms to ultraviolet radiation

Abstract

The sensitivity of a variety of micro-organisms to the lethal effects of u.v. irradiation has been studied at low temperatures in the frozen state. In the majority of instances micro-organisms were found to be supersensitive to u.v. irradiation at low temperatures. This increase in sensitivity only occurred when micro-organisms were irradiated whilst frozen and was independent of the effects of freezing and thawing. Supercooled organisms were not supersensitive. Mutation induction by u.v. 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 u.v. irradiation until a maximum effect was observed at — 79 °C. At — 196 °C there was a decrease in the observed supersensitivity both to the lethal and mutagenic effects of exposure to u.v. The enhancement factors for the lethal effects of u.v. irradiation at — 79 °C compared with irradiation at 22 °C for various micro-organisms are as follows: E. coli B/r, 7.0; Staphylococcus aureus , 5.2; Aerobacter aerogenes (r), 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 , 8.5; bacteriophage T 1 , 2.5; Micrococcus radiodurans and the spores of the fungus Aspergillus nidulans were not supersensitive to u.v. irradiation at — 79 °C. Purified DNA (transforming principle, B. subtilis) was not supersensitive to irradiation at low temperatures. The probability that the damage induced by u.v. irradiation at low temperatures does not involve the formation of pyrimidine cyclobutane dimers is discussed.