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Quantitative Microbial Risk Assessment for Escherichia coli O157 on Lettuce, Based on Survival Data from Controlled Studies in a Climate Chamber

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Abstract:

The aims of the study were to determine the survival of Escherichia coli O157 on lettuce as a function of temperature and light intensity, and to use that information in a screening-level quantitative microbial risk assessment (QMRA) in order to evaluate risk-reducing strategies including irrigation water quality guidelines, rinsing, and holding time between last irrigation and harvest. Iceberg lettuce was grown in a climate chamber and inoculated with E. coli O157. Bacterial numbers were determined with the standard plate count method after inoculation and 1, 2, 4, and 7 day(s) postinoculation. The experiments were carried out at 11, 18, and 25°C in light intensities of 0, 400, and 600 mmol (m2)−1 s−1. There was a significant effect of temperature and light intensity on survival, with less bacteria isolated from lettuce incubated at 25 and 18°C compared with 11°C (P < 0.0001), and in light intensities of 400 and 600 mmol (m2)−1 s−1 compared with 0 mmol (m2)−1 s−1 (P < 0.001). The average log reductions after 1, 2, 4, and 7 day(s) were 1.14, 1.71, 2.04, and 3.0, respectively. The QMRA compared the relative risk with lettuce consumption from 20 scenarios. A stricter water quality guideline gave a mean fivefold risk reduction. Holding times of 1, 2, 4, and 7 day(s) reduced the risk 3, 8, 8, and 18 times, respectively, compared with harvest the same day as the last irrigation. Finally, rinsing lettuce for 15 s in cold tap water prior to consumption gave a sixfold risk reduction compared with eating unrinsed lettuce. Sensitivity analyses indicated that variation in bacterial inactivation had the most significant effect on the risk outcome. A QMRA determining the relative risks between scenarios reduces uncertainty and can provide risk managers with decision support.

Document Type: Research Article

DOI: http://dx.doi.org/10.4315/0362-028X.JFP-10-563

Affiliations: 1: Swedish University of Agricultural Sciences, Department of Biomedical Sciences and Veterinary Public Health, Box 7009, 75007 Uppsala, Sweden; National Veterinary Institute, Department of Chemistry, Environment and Feed Hygiene, 75189 Uppsala, Sweden 2: National Veterinary Institute, Department of Chemistry, Environment and Feed Hygiene, 75189 Uppsala, Sweden 3: National Food Administration, Department of Research and Development, 75126 Uppsala, Sweden; Swedish University of Agricultural Sciences, Department of Microbiology, Box 7025, 75007 Uppsala, Sweden

Publication date: December 1, 2011

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