title: Nalidixic acid resistance influences sensitivity to ionizing radiation among Salmonella isolates.
abstract: Nalidixic acid (Nal) resistance has been used as a selective marker for studies of pathogen-inoculated fruits and vegetables. A collection of 24 Salmonella isolates were screened for natural resistance to Nal (50 microg/ml). The resistance to ionizing radiation was determined and compared for i) three naturally Nal-resistant (Nal(R)) strains, ii) three naturally Nal-sensitive (Nal(S)) strains, and iii) three strains derived from Nals strains that were made resistant to Nal (Nal(Ri)) by successive culturing and selection in Nal-amended broth. The radiation D10-values (the radiation dose required to achieve a 1-log reduction in population) were determined in buffer solution and in orange juice. D10-values were significantly (P < 0.05) different among the Salmonella isolates tested. When considered as a group, Nal(R) isolates were significantly more sensitive to ionizing radiation than Nals isolates in both media tested. In buffer, D10 of Nal(R) was 0.210 kGy versus 0.257 kGy for Nals. In orange juice, D10 of Nal(R) was 0.581 versus 0.764 for Nals. Inducing resistance to Nal altered the response to irradiation. D10-value of Nal(Ri) was 0.234 kGy in buffer, a 9% reduction relative to Nals parents. In orange juice, the D10-value of Nal(Ri) was 0.637 kGy, a reduction of 17% relative to Nals parents. These results suggest that natural and/or induced resistance to Nal may predispose Salmonella isolates to greater sensitivity to ionizing radiation,and that this effect is influenced by the suspending medium and by the nature of the isolates evaluated.
authors.affiliation: Food Safety Intervention Technologies Research Unit, U.S. Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, Pennsylvania 19038, USA. bniemira@errc.ars.usda.gov
authors.forename: Brendan A
authors.initials: BA
authors.lastname: Niemira
authors.forename: Kelly A
authors.initials: KA
authors.lastname: Lonczynski
mesh_terms: D015169:Colony Count, Microbial; D003257:Consumer Product Safety; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D005514:Food Irradiation; D005516:Food Microbiology; D006801:Humans; D008826:Microbial Sensitivity Tests; D009268:Nalidixic Acid / Q000494:pharmacology*; D011836:Radiation Tolerance; D012475:Salmonella / Q000187:drug effects* / Q000528:radiation effects*; D013045:Species Specificity
chemical_list: D009268:Nalidixic Acid