Radiation Physics and Chemistry 75 (2006) 1080–1086 Radiation sensitivity of Salmonella isolates relative to resistance to ampicillin, chloramphenicol or gentamicin $ Brendan A. Niemira , Kelly A. Lonczynski, Christopher H. Sommers US Department of Agriculture, Agriculture Research Service, Eastern Regional Research Center, Food Safety Intervention Technologies Research Unit, 600 East Mermaid Lane, Wyndmoor, PA 19038, USA Received 2 November 2005; accepted 11 March 2006 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 1C) 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 1C) in TSB, centrifuged, and the cell pellets were re-suspended in phosphate buffer. The samples were then gamma irradiated at doses up to 1.0 kGy. The D 10 values (the ionizing radiation dose required to reduce the viable number of microorganisms by 90%) were determined for the 24 isolates and they ranged from 0.181 to 0.359 kGy. No correlation was found between the D 10 value of the isolate and its sensitivity or resistance to each of the three antibiotics. Resistance to Amp or Chl is suggested as appropriate resistance marker for Salmonella test strains to be used in studies of irradiation. r 2006 Elsevier Ltd. All rights reserved. Keywords: Antibiotic resistance; Irradiation; Nalidixic acid; Salmonella; D 10 value; Selective marker 1. Introduction In the past two decades, the consumption of fresh fruits and vegetables in the US has increased, and the geographic sources and distribution of fresh produce have expanded greatly (Tauxe et al., 1997). This increase is due, in part, to consumer demand for more ready-to- eat foods that require little preparation, as well as a trend towards healthier foods. However, most produce receives minimal processing and can be a source of bacterial contamination. The demand for these fresh products has placed greater requirements on producers to ensure food safety (Blackburn and Davies, 1994). Public health officials have documented an increase in the number of produce-associated foodborne illnesses (Thayer and Rajkowski, 1999; Sivapalasingam et al., ARTICLE IN PRESS www.elsevier.com/locate/radphyschem 0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2006.03.004 $ Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorse- ment by the US Department of Agriculture. Corresponding author. Tel.: +1 215 836 3784; fax: +1 215 233 6405. E-mail address: bniemira@errc.ars.usda.gov (B.A. Niemira).