Purpose: Assess the impact of inoculum type, solar UV exposure, and time on persistence of surface-inoculated bacteria under conditions of full environmental exposure.
Methods: Navel oranges (n = 150) were inoculated with Salmonella Enteritidis using dry (Feclone- and chalk-based) and liquid inoculum and exposed to shade or sun for 20 d. Whole oranges were quantified for Salmonella 0, 3, 6, 11, and 20 days post-inoculation (DPI). Oranges (n = 500) were similarly inoculated under field conditions with attenuated Salmonella Typhimurium c3985 and generic Escherichia coli. Oranges were assessed for presence of target microorganisms at 0, 5, 12, and 19 DPI. Additional oranges (n = 110) were spray-inoculated with attenuated Salmonella Typhimurium (target 6 log CFU/orange) and processed 17, 27, and 49 DPI for spatial surface mapping of stress-adapted survivors. Each rind was dissected into 5 latitudinal sections and enriched for Salmonella presence.
Results: Extended survival of Salmonella Enteritidis was observed for shaded oranges, with 40 and 20% of the samples positive for dry inoculum (Feclone- and chalk-based, respectively) and 20% for wet inoculation, whereas only Feclone-based inoculation had 10% positives 20 DPI with solar UV exposure. Under field conditions, generic E. coli declined faster than attenuated Salmonella Typhimurium for both inoculation methods. Twenty-one DPI, E. coli was present in 1.7 and 2% of samples, while Salmonella was present in 15% and 12.2% for wet and dry inocula, respectively. Spatial mapping of Salmonella Typhimurium 49 DPI revealed greatest survival at the pedicel and stylar ends, with 30 and 25% positive samples, respectively.
Significance: Bacterial pathogen populations from irrigation water are likely to experience a log-die-off exceeding the proposed federal allowance if fruit contact occurs. Regardless, extended survival of low populations persisting into postharvest handling is possible.