Purpose: Evaluate the transfer of Shiga toxigenic E. coli (STEC) from contaminated surface water to cantaloupe.
Methods: Cantaloupe plots containing cross-classified combinations of overhead or surface drip irrigation along with bare ground or plasticulture raised bed preparation were irrigated from a pond naturally harboring STEC. Surface water was sampled weekly for enumeration of STEC, generic E. coli, and coliforms using routine enumeration methods. Cantaloupes were harvested, enriched in mTSB with 8 mg/l sodium novobiocin, DNA extracted and tested for the presence/absence of stx and eae genes using multiplex PCR.
Results: Over six weeks, STEC populations in irrigation were found to fluctuate between 0.7 to 2.68 log CFU/100 ml. There was no significant correlation between populations of STEC and coliforms (r2 = 0.56; P < 0.25) in irrigation water. A significant correlation was not observed between generic E. coli and STEC (r2 = 0.41; P < 0.45). Over a four-week harvest period, 210 cantaloupes were sampled for STEC contamination. Bare ground plots with drip irrigation and plasticulture plots with overhead irrigation had the highest occurrence of STEC contamination at 20.4% and 19.7%, respectively. Lower percentages of positive samples were found on overhead irrigated bare ground plots (14%) and drip irrigated plasticulture plots (12%).
Significance: These data suggest that the population of generic E. coli in irrigation water does not correlate with STEC concentration. Regardless of irrigation method and bed preparation, when high levels of STEC persist in irrigation water, transfer to cantaloupe can occur.