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Colonization and bacteriophage biocontrol of Salmonella on fresh produce Wong, Catherine Wai Yee


Multiple outbreaks caused by Salmonella have been linked to fresh produce. Washing in sanitizing solutions has been shown to reduce microbial populations by <90%. Bacteriophage cocktails have been suggested as an alternative to chemical sanitizers due to their effective and specific antimicrobial activity, safety and lack of effects on organoleptic properties. Lettuce and tomato plants were separately inoculated with 43 Salmonella strains, 26 most commonly associated and 17 not commonly associated with fresh produce outbreaks. Salmonella populations were measured immediately after inoculation and after 5 days. Laser Scanning Confocal Microscopy (LSCM) was performed after staining with a fluorescein isothiocyanate (FITC) labelled anti-Salmonella antibody. Romaine lettuce was inoculated with a phage cocktail + 1.0mM calcium chloride or only 1.0mM calcium chloride one day before inoculation with 3 separate Salmonella strains. Salmonella populations were measured immediately after inoculation and after 1 and 2 days. Populations of 26 strains (60.5%) increased on all plant species and cultivars, although there were significant differences (p<0.05) in the extent of population increase by different strains on the same plant species/cultivar. The remaining strains displayed differential ability to colonize lettuce and tomato plants depending on plant species or cultivar. Most strains not commonly associated with fresh produce outbreaks were able to colonize the plants. LSCM showed that cells or cellular aggregates were located within stomates, in surface depressions adjacent to stomata or in random microsites not associated with specific anatomical features. Application of a bacteriophage cocktail to Romaine lettuce leaf sections 24 hours before inoculation significantly reduced (P<0.05) populations of Salmonella Saintpaul S204, Saintpaul S205 and Typhimurium S441 by 2-4 log CFU/cm². The results of this study showed that the interaction between plant host and colonizing Salmonella is complex and subject to several interacting factors. Moreover, the colonization potential of Salmonella is highly variable and should be carefully considered in the selection of experimental strains for future research on the ecology of this bacterial species on growing food plants.

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