Nalidixic Acid Resistance Increases Sensitivity of Escherichia coli O157:H7 to Ionizing Radiation in Solution and on Green Leaf Lettuce

Nalidixic acid resistance has been used as a selective marker for studies of pathogen-inoculated fruits and vegetables. Three nalidixic acid–sensitive outbreak strains of Escherichia coli O157:H7 were used to generate mutants resistant to nalidixic acid (Nal, 50 g/mL) by successive culturing and selection in nalidixic acid–amended broth. The resistance to ionizing radiation of the parent and Nal strains was determined (a) in a phosphate buffer solution and (b) on green leaf lettuce. The Nal strains of each of the 3 isolates were significantly (P < 0.05) more sensitive to ionizing radiation than the nalidixic acid–sensitive (Nal) parent strains in both systems. D10 values (the amount of ionizing radiation required to achieve 1 log10 reduction) determined in buffer for the parent strains ranged from 0.18 to 0.33 kGy, whereas for the Nal strains, D10 were approximately 0.10 kGy, a reduction of up to 69%. When evaluated on green leaf lettuce, the D10 for the Nal S strains was approximately 0.18 kGy as opposed to 0.10 to 0.12 kGy for the Nal strains, a reduction of up to 45%. The D10 values obtained on lettuce were significantly different than those obtained in buffer for 4 of the 6 isolates examined. The magnitude of the increase in radiation sensitivity resulting from resistance to nalidixic acid varied among the strains tested and also varied depending on the suspending medium. These results suggest that the use of nalidixic acid resistance as a selective marker may result in significant overestimates of the antimicrobial efficacy of ionizing radiation against E. coli O157:H7.