Investigation of possible situation of internalization of Salmonella Enteritidis in tomato fruits and bacterial survival during tomato plant cultivation

Tomatoes have recently been implicated as an important vehicle in outbreaks of produce-associated salmonellosis. Traceback reports suggested that pre-harvest contamination of Salmonella enterica might be the main reason for these outbreaks; however, the site of pathogen attachment remains unclear. Therefore, it is important to investigate the mechanisms of Salmonella contamination of fresh produce. To trace the presence of Salmonella in soil and plants, Salmonella Enteritidis transformed with a pEGFP plasmid vector (S. Enteritidis-EGFP) was used. Soil was artificially contaminated with S. Enteritidis-EGFP at 10⁴, 10⁶ or 10⁸ CFU/g, followed by cultivation of tomato plants in the contaminated soil. Samples of the soil and each organ of the tomato (fruit, stems/leaves, and root) were assayed for Salmonella by plating onto Tryptic Soy Agar and using the MPN method. Salmonella levels in the soil gradually decreased over time, and soil persistence was dependent on the initial inoculation level. Salmonella levels were below the detection limit (< 100 CFU/g) in fruits and stems/leaves, regardless of the level of soil contamination. Moreover, S. Enteritidis-EGFP was not detected in the tomato fruits after root injury. These results indicate that the internalization of Salmonella in tomato fruits might not occur after cultivation in contaminated soil.