title: Quantitative microbial risk assessment for Escherichia coli O157 on lettuce, based on survival data from controlled studies in a climate chamber.
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 (m(2))(-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 (m(2))(-1) s(-1) compared with 0 mmol (m(2))(-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.
authors.affiliation: Swedish University of Agricultural Sciences, Department of Biomedical Sciences and Veterinary Public Health, Box 7009, 75007 Uppsala, Sweden. jakob.ottoson@slu.se
authors.forename: Jakob R
authors.initials: JR
authors.lastname: Ottoson
authors.forename: Karin
authors.initials: K
authors.lastname: Nyberg
authors.forename: Roland
authors.initials: R
authors.lastname: Lindqvist
authors.forename: Ann
authors.initials: A
authors.lastname: Albihn
mesh_terms: D015169:Colony Count, Microbial; D019453:Escherichia coli O157 / Q000254:growth & development*; D005506:Food Contamination; D005511:Food Handling / Q000379:methods*; D005516:Food Microbiology; D059022:Food Safety; D006801:Humans; D018545:Lactuca / Q000382:microbiology*; D008027:Light; D050296:Microbial Viability; D018570:Risk Assessment; D013696:Temperature; D013997:Time Factors