Purpose: The study aimed to investigate the potential transfer of Escherichia coli ATCC 25922 and Listeria innocua ATCC 33090, surrogate microorganisms for E. coli O157:H7 and L. monocytogenes, respectively, from artificially contaminated soil into the edible portion of vegetables, as affected by the presence of phytopathogen Ralstonia solanacearum or plant commensal Pseudomonas fluorescens.
Methods: Tomatoes, bell peppers and green onions were cultivated in sterile soil (aw ~ 0.98, pH ~ 7.45) in a greenhouse for approximately 60 days. Each potted vegetable was soil-inoculated with a suspension (ca. 108 CFU/ml) of nalidixic-acid resistant E. coli (EC) or L. innocua (LI). In addition, selected vegetables were co-inoculated with R. solanacearum (RS) or P. fluorescens (PF) on alternate days. A total of six inoculation treatments were considered: EC, EC+PF, EC+RS, LI, LI+PF and LI+RS. Mature tomatoes (110), peppers (86) and green onions (63) were harvested and analyzed by traditional culturing methods using selective media supplemented with 50 µg/ml of nalidixic acid.
Results: EC and LI were undetectable in tomato and pepper fruits analyzed by both plating (< 1.7 - 2.2 log CFU/g) and enrichment methods. EC and LI were isolated from the foliar parts of green onions at a maximum population density of 3.4 and 3.7 log CFU/g, respectively. Uptake of these microorganisms in green onions was not significantly affected by the presence of RS or PF (P > 0.05).
Significance: Findings point to the possible transference of zoonotic or geonotic pathogens from soil into onion plants and the health risks associated with the consumption of raw green onions.