Recovery of Salmonella from bermudagrass exposed to simulated wastewater

Authors: McLaughlin, Michael; Brooks, John

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/2008
Publication Date: 1/1/2009
Citation: McLaughlin, M.R., Brooks, J.P. 2009. Recovery of Salmonella from bermudagrass exposed to simulated wastewater. Journal of Environmental Quality. 38:337-342.

Interpretive Summary: In Mississippi and most of the southeastern USA, manure from confined swine feeding operations is held in anaerobic treatment lagoons and eventually discharged by spray application as fertilizer on bermudagrass hay crops. Although Salmonella is widely known to occur in swine and in manure lagoons, levels of Salmonella in commercial Mississippi swine farm lagoons and potential risk for contamination of bermudagrass were unknown. In the research reported here the levels of Salmonella in selected Mississippi commercial swine farm lagoons were determined, and a laboratory simulation model for assessment of the potential risk for bermudagrass contamination by Salmonella was developed. The model was developed using EPA worst case water to simulate lagoon effluent, and is believed to be a new application of this EPA testing matrix. The model showed the relationship between level of Salmonella exposure on bermudagrass leaves and the resulting level of contamination. Salmonella levels measured in commercial swine farm manure lagoons were below threshold limits for contamination predicted by the simulation model. Based on these findings, effluents from these lagoons would not be expected to pose a Salmonella contamination hazard for bermudagrass. This new risk assessment tool will be useful to animal waste management and food safety researchers, and the findings from its application will be important to the livestock production industry, regulatory bodies, and consumers.

Technical Abstract: Most confined swine feeding operations in Mississippi and the southeastern USA hold manure in lagoons and apply the effluent on bermudagrass, Cynodon dactylon (L.) Pers., as fertilizer. Salmonella enterica subsp. enterica (ex Kauffman and Edwards) Le Minor and Popoff, has been reported in Mississippi lagoons, but levels and potential for contamination of bermudagrass were unknown. Therefore, research was conducted to assess potential risk of contamination. A simulation model was developed for Salmonella contamination of bermudagrass in the laboratory, and levels of Salmonella were determined in commercial farm lagoons. EPA (U.S. Environmental Protection Agency) worst case water was used to simulate effluent in laboratory exposures of bermudagrass leaves to Salmonella. Exposed leaves were held overnight at 25C and 85-90% relative humidity, washed, and bacteria enumerated. Salmonella contamination of leaves exposed to 106 cfu mL-1 varied from 0 to 104 cfu per leaf within and among bermudagrass cultivars and Salmonella isolates, but no differences (P<0.05) occurred between eight cultivars (n = 20) or five isolates (n = 10). Data were fitted (R2 = 0.93) to a contamination simulation model (y = 5x10-6x6.623) to describe the relationship between levels of exposure (x, Log10 cfu mL-1) and contamination (y, Log10 cfu per leaf). In fall 2007 Salmonella was isolated in Mississippi from six of six farm lagoons tested. Levels ranged from 1.9 to 2.8 log10 MPN 100 mL-1 and were below the threshold for bermudagrass contamination predicted by the simulation model. Based on this model and other work, which reported levels in lagoons to be highest in fall, effluents from these lagoons would not be expected to produce measurable Salmonella contamination on bermudagrass.