MICROBIAL ECOLOGY AND SAFETY OF FRESH ON-FARM ORGANICALLY GROWN PRODUCE
Title: Comparison of generic E. coli vs. pathogenic E. coli virulence factors in an agricultural watershed: implications for irrigation water standards and leafy green commodities
Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 13, 2010
Publication Date: January 3, 2011
Citation: Shelton, D.R., Karns, J.S., Coppock, C.R., Patel, J.R., Sharma, M., Pachepsky, Y.A. 2011. Comparison of generic E. coli vs. pathogenic E. coli virulence factors in an agricultural watershed: implications for irrigation water standards and leafy green commodities. Journal of Food Protection. 74:18-23.
Interpretive Summary: In the past few years, several food-borne outbreaks of pathogenic E. coli have been reported as a consequence of contaminated fresh produce. In a few instances, contamination was attributable to surface irrigation waters. In an effort to eliminate future contamination events, the fresh produce industry has adopted standards for irrigation waters based generic E. coli. These standards are essentially identical to standards established by the U.S. Environmental Protection Agency (EPA) for recreational waters, which are based on data derived from observed human exposures at swimming beaches impacted by sewage. However, all reported cases of fresh produce contamination from irrigation waters have been due to cattle. In addition, generic E. coli are excreted by wildlife and waterfowl, the vast majority of which are not pathogenic to humans. Consequently, the adequacy of the adopted standards is unclear. In this study, we analyzed water samples for both generic E. coli and virulence factors associated with pathogenic E. coli (genes that encode for proteins or toxins that make the bacterium pathogenic) from a predominantly agricultural watershed. There were relatively high concentrations of E. coli and prevalence of virulence factors; however, we were unable to establish any relationship between the two. These results suggest that generic E. coli may not be a reliable indicator of irrigation water quality. These findings are relevant to the fresh produce industry, research scientists, and to consumers.
The California Leafy Greens Marketing Agreement (LGMA) was adopted in an effort to minimize the risk of contamination of leafy greens with enteric pathogens from a variety of sources, including ground and surface irrigation waters. The LGMA contains standards similar to those established for recreational waters based on generic Escherichia coli concentrations. However, no correlation between generic E. coli and any specific water-borne pathogen(s) has been reported. We conducted this monitoring study in a predominantly agricultural watershed to (i) evaluate spatial and temporal fluctuations in generic E. coli populations and virulence factors associated with pathogenic E. coli and to (ii) investigate whether a relationship could be established between generic E. coli and virulence factors. The virulence factors targeted for analysis were the eae and stx genes, encoding for intimin and shiga-like toxins, respectively; they were detected using polymerase chain reaction (PCR) methods. Generic E. coli concentrations and eae and stx prevalence varied both spatially and temporally; in general, both were higher in agricultural than in forested areas, and were higher in summer/fall seasons than in winter. The eae and stx genes were prevalent throughout the watershed; however, in the absence of actual isolates, no conclusions could be drawn regarding the prevalence of specific pathogenic E. coli. No correlation was observed between generic E. coli concentrations and virulence factors; lower generic E. coli concentrations were not necessarily associated with decreased prevalence of eae and stx genes. These results suggest that the LGMA standards may not adequately address the issue of water-borne contamination and that alternative criteria may be more appropriate for determining when surface waters can be used for irrigation.