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United States Department of Agriculture

Agricultural Research Service

Research Project: MICROBIAL ECOLOGY AND SAFETY OF FRESH ON-FARM ORGANICALLY GROWN PRODUCE Title: The Effect of Repeated Irrigation with Water Containing Varying Levels of Total Organic Carbon on the Persistence of Escherichia coli O157:H7 on Baby Spinach

Authors
item Ingram, David
item Patel, Jitu
item Sharma, Manan

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 5, 2011
Publication Date: May 1, 2011
Repository URL: http://handle.nal.usda.gov/10113/53796
Citation: Ingram, D.T., Patel, J.R., Sharma, M. 2011. The Effect of Repeated Irrigation with Water Containing Varying Levels of Total Organic Carbon on the Persistence of Escherichia coli O157:H7 on Baby Spinach. Journal of Food Protection. 74(5):709-717.

Interpretive Summary: Outbreaks of Escherichia coli O157:H7 on leafy greens have been associated with irrigation water. E. coli is widely used as an indicator for the presence of bacterial foodborne pathogens in the Leafy Greens Marketing Agreement (LGMA), being implemented by numerous leafy green handlers. The LGMA state that irrigation water for leafy greens contain a maximum E. coli population of 126 Most Probable Number (MPN) per 100 ml irrigation water. This study was conducted to determine the fate of the foodborne pathogen E. coli O157:H7 on baby spinach leaves after repeated introduction at low levels through irrigation water containing different concentrations of total organic carbon (TOC), another water quality indicator. Baby spinach plants were repeatedly irrigated by overhead spray with water containing very high (5 – 6 log CFU / 100 ml) or low populations of E. coli O157:H7 (0 -1 log CFU / 100 ml) in a growth chamber. Plants irrigated with high E. coli O157:H7 populations, regardless of TOC levels, showed a 3-log reduction within the first 24 h. Low levels of E. coli O157:H7 persistence were observed for up to 16 days on all TOC treatments, ranging from 76.4 MPN per plant (day 1) to 0.40 MPN per plant (day 16). No viable cells were detected on spinach tissue 24 h after irrigation with water containing < 126 CFU / 100 ml E. coli O157:H7. The fate of pathogens on foliar surfaces should be considered when evaluating irrigation water quality.

Technical Abstract: The California lettuce and leafy greens industry has adopted the Leafy Greens Marketing Agreement (LGMA), which allows for 126 Most Probable Number (MPN) generic E. coli/100ml in irrigation water. Repeat irrigation of baby spinach plants with water containing E. coli O157:H7 and different levels of total organic carbon (TOC) were used to determine the epiphytic survival of E. coli O157:H7. Three irrigation treatments (A: 0ppm TOC, B: 12ppm or 15ppm TOC and C: 120ppm or 150ppm TOC) were prepared using bovine manure containing E. coli O157:H7 at either low (0 - 1 log CFU/100ml) or high (5 - 6 log CFU/100ml) populations and were sprayed onto baby spinach plants in growth chambers using a fine mist air-brush. Most Probable Number (MPN) and direct plating techniques were used to determine the E. coli O157:H7 populations on the aerial plant tissue. Plants irrigated with high E. coli O157:H7 populations, regardless of TOC levels, showed a 3-log reduction within the first 24 h. Low levels of E. coli O157:H7 persistence were observed for up to 16 days on all TOC treatments, ranging from 76.4 MPN per plant (day 1) to 0.40 MPN per plant (day 16). No viable cells were detected on spinach tissue 24 h after irrigation with water containing < 126 CFU/100ml E. coli O157:H7. Under growth chamber conditions in this study, E. coli O157:H7 populations in irrigation water which comply with the LGMA standards will not persist for more than 24 h on foliar surfaces of spinach plants.

Last Modified: 4/19/2014
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