Purpose: Currently, no microbial indicator standards exist for irrigation waters used for produce production in the US. The produce industry suggests that the recreational water guideline (126 E. coli/100 ml) established by the United States Environmental Protection Agency (USEPA) be used. 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.
Methods: Three commercially available media for E. coli detection were evaluated in Yuma and Maricopa, AZ, and Imperial Valley, CA, and then assessed false positive rates by utilizing polymerase chain reaction (PCR) and DNA sequencing. The media chosen for evaluation were (1) MI Agar, (2) IDEXX Colilert Quanti-Tray® and (3) m-ColiBlue24® broth, accepted by the USEPA and widely used by the produce industry. One-L irrigation water grab samples (n = 450) were collected between March and November 2012. The samples were analyzed for both cultural counts and water quality parameters including temperature, salinity and pH. Isolates positive and negative for E. coli were then selected and assessed utilizing PCR and DNA sequencing.
Results: The false positive rate of each method was found to be high, with MI Agar, m-ColiBlue24® broth and the IDEXX Colilert Quanti-Tray® at an accuracy of 67%, 72%, and 51%, respectively. A false positive result is reported when presumptive E. coli sub cultured from the media is found to be non-E. coli through molecular analysis.
Significance: Overall the IDEXX Colilert Quanti-Tray® performed at a greater rate of accuracy than the other two media evaluated, however, high false positive rates may lead to inaccurate assessment of water quality.