Disposition of Salmonella and Escherichia coli O157:H7 following Spraying of Contaminated Water on Cucumber Fruit and Flowers in the Field

Abstract

Cucumbers are frequently consumed raw and have been implicated in several recent foodborne outbreaks. Because this item may become contaminated at the farm, it is vital to explore the fate of attenuated Salmonella Typhimurium or Escherichia coli O157:H7 sprayed onto foliage, flowers, and fruit in fields and determine whether pre- or postcontamination spray interventions could minimize contamination. After spraying cucumber plants with contaminated irrigation water (3.8 log CFU/mL of Salmonella Typhimurium and E. coli O157:H7), 60 to 78% of cucumber fruit were not contaminated because the plant's canopy likely prevented many of the underlying fruit from being exposed to the water. Subsequent exposure of contaminated cucumber plants to a simulated shower event did not appear to dislodge pathogens from contaminated foliage onto the fruit, nor did it appear to consistently wash either pathogen from the fruit. Spraying flowers and attached ovaries directly with a pathogen inoculum (4.6 log CFU/mL) initially led to 100% and 65 to 90% contamination, respectively. Within 3 days, 30 to 40% of the flowers were still contaminated; however, contamination of ovaries was minimal (≤10%), suggesting it was unlikely that internalization occurred through the flower to the ovary with these pathogen strains. In another study, both pathogens were found on a withered flower but not on the fruit to which the flower was attached, suggesting that this contaminated flower could serve as a source of cross-contamination in a storage bin if harvested with the fruit. Because pre- and postcontamination acetic acid-based spray treatments failed to reduce pathogen prevalence, the probability that fruit initially contaminated at 1.3 to 2.8 log CFU of Salmonella Typhimurium or E. coli O157:H7 per cucumber would be positive by enrichment culture decreased by a factor of 1.6 and 1.9 for Salmonella Typhimurium and E. coli O157:H7, respectively, for every day the fruit was held in the field ( P ≤ 0.0001). Hence, to reduce the prevalence of Salmonella Typhimurium on cucumbers below 5%, more than 1 week would be required.