Student Research: Amy Leang
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 Salmonella and E. coli as etiologic agents associated with raw produce-related illness; lettuce and tomatoes in particular have been implicated as common vehicles for Salmonella and E. coli. Over 120 samples of lettuce and tomatoes were collected from five major farmers markets in the Seattle metropolitan area throughout the 2012 summer/fall harvest season. Samples were analyzed for surface contamination of Salmonella spp. and generic E. coli 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 Salmonella spp., 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 Salmonella invA gene. Rapid Colilert® tests, paired with Quanti-Tray®/2000 (IDEXX), were adapted for qualitative and quantitative detection of generic E. coli on produce. Following elution with phosphate buffered saline (PBS), positive wells were enumerated after 24 hrs and E. coli levels expressed as MPN/100 mL. No Salmonella was detected; presumptive Salmonella isolates were confirmed to be Citrobacter. E. coli, indicating fecal contamination, was present on 62% of lettuce and 6.4% of tomatoes samples. No significant differences were found in E. coli contamination by temporal variation, between organic and conventional production methods, or between vendors. There were significantly more lettuce samples contaminated with E. coli than tomatoes (p < .05).