Prevalence of Salmonella and E. coli on Produce from Seattle Farmers Markets
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This study surveyed the extent of microbial contamination on lettuce and tomatoes purchased from farmers markets, an increasingly popular source of local produce. Foodborne outbreaks in the U.S. have implicated <italic>Salmonella</italic> and <italic>E. coli</italic> as etiologic agents associated with raw produce-related illness; lettuce and tomatoes in particular have been implicated as common vehicles for <italic>Salmonella</italic> and <italic>E. coli</italic>. 126 samples of lettuce and tomatoes were collected from 5 major farmers markets in the Seattle metropolitan area throughout the 2012 summer/fall harvest season. Samples were analyzed for surface contamination of <italic>Salmonella spp.</italic> and generic <italic>E. coli</italic> as an indicator for fecal contamination. Whole tomato and 50 g lettuce samples were enriched or eluted in sterile Whirl-Pak® bags. Surfaces of samples were manually rubbed in liquid media to aid in removal of surface attached particles. Samples were pre-enriched in Universal Pre-enrichment Broth for the detection of <italic>Salmonella spp.</italic>, followed by selective enrichment with Tetrathionate (TT) broth and plating onto XLD agar for isolation. Presumptive colonies were confirmed with API and qPCR targeting the <italic>Salmonella</italic> invA gene. Rapid Colilert® tests, paired with Quanti-Tray®/2000 (IDEXX), were adapted for qualitative and quantitative detection of generic <italic>E. coli</italic> on produce. Following elution with phosphate buffered saline (PBS), positive wells were enumerated after 24 hrs and <italic>E. coli</italic> levels expressed as MPN/100 mL per sample. No <italic>Salmonella</italic> was detected; presumptive <italic>Salmonella</italic> isolates were confirmed to be <italic>Citrobacter</italic>. <italic>Escherichia coli</italic>, indicating fecal contamination, was present on 62% of lettuce and 6.4% of tomatoes samples. No significant differences were found in <italic>E. coli</italic> contamination by sampling date or between organic and conventional production methods. There were significantly more lettuce samples contaminated with <italic>E. coli</italic> than tomatoes (<italic>p</italic> < .05). This was the first study investigating the extent of microbial contamination on farmers markets produce in the United States. Results from this study and other fresh produce surveys indicate a low baseline occurrence of <italic>Salmonella</italic> on farmers markets produce. However, good agricultural practices should be promoted to reduce levels of <italic>E. coli</italic> indicating fecal contamination on produce.
- Environmental health