Factors Influencing Salmonella Survival in Agricultural Soils and Internalization through Solanaceous Crop Roots

Solanaceous crops such as peppers and more commonly tomatoes have been linked to Salmonella outbreaks that have occurred in the United States. The source of contamination for some of these outbreaks was traced back to the preharvest environment. Sources of contamination at the preharvest level of production often include irrigation water, soil, and the use of biological soil amendments of animal origin (BSAAO). This dissertation aims to (i) determine how factors such as the use of BSAAO's, Salmonella strain, soil type, and irrigation influence the survival of Salmonella in agricultural soils and (ii) determine the risk of Salmonella internalization in to transplanted solanaceous crops (tomatoes and bell peppers) and identify factors that influence internalization through the roots such as mode of contamination, soil contamination level, and root damage. Sandy loam (SL) and clay loam (CL) soils both unamended and amended with poultry litter (PL), irrigated either daily or weekly were inoculated with one of twelve strains of Salmonella enterica and monitored for growth and survival until no longer detected. Tomato plants were transplanted into one of three contamination treatments: contaminated soil (A) or irrigation water administered either once (B, single exposure event) or for 7 d following transplantation (C, repeated exposure event). Plants were sampled 1, 2, 3, 7 and 90 d post-transplantation for Salmonella internalization. Pepper plant roots were left intact, damaged, or removed and placed in inoculated water (6.2 log CFU/ml). Pepper plants in the soil contamination study, were transplanted into soil inoculated with Salmonella at different contamination levels (High, Mid, and Low). Samples were collected 1, 2, 3, and 7d post-transplant. Plants were surface sanitized and enumerated for Salmonella internalization by plant section. Strains in soils treated with PL survived significantly (P<0.05) longer (56 and 112 days for SL and CL respectively), than Salmonella strains in unamended soils. In PL amended SL and CL soils, most strains grew significantly (P<0.05) within the first week following inoculation and ranged from 84 to 210 days in the point it was last detected. Strains survival increased significantly (P<0.05) in soil irrigated weekly compared to soils irrigated daily, weekly irrigation increased survival by 140 to 224 and 42 to 168 days in SL and CL soils respectively depending on the strain. Root sections in the tomato plants of treatment A had significantly (P<0.05) higher recovery of Salmonella internalization compared to the other treatments. No tomato fruit sampled were positive for Salmonella internalization. Pepper plants with damaged roots had significantly higher (P<0.05) Salmonella internalization in the stem than plants with intact roots, while plants with no root stalk had significantly higher (P<0.05) Salmonella internalization in the stem and leaves of plants with intact and damaged roots. Pepper plants exposed to high concentrations of Salmonella had significantly more internalization occur in the roots than plants exposed to mid and low concentrations of Salmonella. Due to the results of these studies assessing the risk of using PL and irrigation regime should be considered in the Salmonella growth and survival in agricultural soils. Though it is unlikely, steps should be considered to limit Salmonella contamination of soil and irrigation water and root damage to prevent the internalization of Salmonella in tomato and bell pepper plants through intervention measures such as composting, water treatment, and effective transplanting techniques.