EFFECT OF TEMPERATURE ON RADIATION RESISTANCE OF SPORES OF CLOSTRIDIUM BOTULINUM 33A

Abstract

Spores of Clostridium botulinum 33A in two suspending media, phosphate buffer and pork pea broth, were gamma irradiated at radiation temperatures from +20 °C to −196 °C. The liquid–solid transition around 0 °C introduced a one-step plateaulike change in radiation sensitivity of spores. The spores were considerably more resistant in the solidly frozen medium than in the liquid medium. The solid state reduced indirect effects of radiation by physical blocking of secondary radicals. The one-step transition was much smaller in pork pea broth than in buffer. This indicated that scavenging of secondary radicals by pork pea broth caused a considerable reduction of indirect effects of radiation, but did not completely eliminate them.Freezing alone did not eliminate all indirect effects. This was evident from the gradual reduction in radiation sensitivity of spores in the solidly frozen phosphate buffer betweeen −25 °C and −196 °C. Furthermore, even at −196 °C indirect effects of radiation remained active since (a) spore survival did not level off with respect to temperature and (b) the lethal efficiency of radiation was still appreciably greater in phosphate buffer than in pork pea broth at −196 °C.In frozen pork pea broth at −25 °C to −196 °C temperature had no effect on radiation sensitivity of spores. This indicated that spores were killed by direct hits; apparently physical blocking by the solidly frozen medium, plus radical scavenging by pork pea broth eliminated all indirect effects of radiation.