Migration, PCR detection and molecular typing of Salmonella during preharvest production, postharvest storage of produce using tomato as a model
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Salmonellae have been increasingly identified as a cause of outbreaks of infections associated with raw produce. However, lack of information on sources of contamination before and after harvest, location of cells on or in raw fruits and vegetables, and sensitive detection techniques make it difficult to develop efficient intervention steps. |A PCR assay derived from hilA, a positive regulator of Salmonella invasive genes, was developed for detecting the pathogen on tomatoes. The assay was tested on eighty-three Salmonella and twenty-two non- Salmonella strains, and validated for detecting Salmonella Montevideo in and on inoculated tomatoes; 10 2 and 10 1 CFU/g, respectively, were detected, using a 6-h enrichment at 37 o C. Results indicate that the hilA-based PCR assay is sensitive and specific. |To investigate the establishment of Salmonella in tomato plants through different routes before harvest, a five-serotype mixture of Salmonella was used to inoculate plants, either by injecting stems or brushing flowers. Microbiological analysis of ripe fruits showed that 43 and 40% of stems inoculated before and after fruits set, respectively, were positive for Salmonella; 25% of fruits produced from inoculated flowers were positive. Uptake of Salmonella by roots of tomato seedlings grown hydroponically was also studied using Salmonella tagged with green fluorescent protein. The pathogen was detected in cotyledons, stems, and leaves within 9 days of inoculation of nutrient solution in which cut and intact roots were submerged. |Salmonella survived for at least 45 days in inoculated moist soil. The population of Salmonella on tomatoes in contact with soil increased by 2.5 log10 CFU/tomato during storage at 20 o C for 4 days, and remained constant for an additional 10 days. The pathogen decreased by ca. 4 log10 CFU/tomato during storage for 14 days at 70% relative humidity. Imaging of cut tomatoes revealed that more cells were in the stem scar and subsurface areas of tomatoes as the time of storage in contact with inoculated soil increased. PCR fingerprinting showed that among five serotypes tested, S. Montevideo was the most persistent serotype detected on tomatoes. |This investigation demonstrates that tomato roots, stems, and flowers are possible sites at which Salmonella may establish and remain viable during plant growth and fruit development.