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Title: NALIDIXIC ACID RESISTANCE INFLUENCES SENSITIVITY TO IONIZING RADIATION AMONG SALMONELLA ISOLATES

Author
item Niemira, Brendan
item LONCZYNSKI, KELLY - FORMER ERRC EMPLOYEE

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2006
Publication Date: 7/20/2006
Citation: Niemira, B.A., Lonczynski, K.A. 2006. Nalidixic acid resistance influences sensitivity to ionizing radiation among salmonella isolates. Journal of Food Protection. 69(7):1587-1593.

Interpretive Summary: Resistance to antibiotics, such as nalidixic acid (Nal), has been used as a selective marker for studies of pathogen-inoculated fruits and vegetables. A collection of 24 Salmonella isolates were screened for native resistance to Nal. Of the 24 isolates, three shown to be resistant to Nal and three shown to be sensitive to Nal were selected for further study. The resistance to ionizing radiation was determined and compared for the three natively Nal resistant (NalR) strains and for the three natively Nal sensitive (NalS) strains, and also for three strains derived from NalS which were made resistant to Nal by successive culturing and selection in Nal-amended broth. To distinguish these induced Nal-resistant strains from the native Nal-resistant strains, they were designated NalRi The radiation D10 values (the radiation dose required to achieve a 1 log10, or 90%, 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, NalR isolates were significantly more sensitive to ionizing radiation than NalS isolates, with D10 values of the former 18% or 24% lower than the latter in buffer or orange juice, respectively. Inducing resistance to Nal significantly reduced the D10 values of NalRi isolates relative to their NalS parents by 9% in buffer. In orange juice, the D10 value of NalRi was reduced by 17%, making it intermediate between the NalS parent and NalR isolates. These results suggest that native 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.

Technical Abstract: Nalidixic acid 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 native resistance to nalidixic acid (Nal, 50 'g/ml). The resistance to ionizing radiation was determined and compared for a) three natively Nal resistant (NalR) strains, b) three natively Nal sensitive (NalS) strains, and c) three strains derived from NalS which were made resistant to Nal (NalRi) by successive culturing and selection in Nal-amended broth. The radiation D10 values (the radiation dose required to achieve a 1 log10 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, NalR isolates were significantly more sensitive to ionizing radiation than NalS isolates, with D10 values of the former 18% or 24% lower than the latter in buffer or orange juice, respectively. Inducing resistance to Nal significantly reduced the D10 values of NalRi isolates relative to NalS parents by 9% in buffer. In orange juice, the D10 value of NalRi was reduced by 17%, making it intermediate between the NalS parent and NalR isolates. These results suggest that native 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.