Submitted to: Food Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/28/2013
Publication Date: 7/6/2013
Citation: Patel, J.R., Singh, M., Macarisin, D., Sharma, M., Shelton, D.R. 2013. Differences in biofilm formation of produce and poultry Salmonella enterica isolates and their persistence on spinach plants. Food Microbiology. 36:388-394. Interpretive Summary: In recent years, the number of foodborne illness outbreaks linked to fresh produce has increased in the U.S. Irrigation water may be a source of pathogens that contaminate fresh produce at the farm level. We evaluated the effect of repeated contamination of spinach plants with spray irrigation containing Salmonella. The goal was to evaluate persistence of Salmonella on spinach leaves. Spinach plants were contaminated biweekly with ca. 100 or 10,000 cells/100 ml of irrigation water in a growth chamber. Surviving Salmonella populations on spinach leaves were determined by direct plating and most-probable-number (MPM) technique. We found that Salmonella were unlikely to persist on spinach leaves when irrigation water was contaminated at levels below the Leafy Greens Marketing Agreement Standards (less than 126 cells generic E. coli per 100 ml water). In a parallel study, Salmonella isolated from produce and poultry were evaluated for their ability to form strong biofilms on inert surfaces (polystyrene, polycarbonate, and stainless steel), and their persistence on spinach leaves. Salmonella strains from produce were observed to produce the stronger biofilm. These strains from produce also persisted at significantly higher numbers than those from poultry on spinach leaves during the entire 35 days study. This study indicates that Salmonella persistence on spinach leaves is affected by the source of contamination and the biofilm forming ability of the strain. This information should be useful to other scientists and the produce industry.
Technical Abstract: Repeat irrigation of spinach plants with water containing Salmonella was used to determine Salmonella persistence on spinach leaves. Spinach plants were irrigated four times (biweekly) with water containing ca. 2.1 log CFU Salmonella per 100 ml water (the maximum generic E. coli MPN recommended by the Leafy Greens Marketing Agreement; LGMA), or 4.1 CFU Salmonella/100 ml water. Green Fluorescent protein expressing Salmonella were undetectable by most-probable number (MPN) technique at 24 h and 7 days following each irrigation event. This study indicates that Salmonella are unlikely to persist on spinach leaves when irrigation water is contaminated at a level below the LGMA standards. In a parallel study, persistence of Salmonella isolated from poultry or produce was compared following biweekly irrigation of spinach plants with water containing 6 log CFU/100 ml; persistence was also correlated with biofilm formation. Salmonella isolates from produce formed stronger biofilms on polystyrene, polycarbonate and stainless steel surfaces, as determined by microtiter plate assay and bioreactor assay, respectively. Strong biofilm forming Salmonella isolates from produce persisted at significantly higher numbers than those from poultry on spinach leaves during the entire 35 days study. Salmonella from poultry were undetectable (< 1 log CFU/g) on spinach plants 7 days following each irrigation event when assayed by direct plating; although they were subsequently recovered by MPN. This study indicates that Salmonella persistence on spinach leaves is affected by the source of contamination and the biofilm forming ability of the strain.