IAFP 2009 Program Book and Abstract Book - AB153

Purpose: The study aimed to determine whether E. coli O157:H7 would be present in the aerial leaf tissue of a spinach plant when introduced via soil inoculation at different growth stages in a greenhouse setting. Methods: Spinach seeds of a standard commercial variety were sown individually in 8-inch pots and watered daily and fertilized weekly after germination. Soil inoculation at two levels (108 and 104 CFU/ml) of an E. coli O157:H7 green fluorescent protein (GFP)-expressing strain was conducted on a weekly basis for a total of five times after germination. Inoculated spinach samples were harvested weekly and examined for the presence of E. coli O157:H7 on leaves and in surrounding soil. Results: Among 60 plant samples examined for internal leaf contamination, only one plant yielded positive for the GFP-expressing E. coli O157:H7 strain. Surface contamination occurred occasionally and clustered between three to five weeks after germination. No surface contamination occurred among leaves younger than three weeks of age. On the other hand, when inoculated at 108 CFU/ml, the GFP-expressing E. coli O157:H7 survived the entire cultivation period but with gradually reduced levels. Significance: The experiments demonstrated that the internalization of E. coli O157:H7 into a growing spinach plant under greenhouse conditions was a rare event but that contamination did occur, primarily when the plants reached 3 weeks of age. The study provided important data to further assess the association between spinach age and the potential contamination with E. coli O157:H7. P3-14 Fate of Internalized Escherichia coli O157:H7 on Field Grown Spinach Treated with Contaminated Irrigation Water CATHY WEBB, Marilyn Erickson, Juan Carlos Diaz-Perez, Sharad Phatak, John Silvoy, Lindsey McGhin, Alison Payton, Jean Liao and Michael Doyle University of Georgia, Center for Food Safety, Griffin, GA, USA Introduction: Internalization of Escherichia coli O157:H7 into edible portions of leafy green plants has been reported under greenhouse conditions, but whether internalization occurs under field conditions and the associated contributing factors need to be studied. Purpose: To determine the fate of E. coli O157:H7 applied at varying concentrations to spinach fields through drip or spray irrigation water and the influence of plant growth stage on surface and internalized contamination of leaves and roots. Methods: A four strain mixture of Shiga toxin-negative E. coli O157:H7 was added to irrigation water to give concentrations of 102, 104, or 106 CFU/ml. The water was administered once to the soil at the base of early-, mid-, or late-season spinach plants as well as to spinach plants in mid- and late-season through an overhead spray. Soil and plants (roots and leaves) were sampled up to 3 weeks following exposure and analyzed using either direct plate count or enrichment culture. A mercury chloride/ethanol wash of plant tissue prior to grinding and analysis targeted internalized pathogen. Results: E. coli O157:H7 persisted in soil 21, 22, and 7 days following application of contaminated irrigation water to soil in the early-, mid-, and late-growing seasons. Despite persistence in soil, internalized pathogen was never detected in spinach leaves nor in root samples collected up to 3-weeks following the early- or late-season exposure. For plants exposed to contaminated soil mid-season, 5 of 30 root samples collected 7-days post-exposure were positive for internalized E. coli O157:H7. Following spray inoculation, the pathogen could only be detected in or on leaves through enrichment culture on the day of inoculation in mid- and high-dose treated spinach plants. Seven days post-spraying, all spinach leaves tested negative for surface or internal contamination. Significance: This data will be useful in risk assessment studies involving contamination of leafy green fields with contaminated irrigation water. P3-15 The Survival of Escherichia coli O157:H7 in Cucumber Fermentation Brines FRED BREIDT Food, Bioprocessing and Nutrition Sciences, NC State University, USDA/ARS, Raleigh, NC, USA Introduction: Bacterial pathogens have been reported on fresh cucumbers and other vegetables used for commercial fermentation. Fermented vegetables have an excellent safety record, and are exempt from acidified food regulations. However, there is little data showing how bacterial pathogens are killed in cucumber fermentations. Purpose: Lactic acid bacteria produce a variety of antimicrobial metabolites, and recycled brines are commonly used for commercial fermentation. The objective was to determine how acid resistant Escherichia coli O157:H7 strains are killed in these brines. Methods: We obtained brine samples from six 40,000 L commercial cucumber fermentation tanks at different stages of fermentation. The brines were sterilized by filtration and inoculated with a cocktail (106 CFU/ml) of E. coli O157:H7 strains prepared to induce acid resistance. Brine composition was determined by HPLC. Inoculated brines were incubated anaerobically at 30°C. The surviving cell numbers were determined on non-selective media using a spiral plater and automated plate reader. Results: Brines contained up to 50 mM sugar, 30 to 150 mM lactic acid, 4 to 8 percent NaCl, and had pH values of 3.0 to 4.2. Most samples contained sorbic acid (< 0.04 %), used to inhibit spoilage microorganisms in commercial fermentations with air purging. In a brine sample with 55 mM sugar, 32 mM lactic acid, 0.02% sorbic acid and pH 4.1, the cell numbers of E. coli decreased one log after seven days, although by 15 days there were no detectable cells (< 102). Most samples had no detectable cells after seven days. There was no apparent change in acid or sugar concentrations or pH in any of the samples during incubation, indicating there was no growth of the E. coli strains. Abstract Book — 153 Poster

IAFP 2009 Program Book and Abstract Book

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