Agriculture Reference
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more adherent than E. coli O157:H7 when 3-day-old alfalfa sprouts were used in a
4 h adhesion assay, with greater growth of S. enterica also observed on alfalfa seeds
2 days after sprouting (Charkowski and others 2002). Using GFP-labeled strains,
increased colonization of alfalfa seeds and roots by S. enterica as opposed to E. coli
O157:H7 (Charkowski and others 2002; Dong and others 2003) suggests that alfalfa
sprouts are more prone to invasion by Salmonella than E. coli O157:H7. However,
the same is not true for bean sprouts, with Warriner and others (2003b) reporting that
bioluminescent strains of E. coli and S . Montevideo labeled with the lux gene similarly
infi ltrated bean sprouts, particularly through the roots, after 24 h of germination. Given
that exposing the alfalfa seedling root area to as few as 1 Salmonella CFU led to
infi ltration and growth to high numbers 5 days later (Dong and others 2003), the
chemical sanitizers currently used by sprout manufacturers do not provide the needed
assurance in terms of sprout safety (Taormina and Beuchat 1999).
Leafy Greens
The safety of leafy greens, particularly lettuce and spinach, has become a top food
safety priority with 20 of 24 leafy green outbreaks reported since 1996 traced to
E. coli O157:H7. Consequently, three common reservoirs for E. coli O157:H7 -
contaminated irrigation water, manure composts, and soil—have been targeted as
probable routes for dissemination of this pathogen in the fi eld. However, contamina-
tion can also occur during harvesting, fi eld coring, and further processing (e.g., shred-
ding, conveying, washing, drying) with the ice used in cooling lettuce recently
identifi ed as another means for the spread of pathogens during melting (Kim and
Harrison 2008 ).
Using a series of lettuce and parsley plots treated once with E. coli O157:H7 -
inoculated manure compost (10 7 CFU/g) before transplanting or spraying with E. coli
O157:H7-inoculated irrigation water (10 5 CFU/ml) 3 weeks after transplanting, Islam
and others (2004) reported that the pathogen persisted 126 to 154 and 154 to 217 days
in soil samples in which lettuce and parsley were grown, respectively, with E. coli
O157:H7 detected up to 77 days on lettuce and 177 days on parsley. In a similar study
by Solomon and others (2003), E. coli O157:H7 persisted 10 to 22 days on 30-day-old
lettuce plants after a single spray irrigation treatment with 100 ml of natural irrigation
water containing 10 2 CFU/ml of a GFP - labeled E. coli O157:H7 strain. When plants
were subjected to two or three spray irrigation treatments during the fi rst 2 weeks, the
pathogen persisted on the leaf surface for the entire 30-day duration of the study,
underscoring safety concerns surrounding the potential for internalization of E. coli
O157:H7 through surface stomata (Seo and Frank 1999; Takeuchi and Frank 2000).
Given a choice, surface irrigation is preferred with Solomon and others (2002a) report-
ing that 29 of 32 as opposed to 6 of 32 similarly grown lettuce plants yielded the same
GFP-labeled strain of E. coli O157:H7 after spray and surface irrigation, respectively,
1, 10, or 20 days before harvest.
Working at Rutgers University, Solomon and others (2002b) were fi rst to conclu-
sively demonstrate uptake and subsequent internalization of a GFP-expressing E. coli
O157:H7 from inoculated cow manure and irrigation water into leaf lettuce plants via
the plant vascular system. Using soil inoculated at 10 6 CFU/g, E. coli O157:H7 was
cultured from the interior of 6- and 9-day-old HgCl 2 - treated lettuce seedlings with
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