Purpose: Determine genes required for survival of Salmonella on tomatoes during simulated environmental conditions and postharvest washing.
Methods: Dwarf tomato plants were placed in an environmental chamber with Salmonella Newport inoculated fruit for 2 h (29.4°C, 60% RH) or for a 24 h cycle mimicking average July field conditions in east TN (day- 29.4°C, 60% RH: morning/evening- 23.9°C, 75% RH: night-20°C, 89% RH). Tomatoes were then harvested and washed in water, 50 ppm chlorine, 40 ppm peroxyacetic acid (PAA), or a no rinse (NR) control for 1 min to simulate postharvest washing. Salmonella RNA was stabilized, extracted, sequenced and relative expression of the transcriptome was compared.
Results: When compared to the 2 h NR control, 24 h NR Salmonella upregulated genes required for TCA cycle II 4-fold and aerobic respiration 3.4-fold. Glycolytic pathway associated genes displayed a range of response from up to 36.9 fold for 24 h water; 2 h water treatments only had downregulation (up to 5-fold) of glycolytic pathways. Twenty-four hour chlorine downregulated UMP biosynthesis 65.9-fold, which was not evident after only 2 h on tomato fruit. Arginine and aminobutyrate degradation pathways were slightly upregulated (3.2-fold), while arginine biosynthesis pathways were downregulated up to 67.3-fold for 24 h PAA treatment. Two hour PAA treatment resulted in 16.4-fold upregulation of autotrophic ammonia oxidation and an 8-fold downregulation of aerobic ammonia oxidation.
Significance: Understanding the mechanisms of Salmonella survival on plants will provide insight which can lead to better food safety interventions for the produce industry.