Apr. 1, 2009 - Mar. 31, 2010Award Number
Linda J. Harris, Ph.D.
University of California, Davis
From 1995 through 2006, 22 outbreaks of Escherichia coli O157:H7 were associated with consumption of leafy green vegetables. For reasons unknown, most of the outbreaks have been associated with lettuce that was harvested in late summer or early fall. To reduce the potential for further outbreaks we need to have a better understanding of how E. coli O157:H7 survives in fields of growing lettuce. To accomplish this we have used a non-pathogenic strain of E. coli characteristics that are similar to the pathogen to inoculate growing lettuce plants in the field. Lettuce plants were inoculated (mock contamination event) a single time 4 weeks after planting. Our preliminary data suggest that after a contamination event E. coli does not survive very well. Large, rapid decreases in numbers of the organism are observed on each lettuce plant. However, small number of E. coli do survive on the lettuce plants for much longer periods and in two trials up to and including the time of harvest (about 7 to 9 weeks after planting and 3 to 4 weeks after introduction of E. coli). The proposed research will build on these preliminary data with two field trials in 2009 that will compare both drip and sprinkler irrigation and spring (mid-May) and late summer (mid-July) plantings. Our long-term goal is to generate data that will help inform growers of strategies that could mitigate the risk of the organism surviving on lettuce after a contamination event.
From 1995 through 2006, 22 outbreaks of Escherichia coli O157:H7 were associated with consumption of leafy green vegetables. Where traceback investigations were possible, lettuce and leafy greens grown in Kern County, San Benito County and particularly the Salinas Valley have been identified. For reasons unknown, most of the outbreaks have been associated with lettuce harvested in late summer or early fall. In order to reduce the potential for further outbreaks a better understanding of the fate of E. coli O157:H7 in the lettuce field is needed to provide information on post- contamination risks and to develop sound strategies to avoid future outbreaks of foodborne illness. To date, research has primarily focused on the behavior of E.coli on cut leafy greens, in part because of the challenges associated with conducting field trials. In 2007 and 2008, we conducted three field trials in the Salinas valley to assess the fate of inoculated, attenuated non-pathogenic (stx1 and 2 negative, BLS1) E. coli O157:H7 ATCC 700728 on lettuce. Drip and sprinkler irrigation were compared in May and July (early and late summer) plantings. For all trials, a rapid decline of E. coli O157:H7 was observed during the first two days after inoculation and by day 7 or 14 levels were below the limit of detection by filtration (10 CFU/plant). In 2008 trials E. coli O157:H7 was detected by enrichment 7 to 9 weeks after planting, which corresponds to typical harvest times. During the June 2008 trial, the number of E. coli O157:H7-positive lettuce plants was significantly higher in those blocks that were sprinkler irrigated when compared to those that were drip irrigated but this difference was not observed in either of the two other August trials. Significant differences between early and late summer plantings were not observed. The current proposal seeks funds to repeat the early and late summer planting with drip and sprinkler irrigation in order to further elucidate effects of season and irrigation on survival of E. coli O157:H7. The results of the combined work should help to inform growers of strategies that would mitigate the risk of the organism surviving after a contamination event.