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United States Department of Agriculture

Agricultural Research Service

Title: Sensitivity of Planktonic and Biofilm-Associated Salmonella to Ionizing Radiation

Authors
item NIEMIRA, BRENDAN
item Solomon, Ethan

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 8, 2004
Publication Date: May 1, 2005
Citation: Niemira, B.A., Solomon, E.B. 2005. Sensitivity of planktonic and biofilm-associated salmonella to ionizing radiation. Applied and Environmental Microbiology. 71(5): 2732:2736.

Interpretive Summary: Salmonella forms tightly knit clusters of cells called biofilms which adhere to the surfaces of foods and food processing equipment. These cell clusters are relatively resistant to chemical sanitizing treatments. Ionizing radiation has been used to inactivate Salmonella on a variety of foods and contact surfaces, but the relative efficacy of the process against surface-associated cell clusters (biofilms) vs. free-living (planktonic) cells is not well documented. The radiation sensitivity of free-living or surface-associated cells was determined for three foodborne illness-associated isolates of Salmonella. The surface-associated cell clusters were formed on sterile glass slides in a co-incubation apparatus, using inoculated tryptic soy broth. The free-living and surface-associated cultures were gamma irradiated to doses up to 2.5kGy. The dose of radiation required to reduce the bacterial population by 90% ('D10 value') was calculated for each isolate/culture. The D10 values of S. Anatum were not significantly different for surface-associated and free-living cells. In contrast, the surface-associated cells of S. Stanley and S. Enteritidis were significantly more sensitive to ionizing radiation than respective free-living cells, with D10 values reduced by 10.2% (S. Stanley) or 18.5% (S. Enteritidis). The ability of ionizing radiation to inactivate pathogens is therefore preserved or enhanced when treating surface-associated bacteria. These results will assist regulatory agencies and commercial food processors in designing antimicrobial sanitation processes that are effective against both free-living and surface-associated bacteria, thereby reducing the risk of food-borne illness so as to protect consumers.

Technical Abstract: Salmonella forms biofilms that are relatively resistant to chemical sanitizing treatments. Ionizing radiation has been used to inactivate Salmonella on a variety of foods and contact surfaces, but the relative efficacy of the process against biofilm-associated cells vs. free-living planktonic cells is not well documented. The radiation sensitivity of planktonic or biofilm-associated cells was determined for three foodborne illness-associated isolates of Salmonella. Biofilms were formed on sterile glass slides in a co-incubation apparatus, using inoculated tryptic soy broth, incubated at 37 C for 48 h. Resulting biofilms were 18-24 microns in height as determined by confocal scanning laser microscopy. The planktonic and biofilm cultures were gamma irradiated to doses of 0.0 (control), 0.5, 1.0, 1.5, 2.0 and 2.5 kGy. The D10 value (the dose of radiation require to reduce population by one log10, or 90%) was calculated for each isolate/culture based on surviving populations at each radiation dose. The D10 values of S. Anatum were not significantly (P<0.05) different for biofilm-associated (0.645 kGy) and planktonic cells (0.677 kGy). In contrast, the biofilm-associated cells of S. Stanley were significantly more sensitive to ionizing radiation than respective planktonic cells, with D10 values 0.531 and 0.591 kGy, respectively. D10 values of S. Enteritidis were similarly reduced for biofilm-associated (0.436 kGy) vs. planktonic (0.535 kGy) cells. The antimicrobial efficacy of ionizing radiation is therefore preserved or enhanced when treating biofilm-associated bacteria.

Last Modified: 9/10/2014
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