TRANSFER MECHANISM AND TRANSCRIPTOMIC PROFILING OF SALMONELLA IN FRESH-CUT FRUITS
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Salmonella has been recognized as an increasing concern for food safety of fresh fruits because of their constant involvement in associated foodborne outbreaks. However, knowledge is relatively limited about the transfer pattern of this particular pathogen from contaminated to uncontaminated fruits, nor do we know about the molecular mechanisms incorporated by it to address the environment of fresh fruits. In this study, we investigated the transfer mechanism of Salmonella enterica serovars Newport and Typhimurium between fresh gala apples and cantaloupes, respectively, via consecutive fresh cuttings and the subsequent survival of the strains in juice and cube fruit products under 4oC for up to 7 days. We also used RNA-seq and bioinformatics approaches to explore the transcriptome of Salmonella Newport in cantaloupe in comparison with 0.1% BPW (control group). Our results demonstrated fresh cutting as a practical way for bacterial transfer from contaminated to up to 4 uncontaminated fruits, and the transfer rate decreased drastically as the number of fresh cuts increased. The relative distribution of Salmonella cells in the 1st un-inoculated cantaloupe sample was significantly higher than the corresponding gala apple sample (p ≤0.05), while the transfer rate to the following un-inoculated samples in the cantaloupe group was visibly lower than the gala apple group. In general, cantaloupe, rather than gala apple, was capable to at least maintain a constant level of bacterial population during the 7 days storage at 4oC, with juice sample being even more supportive than cube sample (p ≤0.05). Moreover, Salmonella Typhimurium had a better adaptation in cantaloupe environment than Salmonella Newport. According to the results of transcriptome analysis, metabolism activities were promoted for essential nutrient requirements including carbon, energy, nitrogen, phosphorus, iron and zinc comparing to the control group, while suppressed for others such as lipid and fatty acids (FDR ≤0.05, |log2 FoldChange| ≥1). The differential regulation of the corresponding genes of the biological processes was possibly achieved in a time much less than 1 hour.