Survival and growth of wild-type and rpoS-deficient Salmonella Newport strains in soil extracts amended with heat-treated poultry pellets

Authors: SHAH, MANOJ - North Dakota State University; Bradshaw, Rhodel; NYARKO, ESMOND - University Of Delaware; Millner, Patricia; NEHER, DEBORAH - University Of Vermont; WEICHT, THOMAS - University Of Vermont; BERGHOLZ, TERESA - North Dakota State University; Sharma, Manan

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2018
Publication Date: 3/1/2019
Citation: Shah, M.K., Bradshaw, R.N., Nyarko, E., Millner, P.D., Neher, D., Weicht, T., Bergholz, T.M., Sharma, M. 2019. Survival and growth of wild-type and rpoS-deficient Salmonella Newport strains in soil extracts amended with heat-treated poultry pellets. Journal of Food Protection. https://doi.org/10.4315/0362-028X.JFP-18-465.
DOI: https://doi.org/10.4315/0362-028X.JFP-18-465

Interpretive Summary: Salmonella Newport is a bacterial pathogen which has caused outbreaks of foodborne illness associated on produce commodities like tomatoes and cucumbers. Biological soil amendments (BSAs) of animal origin, include manure, are applied as fertilizer, and manure runoff in agricultural fields, can be a source of Salmonella Newport on farms which can contaminate produce commodities through a variety of routes, including irrigation water and BSA. Our study examined the growth and survival of Salmonella in simulated runoff (extracts) which were made with heat-treated poultry pellets (HTPP), a commonly used organic fertilizer derived from poultry litter stabilizes nutrient levels. Our results from this study show soil extracts which contained HTPP (amended) were more likely to support higher growth rates and populations of S. Newport compared to extracts which did not contain HTPP. When amended extracts were filtered to remove some of the naturally present, non-pathogenic (indigenous) bacteria from soil or HTPP, S. Newport populations increased quickly without the presence of microbial competition. In unamended extracts, the Salmonella populations declined quickly, either due to a lack of nutrients or microbial competition from indigenous microorganisms. Our results also showed that gene rpoS was critical to the survival and growth of Salmonella Newport in unamended soil extracts. This work benefits farmers by identifying a risk factor for foodborne contamination in the on-farm environment and allow them to fashion specific, appropriate mitigation to reduce bacterial contamination.

Technical Abstract: Manure runoff can transfer foodborne pathogens to farmlands or to water sources leading to subsequent contamination of produce. Untreated biological soil amendments (BSA), like manure, can be contaminated with foodborne pathogens. Salmonella Newport is a major bacterial foodborne pathogen which has been associated with several outbreaks attributed to fresh produce commodities. During cultivation, contaminated soil may lead to transfer of Salmonella to fruits or vegetables. Studies have shown the occurrence and survival of Salmonella in manure or manure slurries. However, data on the survival and growth of S. Newport is lacking in matrices simulating runoff. We quantified the survival/growth of wild type (WT) S. Newport and rpoS-deficient ('rpoS) strain in sterile and non-sterile soil extracts prepared with (amended) or without (unamended) heat-treated poultry pellets at 25°C. It was observed that S. Newport WT and 'rpoS strains could grow in amended and unamended soil extracts to a population of 6 to 8 log CFU/ml in 24-30 h and remained in stationary phase up to 4 days. S. Newport in amended soil extracts exhibited a decreased lag phase (2.87±1.01 h) and higher maximum cell densities (6.84±1.25 log CFU/ml) compared to lag phase (20.10±9.53 h) and maximum cell densities (5.22±0.82 log CFU/ml) in unamended soil extracts. In addition, S. Newport had lower cell densities in non-sterile soil extracts (5.94±0.95 log CFU/ml) than in sterile soil extracts (6.66±1.50 log CFU/ml), indicating competition for nutrients between indigenous microbes and S. Newport. Unamended non-sterile soil extracts did not support the growth of S. Newport WT and 'rpoS strains. The most favorable growth conditions were provided by amended sterile and non-sterile soil extracts, followed by sterile unamended soil extracts for both S. Newport WT and 'rpoS strains. S. Newport may grow to greater populations in amended extracts compared to unamended extracts, providing a route for increased Salmonella populations in pre-harvest produce-growing environments.