Purpose: To evaluate the survival of surrogate Escherichia coli on overhead-inoculated sweet onions during conventional field curing.
Methods: Two days after the final irrigation, sweet onions (Allium cepa var. cepa Walla Walla Sweet) (4-month-old transplants, i.e., 6-month-old plants) were overhead-inoculated with a suspension of non-pathogenic rifampicin-resistant E. coli (PTV 353, PTV 354, PTV 355) using a backpack sprayer. One week later, plants were lifted, roots were undercut, and bulbs were placed on the soil surface (cured) for another 2 weeks. E. coli was recovered from individual onions bulbs (n = 10 - 40 per time point) by massaging and shaking in 0.1% peptone for 30 s. E. coli survivors were enumerated on tryptic soy agar with rifampicin and cycloheximide and CHROMagar ECC with rifampicin or by a most-probable-number method. When counts were below the limit of detection, samples were enriched in tryptic soy broth with rifampicin.
Results: E. coli populations declined from 5.2 ± 0.3 to 3.8 ± 0.5 log CFU/onion plant within 2 h after inoculation. At subsequent sampling times (≥ 8 h), the amount of E. coli recovered from each onion was highly variable (range at 8 h: 1.9 to 6.9 log CFU/onion). At the end of curing (22 d postinoculation), E. coli was recovered from 50% of the onions (32% of samples ranged from 3.1 to 7.0 log CFU/onion). E. coli was not detected on any uninoculated onions.
Significance: When applied to onion plants (preharvest) by overhead application, E. coli can survive on sweet onion bulbs throughout the duration of conventional curing in the field.