Surface Destruction of Listeria Monocytogenes by Ozone and Chlorine

NON-TECHNICAL SUMMARY: Listeria monocytogenes is a common contaminant in food processing plants. This project will examine the ability of ozone, chlorine, and combined ozone and chlorine, to kill Listeria monocytogenes on food contact surfaces.
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APPROACH: Two strains of L. monocytogenes will be used (SLCC 5764 and Scott A). Ozone will be produced using a Corona discharge generator and bubbled through buffer. Ozone concentrations will be determined using an ozone sensor. Four levels of ozone (0.5, 2.5, 5.0 and 10.0 ppm) will be tested at 5 and 20 degrees C. Chlorine will be examined as chlorine dioxide at 10, 20 and 50 ppm available chlorine at 5 and 20 degrees C. Chlorine levels will be determined colorimetrically. Listerial cells will be grown as a biofilm on stainless steel chips to approximate food plant conditions. Under controlled conditions, the chips with the attached L. monocytogenes cells will be exposed to: ozone alone; chlorine alone; or a combination of ozone and chlorine. Listerial cells will be enumerated on McBrides Listeria Agar.

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PROGRESS: 2000/10 TO 2006/09<BR>
The effects of ozone (0.25 ppm, 24 degrees C) in distilled water on six strains of L. monocytogenes were evaluated (Fisher et al., 2000). A biphasic death curve was found for all six strains. After 14 minutes of exposure, cell number was reduced by a 5.3 log reduction (strain SLCC 5764); 6.2 log reduction (strain 7644); 6.8 log reduction (strain Scott A); 6.9 log reduction (strain 7644); 7.7 log reduction (strain 19112); while strain LO28 was completely inactivated. These results showed that differences exist in the sensitivity of listerial strains to the killing effects of ozone. Similar differences among the listerial strains also were found after exposure to 0.25 ppm ozone in phosphate buffered saline, with strain SLCC 5764 again the most resistant. Listeria monocytogenes strains SLCC 5764 and 10403S (most resistant to 0.25 ppm ozone) were further examined by exposure to 0.40 and 1.00 ppm ozone in distilled water. After six minutes exposure to 0.40 ppm ozone, strain SLCC 5764 showed a 5.6 log reduction, while strain 10403S showed an 8.0 log reduction. At 1.00 ppm ozone, both strains were completely killed within 1.5 minutes. In general, mid-exponential and late stationary phase cells were more sensitive to ozone exposure than were early stationary phase cells. The effectiveness of ozone treatment on strains SLCC 5764 and 10403S after exposure to 0.25 ppm ozone at 4, 24 and 37 degrees C was examined. Results indicated that ozonation at 4C was more effective than at 24 or 37C. This result may be partly explained due to the increased solubility of ozone at lower temperature. Cabbage was inoculated with listerial strains SLCC 5764, 10403S and Scott A. Cabbage pieces (25 gm) were inoculated with 1.0 mL of an early stationary phase culture and L. monocytogenes counts and total plate counts were made after two and five minutes exposure to 1.00 ppm ozone. After two minutes exposure, strain Scott A, 10403S and SLCC 5764 showed a 100, 75 and 70% reduction on McBride\'s agar, respectively, while total plate counts were reduced by 24%. After five minutes exposure, all three listerial strains were inactivated, while the total plate counts were reduced by 40%.
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IMPACT: 2000/10 TO 2006/09<BR>
Results from this study show that Listeria monocytogenes can grow as biofilms. The growth environment influences the amount of biofilm formation. When grown as biofilms, L. monocytogenes are more resistant to chemical sanitizers. Increasing the concentrations of either ozone or chlorine can kill listerial biofilms.