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United States Department of Agriculture

Agricultural Research Service

Title: RADIATION SENSITIVITY OF SALMONELLA ISOLATES RELATIVE TO RESISTANCE TO AMPICILLIN, CHLORAMPHENICOL OR GENTAMICIN

Authors
item Niemira, Brendan
item Lonczynski, Kelly
item Sommers, Christopher

Submitted to: Journal of Radiation Physics and Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 11, 2006
Publication Date: July 20, 2006
Citation: Niemira, B.A., Lonczynski, K., Sommers, C.H. 2006. Radiation sensitivity of salmonella isolates relative to resistance to ampicillin, chloramphenicol or gentamicin. Journal of Radiation Physics and Chemistry. 75:1080-1086.

Interpretive Summary: Antibiotic resistance of inoculated bacteria is a commonly used selective marker. Bacteria resistant to the antibiotic nalidixic acid (Nal) have been used in studies of ionizing radiation, but Nal-resistant bacteria have recently been shown to have an increased sensitivity to irradiation than Nal-sensitive isolates. The purpose of this research was to screen a collection of Salmonella isolates for antibiotic resistance and determine the association, if any, of antibiotic resistance with radiation sensitivity. Twenty-four clinical isolates of Salmonella were screened for native resistance to multiple concentrations of the antibiotics ampicillin (Amp), chloramphenicol (Chl), or gentamicin (Gm). Test concentrations were chosen based on established clinical minimum inhibitory concentration (MIC) levels, and isolates were classified as either sensitive or resistant based on their ability to grow at or above the MIC. Salmonella cultures were grown overnight at (37°C) in antibiotic amended tryptic soy broth (TSB). Each of the 24 isolates (100%) were resistant to Gm and were able to grow when exposed to the highest level of antibiotic. Eight of the 24 isolates (33%) were shown to be resistant to Amp. Seven of the 24 isolates (29%) were shown to be resistant to Chl. In separate experiments, all 24 Salmonella cultures were grown overnight (37°C) in TSB, centrifuged, and the cell pellets re-suspended in phosphate buffer. The samples were then gamma irradiated at doses up to 1.0 kGy to determine their sensitivities to ionizing radiation. The D10values (the ionizing radiation dose required to reduce the viable number of microorganisms by 90%) were determined for the 24 isolates and ranged from 0.181 to 0.359 kGy. No correlation was found between the D10 values of the isolates and their sensitivity or resistance to each of the three antibiotics. Resistance to Amp or Chl are suggested as more appropriate than Nal for use as Salmonella resistance markers in studies of irradiation. The use of validated methods, such as those presented herein, will improve the quality and utility of data obtained in these studies.

Technical Abstract: Antibiotic resistance of inoculated bacteria is a commonly used selective marker. Bacteria resistant to the antibiotic nalidixic acid have been shown to have an increased sensitivity to irradiation. The purpose of this research was to screen a collection of Salmonella isolates for antibiotic resistance and determine the association, if any, of antibiotic resistance with radiation sensitivity. Twenty-four clinical isolates of Salmonella were screened for native resistance to multiple concentrations of ampicillin (Amp), chloramphenicol (Chl), or gentamicin (Gm). Test concentrations were chosen based on established clinical minimum inhibitory concentration (MIC) levels, and isolates were classified as either sensitive or resistant based on their ability to grow at or above the MIC. Salmonella cultures were grown overnight at (37°C) in antibiotic amended tryptic soy broth (TSB). Native resistance to Gm was observed with each of the 24 isolates (100%). Eight isolates (33%) were shown to be resistant to Amp while seven isolates (29%) were shown to be resistant to Chl. In separate experiments, Salmonella cultures were grown overnight (37°C) in TSB, centrifuged, and the cell pellets re-suspended in phosphate buffer. The samples were then gamma irradiated at doses up to 1.0 kGy. The D10values (the ionizing radiation dose required to reduce the viable number of microorganisms by 90%) were determined for the 24 isolates and ranged from 0.181 to 0.359 kGy. No correlation was found between the D10 value of the isolate and its sensitivity or resistance to each of the three antibiotics. Resistance to Amp or Chl are suggested as appropriate resistance markers for Salmonella test strains to be used in studies of irradiation.

Last Modified: 10/31/2014
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