Some Effects of Ultraviolet Radiation on Sporulating Cultures of Bacillus Cereus'

Zelle and Hollaender (1955) summarized data from several sources which showed that spores of aerobic bacilli are usually about twice as resistant to killing by ultraviolet light as are vegetative cells of the same species. Recently Stuy (1956) showed that spores of Bacillus cereus are many times more resistant than their vegetative cells. In seeking an explanation for the discrepancies in these data, we noticed that almost without exception data comparing radiation resistance of spores and their vegetative cells were published prior to the report by Kelner (1949) on photoreactivation. Since only vegetative cells of the sporeforming bacilli are capable of photoreactivation, the differences in sensitivity to ultraviolet light between these cells and their spores would be minimized unless precautions were taken to exclude the photorestoration process. Furthermore, in many of the reports, the vegetative cells and spores were irradiated on the surface of a nutrient medium. Spores rapidly lose their resistance to ultraviolet light when placed in an environment suitable for germination (Mefferd and Wyss, 1951; Stuy, 1956), so it is possible that at the time the spores were irradiated they had actually begun germination, and consequently were more sensitive to destruction. When the spores of Bacillus anthracis were placed in environment favorable for germination, their sensitivity to ultraviolet light began to increase within 5 min and in a relatively short time their sensitivity paralleled that of vegetative cells. Since the change in sensitivity of germinating spores occurs with such dramatic suddenness, it was felt that a system in which these changes occur at a more leisurely pace would be more amenable to experimental manipulations. Accordingly our attention was drawn to the system