Submitted to: Journal of Food Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/27/2004
Publication Date: 2/16/2005
Citation: Niemira, B.A. 2005. Nalidixic acid resistance increases sensitivity of escherichia coli o157:h7 to ionizing radiation in solution and on green leaf lettuce. Journal of Food Science. 70(2): M121-M124. Interpretive Summary: Resistance to antibiotics has been used as a way to distinguish inoculated test pathogens from background microflora for studies of pathogen-inoculated fruits and vegetables. Three strains of the human pathogen Escherichia coli O157:H7, originally antibiotic-sensitive, were made antibiotic-resistant. The response to ionizing radiation was determined for these six strains in a phosphate buffer solution and on green leaf lettuce. All of the antibiotic-resistant daughter strains were significantly more sensitive to ionizing radiation than the antibiotic-sensitive parent strains. The differences in radiation sensitivity between antibiotic-resistant and antibiotic-sensitive strains depended on the isolate and the suspending medium (buffer or lettuce). In buffer, the radiation resistance for the antibiotic-resistant strains was reduced by up to 69%. On lettuce, the radiation resistance for the antibiotic-resistant strains was reduced by up to 45%. These results suggest that the use of antibiotic resistance may result in significant overestimates of the efficacy of irradiation against E. coli O157:H7. These results will assist food scientists in generating data which accurately reflects real-world conditions, thereby assisting regulatory agencies and commercial food processors in the development of vegetable processing protocols that reduce the risk of food-borne illness so as to protect consumers, while preserving food quality.
Technical Abstract: 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 made resistant to nalidixic acid (Nal-R, 50 ug/ml) by successive reculturing (~10E7 cfu per transfer) and selection in tryptic soy broth of increasing nalidixic acid concentration. The resistance to ionizing radiation of the parent and Nal-R strains was determined in Butterfield's phosphate buffer solution and on green leaf lettuce. The Nal-R strains of each of the three isolates were significantly (P<0.05) more sensitive to ionizing radiation than the nalidixic acid-sensitive (Nal-S) parent strains in both systems. The D10 values (the amount of ionizing radiation required to achieve 1.0 log10 reduction) determined in buffer for the parent strains ranged from 0.18-0.33 kGy, while for the Nal-R strains, D10 were ~0.10 kGy, a reduction of up to 69%. On green leaf lettuce, the D10 for the Nal-S strains decreased to ~0.18 kGy vs. 0.10-0.12 kGy for the Nal-R strains, a reduction of up to 45%. The D10 values obtained on lettuce were significantly different than those obtained in buffer for four of the six isolates examined. The magnitude of the increase in radiation sensitivity resulting from resistance to nalidixic acid was strain dependent, and varied depending on the suspending medium i.e., buffer or lettuce. 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.