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Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 3/16/2007 Publication Date: 5/1/2007 Citation: Niemira, B.A. 2007. Irradiation Sensitivity of Planktonic and Biofilm-Associated Escherichia coli O157:H7 Isolates is Influenced by Culture Conditions. Applied and Environmental Microbiology. 73(10): 3239-3244. Interpretive Summary: Escherichia coli O157:H7 is an important human pathogen which is known to form closely knit clusters of cells (biofilms) that are relatively resistant to chemical sanitizing treatments. Ionizing radiation is a food processing technology that effectively inactivates E. coli O157:H7 on a variety of foods and contact surfaces. However, the degree to which the biofilm habitat might protect E. coli O157:H7 cells from irradiation, and the comparative radiation sensitivity of 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 E. coli O157:H7 (C9490, ATCC 35150, ATCC 43894). Biofilms of each culture were grown on sterile glass slides, and incubated at 37C for either 24 h, 48 h or 72 h. The planktonic and biofilm cultures were treated with ionizing radiation at various doses ranging from 0.0 (control) to 1.5kGy. Based on the surviving populations at each radiation dose, the D10 value (the dose of radiation required to reduce population by one log, or 90%) was calculated for each isolate and growth form for each culture maturity. For each of the times sampled, the D10 values of isolate 43894 planktonic cells were significantly higher than those observed for biofilm-associated cells, indicating a significantly increased sensitivity to irradiation for cells in the biofilm habitat. At the 24h sampling time, isolate C9490 showed a similar pattern, with D10 values of planktonic cells significantly higher than for biofilm-associated cells, while isolate 35150 showed the reverse, with D10 values of planktonic cells significantly lower than for biofilm-associated cells. At the 48h and 72h sampling times, there was no difference in radiation sensitivity based on biofilm habitat for C9490 or 35150. These results indicate that the extent to which the biofilm habitat modifies the antimicrobial efficacy of ionizing radiation is dependent on the specific isolate examined and the maturity of the biofilm. Furthermore, it is shown that although the biofilm habitat can protect pathogen cells from chemical sanitizers, it does not protect against irradiation. Technical Abstract: Escherichia coli O157:H7 is an important human pathogen which is known to form biofilms that are relatively resistant to chemical sanitizing treatments. Ionizing radiation effectively inactivates E. coli O157:H7 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 E. coli O157:H7 (C9490, ATCC 35150, ATCC 43894). Biofilms were formed on sterile glass slides in a co incubation apparatus, using inoculated tryptic soy broth, incubated at 37C for either 24 h, 48 h or 72 h. The planktonic and biofilm cultures were gamma irradiated to doses ranging from 0.0 (control) to 1.5kGy. The D10 value (the dose of radiation required to reduce population by one log, or 90%) was calculated for each isolate/culture/maturity based on surviving populations at each radiation dose. For each of the times sampled, the D10 values of isolate 43894 planktonic cells (0.454-0.479 kGy) were significantly (P<0.05) higher than those observed for biofilm-associated cells (0.381-0.385kGy), indicating a significantly increased sensitivity to irradiation for cells in the biofilm habitat. At the 24h sampling time, isolate C9490 showed a similar pattern, with D10 values of planktonic cells (0.653kGy) significantly higher than for biofilm-associated cells (0.479kGy), while isolate 35150 showed the reverse, with D10 values of planktonic cells (0.396kGy) significantly lower than for biofilm-associated cells (0.526kGy). At the 48h and 72h sampling times, there was no difference in radiation sensitivity based on biofilm habitat for C9490 or 35150. These results indicate that the extent to which the biofilm habitat modifies the antimicrobial efficacy of ionizing radiation is dependent on the specific isolate examined and the maturity of the biofilm. |
