Agricultural Practices Influence Salmonella Contamination and Survival in Pre-harvest Tomato Production

Abstract

Between 2000 and 2010 the Eastern Shore of Virginia was implicated in four Salmonella outbreaks associated with tomato. Therefore, a multi-year study (2012–2015) was performed to investigate presumptive factors associated with the contamination of Salmonella within tomato fields at Virginia Tech’s Eastern Shore Agricultural Research and Extension Center. Factors including irrigation water sources (pond and well), type of soil amendment: fresh poultry litter (PL), PL ash, and a conventional fertilizer (triple superphosphate – TSP), and production practices: staked with plastic mulch (SP), staked without plastic mulch (SW), and non-staked without plastic mulch (NW), were evaluated by split-plot or complete-block design. All field experiments relied on naturally occurring Salmonella contamination, except one follow up experiment (worst-case scenario) which examined the potential for contamination in tomato fruits when Salmonella was applied through drip irrigation. Samples were collected from pond and well water; PL, PL ash, and TSP; and the rhizosphere, leaves, and fruits of tomato plants. Salmonella was quantified using a most probable number method and contamination ratios were calculated for each treatment. Salmonella serovar was determined by molecular serotyping. Salmonella populations varied significantly by year; however, similar trends were evident each year. Findings showed use of untreated pond water and raw PL amendment increased the likelihood of Salmonella detection in tomato plots. Salmonella Newport and Typhimurium were the most frequently detected serovars in pond water and PL amendment samples, respectively. Interestingly, while these factors increased the likelihood of Salmonella detection in tomato plots (rhizosphere and leaves), all tomato fruits sampled (n = 4800) from these plots were Salmonella negative. Contamination of tomato fruits was extremely low (< 1%) even when tomato plots were artificially inoculated with an attenuated Salmonella Newport strain (104 CFU/mL). Furthermore, Salmonella was not detected in tomato plots irrigated using well water and amended with PL ash or TSP. Production practices also influenced the likelihood of Salmonella detection in tomato plots. Salmonella detection was higher in tomato leaf samples for NW plots, compared to SP and SW plots. This study provides evidence that attention to agricultural inputs and production practices may help reduce the likelihood of Salmonella contamination in tomato fields.

Diversity and Dynamics of Salmonella enterica in Water Sources, Poultry Litters, and Field Soils Amended With Poultry Litter in a Major Agricultural Area of Virginia

“…As reported in our previous study (Gu et al, 2018), naturally contaminated irrigation water and fertilizer (poultry litter) were potential sources of Salmonella in tomato fields. The persistence of Salmonella in experimental plots (detectable during 4-month growth season) and commercial crop fields (4-6 months) was comparable.…”

“…Briefly, DNA from a pure bacterial culture was isolated using InstaGene ™ Matrix (BioRad, Hercules, CA). Multiplex PCR was set up using Qiagen HotStar Master Mix (Qiagen) and 1 μl of DNA, and thermocycled with the following protocol: 95°C, 15 min; 30 cycles at 94°C for 30 s, 48°C for 90 s, 72°C for 90 s; then 72°C for 10 min (Gu et al, 2018). Primers used for PCR amplification have been reported previously (Fitzgerald et al, 2007;McQuiston et al, 2011).…”

Salmonella enterica Elicits and Is Restricted by Nitric Oxide and Reactive Oxygen Species on Tomato

“…Overall, higher titers of S. Newport were consistently retrieved from leaves compared to fruit, an observation which has been reported elsewhere (Barak et al, 2011;Han and Micallef, 2014;Gu et al, 2018). Supporting evidence can be found in Salmonella field sampling studies.…”

“…Supporting evidence can be found in Salmonella field sampling studies. For example in a multi-year field study, sampling tomato leaves and fruit for wild S. enterica colonization found only leaves returned positive S. enterica result, never fruit (Gu et al, 2018). This tissue-specific variability in Salmonella carrying capacity could be due to relative abundance and composition of nutrients on the surface of the different plant organs, higher relative humidity on leaves as a result of transpiration, and a higher and more rugged surface area or attachment on leaves compared to fruit.…”

Correlation of Salmonella enterica and Listeria monocytogenes in Irrigation Water to Environmental Factors, Fecal Indicators, and Bacterial Communities

“…Antimicrobial resistant S. enterica was only identified from pond water samples, which may indicate a higher food safety risk of applying surface water for agricultural irrigation in this region. The average population densities of S. enterica and L. monocytogenes in tested water samples were lower than 2 log and 1 log MPN/L, respectively, but irrigation with naturally contaminated water might result in transmission of foodborne pathogens to fields ( Gu et al, 2018 ). The results provide growers with baseline information on the risk of irrigation water being contaminated in this region.…”