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Submitted to: Food and Bioprocess Technology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 3/6/2008 Publication Date: 4/1/2008 Citation: Niemira, B.A. 2008. Irradiation Sensitivity of Planktonic and Biofilm-Associated Listeria Monocytogenes and L. Innocua as Influenced by Temperature of Biofilm Formation. Food and Bioprocess Technology. 79-85. Interpretive Summary: Human pathogens can form complex networks of cells called biofilms, which serve to protect them from antimicrobial treatments. Ionizing radiation effectively inactivates Listeria on a variety of foods and contact surfaces, but no information is available on the relative efficacy of the process against biofilm-associated cells vs. free-living planktonic cells. The radiation sensitivity of planktonic (free-living) or biofilm-associated cells was determined for two foodborne illness-associated isolates of the human pathogen Listeria monocytogenes and for a commonly employed surrogate, L. innocua. Biofilms were formed on sterile glass slides in a co-incubation apparatus, incubated for 48h at 22, 28 or 37 degrees C. The cultures were gamma irradiated and the radiation D10 value (the dose required to reduce the population by 90%) was calculated for the planktonic and biofilm cells of each isolate grown at each temperature. Higher D10 values reflect greater resistance to radiation. The effect of temperature of cultivation on the radiation sensitivity of both planktonic cells and biofilm-associated cells varied for each of the isolates. Depending on the isolate and temperature of cultivation, biofilm-associated cells were equally sensitive or more sensitive to irradiation as the respective planktonic cells. D10 values overall tended to increase with temperature of cultivation for L. innocua and for one of the isolates of L. monocytogenes, but tended to decrease with of increasing temperature for the other L. monocytogenes isolate. The D10 values of the various culture/temperature combinations differed significantly among the isolates examined. Irradiation effectively eliminates both planktonic and biofilm-associated cells. The extent to which the biofilm habitat modifies the antimicrobial efficacy of ionizing radiation is dependent on the specific isolate examined and the temperature at which it forms. Development of antimicrobial processes that can overcome the protective effect of the biofilm habitat requires information on how culture conditions may enhance or detract from the efficacy of the process. This study is the first to show that biofilm-associated Listeria cells are as sensitive or more sensitive to ionizing radiation compared with planktonic cells, that this response is dependent on biofilm formation conditions, and to demonstrate the similarity (or lack thereof) in response of the pathogen L. monocytogenes and the surrogate L. innocua. Technical Abstract: Ionizing radiation effectively inactivates Listeria on a variety of foods and contact surfaces, but no information is available on the relative efficacy of the process against biofilm-associated cells vs. free-living planktonic cells. The radiation sensitivity of planktonic or biofilm-associated cells was determined for two foodborne illness-associated isolates of Listeria monocytogenes (ATCC 43256 and 49594) and for a commonly employed surrogate, L. innocua (ATCC 51742). Biofilms were formed on sterile glass slides in a co-incubation apparatus, incubated for 48h at 22, 28 or 37 degrees C. The cultures were gamma irradiated and the radiation D10 value was calculated for each combination of isolate/culture/temperature. The effect of temperature of cultivation on the radiation sensitivity of both planktonic cells and biofilm-associated cells varied for each of the isolates. Depending on isolate and temperature, biofilm-associated cells were equally sensitive or more sensitive to irradiation. D10 values overall tended to increase with temperature of cultivation for L. monocytogenes 49594 and L. innocua 51742, but tended to decrease with of increasing temperature for L. monocytogenes 43256. The D10 values of the various culture/temperature combinations differed significantly among the isolates examined. Irradiation effectively eliminates both planktonic and biofilm-associated cells. The extent to which the biofilm habitat modifies the antimicrobial efficacy of ionizing radiation is dependent on the specific isolate examined and the temperature at which it forms. Development of antimicrobial processes that can overcome the protective effect of the biofilm habitat requires information on how culture conditions may enhance or detract from the efficacy of the process. This study is the first inquiry to show that biofilm-associated Listeria cells are as sensitive or more sensitive to ionizing radiation compared with planktonic cells, that this response is dependent on biofilm formation conditions, and to demonstrate the similarity (or lack thereof) in response of the pathogen L. monocytogenes and the surrogate L. innocua. |
