Effect of Radiation and Heat on Bacterial Spore DNA

Mild irradiation (administered first) is known to sensitize bacterial spores to subsequent heat injury. This project was concerned with the molecular changes underlying this type of synergistic enhancement of lethal effect. Using the alkaline sucrose gradient sedimentation technique it was found that ionizing radiation of 0.05-0.3 Mrad as well as heating at 90°C for 10-30 min (applied individually) induced single-strand breaks in the [3H] DNA of spores of B. subtilis 168 and C. botulinum 62A. In each case more DNA breaks were induced in the more sensitive strain. Combination treatments of radiation (administered first) followed by heating at 90°C showed a distinct synergistic enhancement effect in the observed number of single-strand breaks in the spore [3H] DNA. Depending on the particular treatment schedule, synergistic enhancement of DNA breakage reached up to 95%. The concurrent synergism in the inactivation of spores of B. subtilis under the conditions of this project was in excess of 500 000. It is clear that a combination of radiation and heat enhances both DNA breakage and spore inactivation. It is proposed that synergism may be due to the fact that lethal heat inactivates repair enzymes, while radiation sufficient to injure the spores leaves these enzymes virtually unharmed. (author)