Purpose: This study investigated associations between heat-treated manure-based soil amendments and growth, survival, and transfer of Escherichia coli to spinach and radish.
Methods: Two randomized complete block field experiments (n=4 replications each) with soil amendments [composted poultry litter, composted dairy manure, vermicomposted dairy manure, heat-treated poultry litter pellets (PLP), and urea (control)] were conducted. Plots, spray-inoculated with a three-strain cocktail of rifampicin-resistant generic E. coli isolates (gEcr), were tilled, seeded, overhead irrigated, and assayed 14 times (0 to 91 days) for gEcr. Spinach leaves and radish were assayed (28 to 84 days) for gEcr. Radish globes were washed, trimmed, sanitized (10 to 50 mg/L HOCl, pH6.5) for five minutes, then neutralized, and assayed for gEcr.
Results: Populations of gEcr in plots were ~ three log CFU/g soil (day 0) and increased 10- to 100-fold (day 1) for compost/urea and PLP treatments, respectively. The gEcr population persisted 84+ days in compost- and PLP-amended plots, but these organisms were barely detectable in urea treatments (day 28). Average gEcr populations on radish from compost and PLP plots were 1.17 and 3.29 log MPN/globe, respectively, and significantly (P<0.05) greater than on spinach leaves. Serial washings removed visible soil from radish globes (n=72) and reduced, but did not completely eliminate, gEcr from trimmed, chlorine-sanitized globes (n=48).