Nalidixic Acid Resistance Influences Sensitivity to Ionizing Radiation among Salmonella Isolates

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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 μg/ml). The resistance to ionizing radiation was determined and compared for i) three naturally Nal-resistant (NalR) strains, ii) three naturally Nal-sensitive (NalS) strains, and iii) three strains derived from NalS strains that were made resistant to Nal (NalRi) by successive culturing and selection in Nal-amended broth. The radiation D 10-values (the radiation dose required to achieve a 1-log reduction in population) were determined in buffer solution and in orange juice. D 10-values were significantly (P < 0.05) different among the Salmonella isolates tested. When considered as a group, NalR isolates were significantly more sensitive to ionizing radiation than NalS isolates in both media tested. In buffer, D 10 of NalR was 0.210 kGy versus 0.257 kGy for NalS. In orange juice, D 10 of NalR was 0.581 versus 0.764 for NalS. Inducing resistance to Nal altered the response to irradiation. D 10-value of NalRi was 0.234 kGy in buffer, a 9% reduction relative to NalS parents. In orange juice, the D 10-value of NalRi 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.

Document Type: Research Article

Affiliations: 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

Publication date: July 1, 2006

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