Salmonella Newport in soils amended with heat-treated poultry pellets survived longer and more readily transferred to and persisted on spinach

Authors: SHAH, MANOJ - North Dakota State University; BRADSHAW, RHODEL - US Department Of Agriculture (USDA); NYARKO, ESMOND - University Of Delaware; Handy, Eric; East, Cheryl - Roberts; Millner, Patricia; BERGHOLZ, TERESA - North Dakota State University; Sharma, Manan

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2019
Publication Date: 3/15/2019
Citation: Shah, M.K., Bradshaw, R., Nyarko, E., Handy, E.T., East, C.L., Millner, P.D., Bergholz, T.M., Sharma, M. 2019. Salmonella Newport in soils amended with heat-treated poultry pellets survived longer and more readily transferred to and persisted on spinach. Applied and Environmental Microbiology. 85:e02392-18. https://doi.org/10.1128/AEM.02392-18.
DOI: https://doi.org/10.1128/AEM.02392-18

Interpretive Summary: Heat-treated poultry pellets (HTPP) are a fertilizer commonly used by organic growers of fruit and vegetables. However, subsequent contamination of soil on farms can occur via several routes, such as use of contaminated irrigation water or scat from wild animals. The work reported here shows that HTPP-amended soils led to increased survival of Salmonella Newport, a pathogen which has caused outbreaks of infections associated with tomatoes and cucumbers grown on the Eastern Shore of Maryland. The prolonged survival in the HTTP-amended soils also led to a greater likelihood of transfer and survival of Salmonella Newport to growing spinach plants. Our work also modified a Salmonella Newport strain to eliminate a gene which helps it survive under harsh environmental conditions. However, there were no differences between survival of the modified strain and the original in the examined soils. Finally, similar levels of S. Newport populations were recovered from soil over time as determined by two different methods of quantification (culture recovery and polymerase chain reaction-based method), indicating that this soil environment did not induce a physiological state where S. Newport cells would be undetected by conventional methods. Overall soils fertilized with heat-treated poultry pellets supported longer durations of survival of S. Newport than unfertilized soils. This research benefits farmers and food safety regulators by identifying and characterizing the risk of Salmonella Newport contamination in the pre-harvest environment.

Technical Abstract: Biological soil amendments of animal origin (BSAAO) like untreated poultry litter are commonly used as biological fertilizers, but can harbor bacterial foodborne pathogens like Salmonella enterica. leading to potential transfer of pathogens from soils to fruits and vegetables intended for human consumption. Heat-treated poultry pellets (HTPP) can provide produce growers a slow release fertilizer with a minimized risk of pathogen contamination. However, little is known about the impact of HTTP-amended soil on survival of Salmonella enterica. The contributions of RpoS and potential formation of viable but non-culturable cells to Salmonella survival in soils are also inadaquately understood. We quantified the survival of S. Newport wild-type (WT) and rpoS-deficient ('rpoS) strains in HTPP-amended and unamended soil with or without spinach plants over 91 days using culture and quantitative PCR methods with Propidium Monoazide (PMA-qPCR). A simulated ‘splash’ transfer of S. Newport from contaminated soil to spinach leaves was evaluated at 35 and 63 days post-inoculation (dpi). S. Newport WT and 'rpoS reached the limit of detection - 1.0 log CFU/gdw (gram dry weight) in unamended soil after 35 days, whereas 2-4 log CFU/gdw was observed for both WT and 'rpoS strains at 91 dpi in HTPP-amended soil. Cell densities in soils determined by PMA-qPCR and plate counts methods were similar (p>0.05). Higher levels of S. Newport from HTTP-amended soil transferred to and survived on spinach leaves for longer periods of time compared to S. Newport from unamended soil (p<0.05). Salmonella Newport introduced to HTTP-amended soils survived for longer periods and was more likely to transfer to and persist on spinach plants compared to unamended soils.