Mechanism by which gamma irradiation increases the sensitivity of Salmonella typhimurium ATCC 14028 to heat.

Effects of irradiation and heating on survival of Salmonella typhimurium ATCC 14028 were examined by measuring DNA damage and the integrity of the cytoplasmic membrane. S. typhimurium cells fell into two distinct groups following heating: (i) heat-sensitive cells, which were rapidly inactivated at 65 degrees C and (ii) heat-resistant cells, which were only slowly inactivated at 65 degrees C. Radiation sensitivity of S. typhimurium was greater in the presence of air than in the presence of N2 gas (radiation doses required to inactivate 90% of the cells, 0.394 +/- 0.029 in air and 0.561 +/- 0.035 in N2). Recovery of the covalently closed circular form of plasmid pBR322 from S. typhimurium transformants (Ampr Tetr) was decreased by irradiation but not by heating. Heating prior to irradiation significantly decreased the recovery of plasmid DNA without affecting survival of S. typhimurium. Transformability of the recovered plasmid pBR322 was affected by neither irradiation nor heating, and mutation of antibiotic resistance genes was not detected in S. typhimurium. Heating, but not irradiation, caused destabilization of the cytoplasmic membrane, allowing penetration of hydrophobic dye. These results suggest that lethality of heating followed by irradiation for S. typhimurium was additive, reflecting irradiation-induced DNA damage and heat-induced membrane destabilization. When irradiation preceded heating in the absence of air, more cells were inactivated than was expected, because of heat-inactivating radiation-damaged DNA.