Abstract
This randomized controlled trial characterized the transfer of E. coli from animal feces and/or furrow water onto adjacent heads of lettuce during foliar irrigation, and the subsequent survival of bacteria on the adaxial surface of lettuce leaves. Two experiments were conducted in Salinas Valley, California: (1) to quantify the transfer of indicator E. coli from chicken and rabbit fecal deposits placed in furrows to surrounding lettuce heads on raised beds, and (2) to quantify the survival of inoculated E. coli on Romaine lettuce over 10 days. E. coli was recovered from 97% (174/180) of lettuce heads to a maximal distance of 162.56 cm (5.33 ft) from feces. Distance from sprinklers to feces, cumulative foliar irrigation, and lettuce being located downwind of the fecal deposit were positively associated, while distance from fecal deposit to lettuce was negatively associated with E. coli transference. E. coli exhibited decimal reduction times of 2.2 and 2.5 days when applied on the adaxial surface of leaves within a chicken or rabbit fecal slurry, respectively. Foliar irrigation can transfer E. coli from feces located in a furrow onto adjacent heads of lettuce, likely due to the kinetic energy of irrigation droplets impacting the fecal surface and/or impacting furrow water contaminated with feces, with the magnitude of E. coli enumerated per head of lettuce influenced by the distance between lettuce and the fecal deposit, cumulative application of foliar irrigation, wind aspect of lettuce relative to feces, and time since final irrigation. Extending the time period between foliar irrigation and harvest, along with a 152.4 cm (5 ft) no-harvest buffer zone when animal fecal material is present, may substantially reduce the level of bacterial contamination on harvested lettuce.
Highlights
Fresh fruits and vegetables are recognized worldwide as a source of the foodborne transmission of zoonotic pathogens [1,2,3]
E. coli Rifr Concentration on Romaine Lettuce after Irrigation (Experiment A) As shown in Table 1, the initial concentration of E. coli Rifr applied in 5 g of inoculated feces to each cluster of eight heads of lettuce ranged from 7.72 × to 2.00 × per g chicken feces and
Seventy percent (126/180) of 180 mature heads of Romaine lettuce had detectable concentrations of indicator E. coli Rifr after ~2 h of irrigation based on the high-concentration enumeration assay, with a mean concentration of 88,176 E. coli Rifr per lettuce head
Summary
Fresh fruits and vegetables are recognized worldwide as a source of the foodborne transmission of zoonotic pathogens [1,2,3]. In 1996, a multistate outbreak of E. coli O157:H7 infection associated with mesclun lettuce was identified in Connecticut and Illinois where wash water was the suspected source of the contamination [6]. Since these early reports of lettuce-related E. coli O157:H7 outbreaks, additional commodities of fresh fruits and vegetables have been identified as sources of foodborne pathogens such as strawberries, unpasteurized apple cider, lettuce, spinach, alfalfa sprouts, and bagged mixed salad [6,7,8,9,10,11,12]. Consumption of spinach or lettuce, for example, was associated with over 20 outbreaks and accounted for nearly half of all produce outbreaks during the period between 1995 to 2006 [13,14]
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