Absence of Direct Association between Coliforms and Escherichia coli in Irrigation Water and on Produce

Abstract

Irrigation water is considered a potential source of preharvest pathogen contamination of vegetables. Hence, several organizations have recommended microbiological standards for water used to irrigate edible plants. The purpose of this study was to determine the strength of association between microbial quality indicators (coliforms and Escherichia coli) in irrigation water and on irrigated vegetables. Data analyzed included original results from a cross-sectional study conducted in the Midwestern United States during summer 2009 and information presented in two previously published studies performed in France and Portugal to investigate microbial quality of irrigation water and watered produce. In the cross-sectional study, repetitive PCR (rep-PCR) was used to characterize genetic relatedness of E. coli isolates from water and vegetables. No significant correlations were found between fecal indicators on leafy greens (lettuce and parsley, n = 91) or fruit (tomatoes and green peppers, n = 22) and those found in irrigation water used in the cross-sectional study (P > 0.40) or in the previously published data sets (data set 1: lettuce and waste irrigation water, n = 15, P > 0.40; data set 2: lettuce and irrigation water, n = 32, P = 0.06). Rep-PCR banding patterns of E. coli strains were all distinguishable among the pairs of E. coli isolates recovered from produce and irrigation water on the same farm. From the available data, the concentration of indicator organisms based on a single measure of irrigation water quality was not associated with the presence of these indicators on produce. In the absence of additional information, the use of a single microbial water quality parameter as an indicator of produce safety is of limited value for predicting the safety of the produce.