March 2016
A monthly series reporting to the produce industry on CPS research projects   

Improved sampling and analytical methods for testing agricultural water for pathogens, surrogates and source tracking indicators

UF water sampling technique, microbial surrogates may offer better risk assessment tools

Research led by the Centers for Disease Control and Prevention is examining a new irrigation water sample collection method that is expected to enable better detection of pathogens and fecal indicators to better reflect microbial risks and fecal sources for irrigation water. The project is looking at surrogate organisms that may be better indicators of fecal contamination than E. coli, said Vincent Hill, Ph.D., and an environmental engineer with the CDC's Division of Foodborne, Waterborne and Environmental Diseases.

The goal of the research - now in its second year - is to develop protocols for high-volume ultrafiltration (UF) water sample collection that producers could follow.

"The results from this project will expand the tool box that producers have to characterize the microbial quality of their irrigation water, both in terms of the sample collection methods with ultra-filtration techniques and the analytical methods," he said. "We're looking at alternative fecal indicators as well as the molecular source tracking (MST) analytes, which have the potential to identify human or animal fecal sources."

Key Take-Aways
  • Project seeks to validate large-volume ultrafiltration (UF) water sample collection methods.
  • Researchers are looking at surrogate microbes that may be better indicators of fecal contamination than E. coli.
  • Goal is to develop protocols for large-volume UF water sample collection.
Although the research involves collecting samples from ponds on three different Georgia farms, Hill said the protocols being developed should be applicable to most irrigation sources, regardless of location.

Most water samples collected for E. coli testing are typically 100 milliliters and in some cases may range up to 1 liter. But testing samples of this size may not detect pathogens if they are at very low levels.

The alternative is collecting large-volume samples of 50 to 100 liters. But collecting and transporting them can be cumbersome.

UF sampling, on the other hand, involves a robust filter with minute pores that can trap bacteria, parasites, and viruses. A battery-powered portable peristaltic pump is used to filter water from rivers, lakes or ponds at rates of 2 to 4 liters per minute. Within 15 to 30 minutes, users can collect microbes from the equivalent of 50 liters of water. Afterward, the filter - weighing about a pound -  is capped and sent to the laboratory for testing.

"In the past when we didn't have this UF method, we were shipping tens of liters of water in coolers, which was no fun (and expensive!)," Hill said. "With UF, we get more sensitive detection. If there are relatively few of the microbes we're looking for, we have a better chance of drawing them into the filter."

For this project, study collaborators from the University of Georgia are collecting bi-weekly and monthly samples, including a large-volume UF sample and a small-volume grab sample near the irrigation intake as well as the other side of the main irrigation pond on three different southeast Georgia farms for one year. They are also collecting the same sets of samples after rainstorms. The samples are analyzed for E. coli O157:H7, Salmonella, Cryptosporidium, pathogen surrogates, and other indicators of fecal contamination including MST analytes.

Although some pathogen surrogates have been studied extensively for other applications, such as wastewater impacts on beach water quality, they have not been validated for agricultural uses, he said.

"The new FSMA (Food Safety Modernization Act) law established agricultural water quality standards based on E. coli," Hill said. "But E. coli is a bacterial indicator of fecal contamination, and it's not an ideal indicator for assessing risks associated with viruses or parasites.

"It's a useful tool because it's cheap and easy to measure, but other fecal indicators may be more informative for characterizing irrigation water quality. For example, using MST analytes to identify animal versus human fecal sources can help farmers mitigate contamination issues in their source water."

Co-investigator on the research is George Vellidis, Ph.D., a professor of crop and soil sciences with the University of Georgia, Tifton. Also collaborating is Karen Levy, professor of environmental health and epidemiology at Emory University.

"The University of Georgia and Emory University have been amazing to work with," Hill said. "The University of Georgia has been working with collaborators in southeast Georgia for many years, so they have established a rapport and working relationship with farmers in the area. So with this project, we hit the ground running, which was really great."

In addition, the University of Georgia brought its sampling collection experience. Emory University contributed its expertise in Salmonella testing and characterization as well as data and advanced statistical analyses, Hill said.

Vincent R. Hill, PhD, PE  
Environmental Engineer
Centers for Disease Control and Prevention
National Center for Emerging and Zoonotic Infectious Diseases
Waterborne Disease Prevention Branch
Atlanta, GA

About CPS
The Center for Produce Safety (CPS) is focused exclusively on providing the produce industry and government with open access to the actionable information needed to continually enhance the safety of produce.  Established by public and private partnership at the University of California, Davis, initial funding for CPS was provided by the California Department of Food and Agriculture, the University of California, Produce Marketing Association and Taylor Farms.  Ongoing administrative costs are covered by the Produce Marketing Association, enabling industry and public funds to go exclusively to research. 

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Phone:  530-554-9706

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