Effects of pH and acid resistance on the radiation resistance of enterohemorrhagic Escherichia coli

The effects of pH and the induction of pH-dependent stationary-phase acid r esistance on the radiation resistance of Escherichia coil were determined f or seven enterohemorrhagic strains and one nonenterohemorrhagic strain. The isolates were grown in acidogenic or nonacidogenic media to pH levels of a pproximately 4.7 and 7.2, respectively. The cells were then transferred to brain heart infusion (BHI) broth adjusted to pH 4.0, 4.5, 5.0, and 5.5 (wit h HCl) that was preequilibrated to 2 degrees C, and cultures were then irra diated using a Cs-137 source. Surviving cells and the extent of injury were determined by plating on BHI and MacConkey agars both immediately after ir radiation and after subsequent storage at 2 degrees C for 7 days. Decreasin g the pH of the BHI in which E. coli was irradiated had relatively little e ffect on the microorganism's radiation resistance. Substantial differences in radiation resistance were noted among strains, and induction of acid res istance consistently increased radiation resistance. Comparison of E. coli levels immediately after irradiation and after 7 days of refrigerated stora ge suggested that irradiation enhanced pH-mediated inactivation of the path ogen. These results demonstrate that prior growth under conditions that ind uce a pH-dependent stationary phase cross-protects E. coli against radiatio n inactivation and must be taken into account when determining the microorg anism's irradiation D value.