Summary of Awards to Date

Evaluation of an alternative irrigation water quality indicator


Jan. 1, 2015 - Dec. 31, 2016

Funding Agency

Center for Produce Safety

Amount Awarded



Trevor Suslow, Ph.D.
University of California, Davis


Channah Rock, Ph.D., University of Arizona


Water used in preharvest production of edible horticultural foods, fruits and vegetables, is universally recognized as a key potential hazard for widespread crop contamination with human foodborne pathogens. Industry guidance, standards, and microbiological and sampling interval metrics uniformly identify nonpathogenic E. coli as the practical and cost-effective indicator of fecal contamination of a water source. Drawn from recreational water quality standards, numbers of E. coli can be associated with the presence of human pathogens. However, years of experience and tens of thousands of irrigation water tests demonstrate that the accuracy of current metrics to predicting risk to consumers following consumption of uncooked produce is very poor. Our project objective is to develop an initial baseline of comparative data for indicator E. coli, Total Bacteroides (an alternative and more robust indicator), in surface water sources to the presence of human pathogenic E. coli and Salmonella in water used for irrigation management, ag-chemical sprays, and other preharvest applications. Studies will be conducted in CA, AZ, and selected regional production districts and diverse types of produce. This project would develop data to support academic, industry, and public health evaluations for the replacement of non-functional quantitative irrigation water standards with a simpler semi-quantitative threshold.

Technical Abstract

The lack of consistent correlation between chemical or biological indicators of fecal contamination at the local, regional, national, and global level is well documented in the scientific and public health literature. This situation is extensively described in the environmental and public health science literature and encompasses sources of drinking water, regional watersheds, recreational water, wastewater reclamation, and irrigation water.  FDA clearly recognizes the absence of clarity and consistency relating to, and the practical limitations of, designing any single or multi-level indicator-based approach to establishing compliant and non-complaint conditions that achieve a measureable advancement in public safety and health-based water quality standards. Recent studies assembled from datasets in different parts of the U.S. and other countries underscore the general lack of correlation between a generic E. coli indicator in irrigation sources and the presence of pathogens in the water or on crops irrigated with the water. In addition, ignoring for the moment the limitations of indicator E. coli as the reasonable practical compromise choice, the selection of quantitative regulatory compliance standards is both unnecessary and costly for many producers. There is no dataset or model we are aware of that could be applied to differentiate the risk associated with use of agricultural water under any combination of conditions between a rolling geometric mean of 125 vs. 126 MPN (CFU) / 100 ml or 234 vs. 235 CFU/100ml as a single sample exceedance point. An alternative qualitative approach is hypothesized. A well-established biological indicator of fecal contamination of water sources is the Gram-negative obligate anaerobic bacterium Bacteroides. Tracking and differentiating animal sources of bacteria in the genus Bacteroides has long been a tool for evaluating fecal contamination of surface water with uses in drinking water sources and for recreational purposes. Recent developments in rapid, quantitative, real-time PCR methods and first generation commercial Total Bacteroides qPCR kits make application in service labs for routine agricultural water testing a plausible reality.