Field trials shed light on pathogen
die-off rates

Key Take-Aways
* Project is designed to either strengthen FSMA agricultural-water-die-off provisions or show a need to modify them.
* Research involves replicated field trials conducted in three different locations.
* Effects of weather and environment on E. coli die-off rates will be assessed.
* Results will be used to develop a matrix to better assess risks related to interpreting Microbial Water Quality Profiles.
September 26, 2018

When a sample of pre-harvest agricultural water (as defined by the current Food Safety Modernization Act Produce Safety Rule) tests positive for indicator E. coli above specific thresholds, the FSMA "Rule" calls for a specific waiting period between irrigation and harvest, based on best available pathogen die-off rates. But Renata Ivanek, Ph.D. and associate professor in the College of Veterinary Medicine at Cornell University, said there is an urgent need to further validate those die-off rates for different commodities and under different environmental conditions. With her project, titled "FSMA agricultural-water die-off compliance provisions benefit from condition-specific modifiers," Ivanek said she hopes the results will either strengthen the FSMA regulatory matrix or show a need to modify it.

She also expects the results of the two-year project will help growers and packers of baby leafy greens better manage risk and make more informed decisions about water source quality and harvest timing should they collect a positive water sample. Currently, FSMA allows growers to use a waiting period between irrigation and harvest to achieve an acceptable projected reduction in indicator E. coli levels based on a die-off rate of 0.5 log per day for up to four days. (A 1-log reduction is equivalent to a 90 percent reduction.) "The die-off rate of 0.5 log per day may be actually too low," she said. "If in fact it's higher, growers would be able to harvest their product sooner and without putting the consumer in danger. But if the die-off rate is much lower than 0.5 log per day, then we may be putting consumers at risk." 
Renata Ivanek, Ph.D.
Cornell University

Because of these unknowns, Ivanek said there is an urgent need to conduct replicated trials under open-field conditions in different production regions with different model commodities.
Joining her are co-principal investigators Martin Wiedmann, Ph.D., with Cornell University; Trevor Suslow, Ph.D., with the University of California, Davis; Ana Allende, Ph.D., with the Spanish National Research Council; and Daniel Munther, Ph.D., with Cleveland State University.
The project comprises sets of three replicated field trials conducted in three different locations: California, New York and Spain. Ivanek said she chose the locations to take into account different environmental conditions that might affect pathogen die-offs and to represent distinct and nationally or regionally important produce-growing areas. The locales also were chosen because of their proximity to laboratories and to co-PIs with whom she has rapport. In addition, co-PIs Suslow and Allende have extensive experience working with industry partners and could provide guidance to ensure the field trials represented commercial production practices.  Because the project involves live microbes, all of the fields are experimental and won't be harvested commercially.
The trials involve selected strains of indicator E. coli and non-infectious Salmonella. E. coli is considered an indicator organism under FSMA. But Ivanek said she wanted to determine whether there is a strong correlation between that microorganism and available surrogate Salmonella. "Providing they are both in the water, by following the use of E. coli as a metric, can we assume it's a good metric for Salmonella?" she said. "Maybe the risk is lower or maybe it's higher."
The project involves baby lettuce and baby spinach as the models to represent variations in leaf surface characteristics that might affect pathogen die-off rates, she said. Collaborators in each of the three locations will conduct three trials using the same protocols during the growing seasons. An automated weather station at each site will record weather variations. Researchers will first inoculate water with known quantities of the E. coli and Salmonella. Using research-quality backpack sprayers to mimic overhead irrigation, they will apply the water to the crop. "Then we observe over time, up to four days, how much the contamination levels change," she said. "There is evidence to suggest that the die-off rate isn't constant after irrigation. It's faster right after irrigation and then slows down further on."
As a result, researchers will initially collect samples more frequently the first day after water application, stretching the sampling interval eventually to one per day after 24 hours, for a total of four days.

Researchers at all three locations have already conducted one or two trials. Those in Spain, which was chosen because it has a climate similar to Yuma, Arizona, and California, plan to conduct two additional trials this fall. The third and last trial also is planned for New York this fall.  The die-off results will be married with data collected by the weather stations to determine what effects various weather and environmental factors have on pathogen mortality. The data will be used to develop a predictive mathematical model. From there, Ivanek said the researchers plan to use their data to build a produce- and region-specific matrix to better assess risks related to interpreting Microbial Water Quality Profiles. This study also will provide a template for future studies on other commodities and agro-ecoregions.

  Project abstract may be found on the Center for Produce Safety website:
CPS Minute Video with Dr. Ivanek can be found on the CPS Videos page

Learn more about this project and other CPS funded research by visiting our 
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