The Armed Forces Radiobiology Research Institute (AFRRI) in the U. S. Department of Defense conducts biomedical research on the effects of ionizing radiation. It has the largest radiobiology program in the United States and is a national resource in the response to nuclear and radiation accidents. Bacterial spores are potential biological weapons because they can be prepared and distributed by aerosol, they endure harsh environmental conditions, and they are infectious. Decontamination procedures for large concentrations of spores must be effective and practical. We determined the dose response of bacterial spores to three qualities, or types, of ionizing radiation.
Inactivation of dry and hydrated bacterial spores with gamma radiation has been more thoroughly studied than spore inactivation with neutron radiation. Decimal‐reduction curves were produced at AFRRI for Bacillus atrophaeus (B. subtilis var. niger, B. globigii, “BG”), B. pumilus, B. thuringiensis, and B. anthracis Sterne spores, both wet and dry, using doses of 0.3 to 7.2 kGy neutrons delivered at a dose rate of 44 to 49 Gy/min (Dn/DT=0.95) in the AFRRI training, research, isotope‐producing General Atomic (TRIGA) Mark‐F nuclear reactor, and doses of 0.6 to 24.0 kGy gamma rays delivered at dose rates of 112 to 120 Gy/min in the AFRRI cobalt‐60 (60Co) gamma‐photon irradiation facility. Decimal‐reduction curves were constructed by plotting the spore survival fraction in terms of colony‐forming units vs. radiation dose. All four species showed greater sensitivity to neutron radiation than to gamma radiation, regardless of the state of hydration. Dry spores of all four species were more sensitive to gamma radiation than were hydrated spores. In contrast, the state of hydration, whether dry or hydrated, of spores of B. subtilis and B. pumilus, which were embedded in filter paper strips, did not affect their sensitivity to neutron radiation. Wet B. thuringiensis spores were only slightly more sensitive to neutrons than were B. thuringiensis spores in dry powdered form. Furthermore, the species most resistant to neutron and gamma radiations was a concentrated B. anthracis Sterne spore suspension. When the starting spore concentration, the Bacillus species used, the radiation quality (neutron or gamma), and the state of hydration are known, the radiation decimal‐reduction curves generated in these studies can be used to predict bacterial spore survival.
Electron‐beam radiation (e‐beam) has been used to inactivate microorganisms in spices, fresh food, medical components, and hazardous waste. AFRRI assessed the efficacy of using an e‐beam for decontamination of bulk biological agents and of byproducts of the decontamination procedures such as wipes and aqueous runoff. Biological agent surrogates were tested under controlled conditions to determine the effectiveness of e‐beam for decontamination. Using the AFRRI linear accelerator (LINAC) to deliver doses of 2 to 20 kGy at a dose rate of 1 kGy/min, radiation decimal‐reduction curves were constructed for Bacillus atrophaeus spores in a dry powder and B. anthracis Sterne spores in a slurry. Doses of 0.25 to 1.0 kGy were delivered to vegetative Gram‐negative bacterial cells of Serratia marcescens. The LINAC produced 13‐MeV electrons at 30 pulses/sec with a 4‐µsec pulse width generated through a water scatterer. Spore samples were irradiated in an array of three screw‐capped polystyrene tubes. Dosimetry was performed at the beginning of each experimental run with LiF:Ti,Mg thermoluminescent dosimeters (TLDs), product type TLD‐100 (Bicron®). TLDs were processed with the Bicron®/Harshaw Model 5500 Automatic TLD Reader. The inactivation data for dry B. atrophaeus spores, B. anthracis Sterne spores, and S. marcescens were fitted to a mathematical formula. The e‐beam decimal‐reduction curves for the bacterial spores were similar to our previously generated gamma‐photon radiation curves. The vegetative bacterial cells of S. marcescens were more susceptible to high‐speed electrons than were the bacterial spores. These experimental findings support the concept of using a truck‐mounted transportable LINAC in the field for decontaminating bulk materials that are contaminated with pathogenic bacteria.