The Irradiation of Dry Spores of Bacillus subtilis with Fast Charged Particles

The experiments reported in this paper are designed to investigate the structure of dry spores and at the same time to extend target theory treatment to larger structures. Previous studies on smaller biological materials (enzymes, hormones, viruses, etc.) irradiated in the dry state have recently been reviewed in detail (1). In general the investigations reviewed lend support to a simple target theory modified to consider the physical details of the inactivation process. These studies further suggest the possibility that target theory considerations can be useful in elucidating the structure of any biological entity which can be dried and subsequently assayed. Bacterial spores represent relatively large entities which may be conveniently desiccated and then assayed. They also represent a system where all information is structural (2) and thus form a good model system for relating structure to subsequent function. Previous studies on the action of ionizing radiation on bacterial spores (3-6) usually have indicated an exponential type of dose inactivation curve but contain little information on inactivation as a function of linear energy transfer (LET). Lea (6) states that a-particles are more efficient than X-rays in killing spores of Bacillus mesentericus but mentioned only these two LET's. Stapleton et al. (7) show a similar increase in efficiency of inactivation of spores of the fungus Aspergillus terreus with increasing LET for X-rays, a-particles, thermal and fast neutrons. Studies on inactivation as a function of LET have been carried out for yeast (8), but the conditions of irradiation were such that considerations other than those of simple target theory are necessary. In these studies dry spores of Bacillus subtilis were irradiated with protons, deuterons, and a-particles, covering an LET range from 59 ev per 100 A to 3300 ev per 100 A. The ability to form visible colonies on nutrient agar was assayed.