Purpose: Reduction of inoculated generic E. colilevels was evaluated on Gala and Golden Delicious apple varieties with and without EC water application for up to one week.
Methods: A four-strain, rifampicin-resistant generic E. coli cocktail was inoculated onto apples using a backpack sprayer after sunset on selected varieties (Gala and Golden Delicious) with or without EC in replicated blocks and harvested from at least two canopy positions (high, low) in a three year study. Apples were sampled at 0, 2, 10, 18, 34, 42, 58, 82, 106, and 154 hours after inoculation. Survivors on apples were enumerated on Chromagar ECC-rifampicin with and without filtration; pre-enrichment in TSB was performed as generic E. coli levels declined. Uninoculated control apples were tested for indicator organisms as generic E. coli.
Results: Initial inoculum levels on apples averaged 7.3-7.4 log CFU/apple. Generally, the greatest reduction in generic E. coli levels was observed within the first 8-10 hours after inoculation, with additional reduction at a slower rate between 34-106 hours. Averaged over three years, at 10 hours after inoculation, generic E. coli was reduced 2.1 and 2.5 log CFU/apple for untreated Gala and Golden Delicious, respectively and 2.8-2.9 log CFU/apple for EC treated fruit. The reduction of generic E. coli varied dramatically among individual apples within the same variety at any given time point. For uninoculated control apples, 3 out of 180 apples had detectable levels of generic E. coli.
Significance: Treatment with EC did not appear to enhance survival of generic E. coli on apples compared to the response on control apples that did not receive EC application.