Assessment of E. coli as an indicator of microbial quality of irrigation waters use for produce.

Assessment of E. coli as an indicator of microbial quality of irrigation waters use for produce.

Jan. 1, 2012 - Dec. 31, 2012

2012-205

$84,580.00

Channah Rock, Ph.D.
University of Arizona

Charles P. Gerba, Ph.D., Kelly R. Bright, Ph.D.

Final Report

The goals of this project are to evaluate currently available detection methods for the accurate assessment of Escherichia coli contamination in irrigation waters and provide guidance for interpretation of results through a revised risk based E.coli standard. Currently, there is concern that the false positive rate of E.coli detection may be high in these waters giving false indications of the level of risk from enteric pathogens. This may result in unnecessary costly interventions as well as inaccurate perception of risk among consumers. We propose evaluating three different commercial systems for E.coli detection in irrigation waters and assessing false positive rates by use of molecular technologies. As a secondary objective to evaluating E.coli as a reliable indicator, we propose using a Quantitative Microbial Risk Assessment (QMRA) to assess the use of E.coli as an accurate indicator of food safety risk using data collected in the first stage of this project coupled with existing information found in the scientific literature. Ultimately this work will offer recommendations towards the most reliable methods to be used by the produce industry to assess irrigation water contamination as well as a scientific risk-based E.coli guideline that growers can use to protect public health.

Technical Abstract

The goals of this project are to assess currently used methods for the detection of Escherichia coli in irrigation waters used in Arizona and Southern California, and to provide guidelines for a revised E.coli standard for irrigation waters used for produce. Currently, there is concern that the false positive rate of E.coli detection may be high in these waters giving false indications of the level of risk from enteric pathogens. This may result in unnecessary costly interventions (e.g. disinfection of the water, attempts to limit wildlife access, etc) as well as inaccurate perception of risk among consumers. For this reason it is essential to determine the rate of false positive detection of E.coli in waters used for produce irrigation. E.coli detection methods were originally developed for assessment of treated drinking water quality and not surface/irrigation waters. Recent research by our group and others has indicated that high temperatures and elevated salinity may result in false positives rates as high as 40% in Arizona and similar climates. The first objective of this project will be accomplished by evaluating three commercially available methods for E.coli detection to test irrigation waters from three agricultural areas (Yuma and Maricopa, AZ and Imperial Valley, CA) and assessing false positive rates utilizing Polymerase Chain Reaction (PCR) and sequencing of the bacterial isolates. This unique study will thus determine the usefulness of current detection methods for the accurate assessment of E.coli contamination in irrigation waters and provide guidance for interpretation of results.

The assessment and confirmation work will, however, have little value without applying these data to the currently proposed E.coli guidelines used by the produce industry. Currently, no microbial indicator standards exist for irrigation waters used for produce production in the United States. It has been suggested by the produce industry that the bathing water standard guideline (126 E.coli/100 ml) established by the United States Environmental Protection Agency (USEPA) be used. This guideline was developed from epidemiological studies of bather exposure in recreational waters and has no direct relationship to risk associated with infection or illness rates that might result from produce irrigation waters. Therefore, as a secondary objective to evaluating E.coli as a reliable indicator, we propose using a Quantitative Microbial Risk Assessment (QMRA) to assess the risk from consumption of leafy greens following irrigation with waters containing various levels of E.coli. The QMRA will consider method of irrigation, irrigation timing from harvest time, and other environmental factors that may influence indicator organism or pathogen. This will be done using water quality data collected in the first stage of this project coupled with existing information found in the scientific literature. This effort will result in a suggested E.coli guideline for irrigation waters which reflects irrigation and harvesting practices and is based on human health risk.