Ph. D.

M. S.

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Prior research reveals foodborne pathogens, as well as enteric bacteria, can thrive in agricultural soils. Understanding how macro- and micronutrients, as well as meteorological factors and observational factors, impact pathogen prevalence may promote a better understanding of how pathogens persist in agricultural soils. This study aimed to (i), characterize associations between soil properties (e.g., macro- and micro-nutrient levels) and microbial targets (e.g., S. enterica and L. monocytogenes prevalence, fecal indicator bacteria concentration). Three produce farms in Virginia were selected from different regions (i.e., Blue Ridge Highlands, Piedmont, Coastal Plains). Farms were sampled four times to capture seasonal differences. Five soil samples were collected from 20 plots (25m2) and pooled in equal quantities to form one sample per plot. A total of 240 samples were collected. Listeria and S. enterica samples (25g) were processed using a modified FDA BAM method, while generic Escherichia coli (gEC) and total coliform (TC) samples (5g) were enumerated using Petrifilm. Presumptive Listeria and S. enterica positive samples were confirmed by PCR using a single gene. Bayesian mixed models were used to evaluate associations with each foodborne pathogen and indicator organism with factors of interest. S. enterica prevalence was 4.2% (10/240) in soil samples. Of the ten S. enterica positive samples, nine samples (90%) were from one farm in eastern VA. Listeria spp. prevalence was 10% (24/240) with L.monocytogenes prevalence being 2.5% (6/240). The average gEC and TC concentrations in soil samples were 1.53 (range 0.95-4.01) and 4.21 (range 1.23-7.12) log CFU/g, respectively. Bayesian mixed models revealed that pH impacted prevalence of L. monocytogenes and gEC (MAP=5.48, 95% CI=0.75,345.39, PD=0.98, ROPE=0.01), and (MAP=4.87, 95% CI=2.31,12.22, PD=1.00, ROPE=0.00). There was no evidence of an association between S. enterica prevalence and factors of interest. S. enterica was 11.55 times more likely to be detected on Farm C (where prevalence was highest) compared to other farms (95 % CI= 1.36, 1155.27, PD=0.98, ROPE=0.00). Findings show that while soil nutrient trends differ across all farms, it is difficult to determine the strength of these trends due to strong regional distinctions. / Master of Science in Life Sciences / Fresh produce is essential to our food supply but is often a source of foodborne outbreaks since they are often consumed raw and have absence of a "kill step". Importantly, numerous produce outbreaks are often traced back to the production environment with water and soil and common methods of contamination. Additionally, growers are required to identify potential sources of contamination to minimize public health risks. For example, the FDA Produce Safety Rule mandates that growers identify measures that can be taken to prevent contamination from sources including soil and water. Many growers will often test their soils to determine nutrient levels so they can determine appropriate fertilizer amounts to apply to their crops. By understanding how the level of nutrients as well as weather patterns and management factors impact microbial detection, scientists and growers can gain a better understanding of how pathogens persist in agricultural soils. This study looked at sampling soil from three farms in three different regions of Virginia (i.e., Blue Ridge Highlands, Piedmont, and Coastal Plains). Sampling occurred four times across two seasons (fall and summer). Five soil samples were pooled to form one composite sample per plot which totals twenty samples per farm. A total of 240 samples were collected overall for this study. Samples were processed for Listeria and Salmonella (foodborne pathogens) using a modified method from the FDA Bacteriological Analytical Manual (BAM), while generic Escherichia coli and total coliforms (indicator organisms) were computed using Petrifilm. Polymerase Chain Reaction (PCR) was used to confirm the presence of Listeria and Salmonella. Key results from this study revealed that different regions had a significant impact on the presence or absence of Listeria and Salmonella in Virginia agricultural soils. For example, Salmonella was more likely to be detected in the Coastal Plains region, where interestingly, the prevalence was highest (90%; 9/10). The prevalence of Listeria spp. (63%; 15/24) and L. monocytogenes (83%; 5/6 ) were highest in the Piedmont region. Additionally, there was no association between the occurrence of Salmonella and factors of interest (e.g., soil properties, weather factors, observational factors). Additionally, the study found L. monocytogenes was more likely to be detected when soil pH increased. These findings reveal that while soil nutrient, weather, and observational trends differ across all farms, sampling region and time of year create challenges in determining trends due to clear regional differences. This study offers insights into how growers can potentially utilize soil testing (a practice they are already doing) to identify how pathogens may be present in their agricultural soils.