Evaluation of a Simplified E. coli/Coliform Method for a Variety of Water and Produce Rinse Samples

Introduction: Water and produce rinse samples are tested for the presence of generic E. coli and coliform as sanitary and process control measurements.   Peel Plate EC (E. coli and Coliform) is a pre-prepared self-wicking method with β-galactosidase (GAL) and β-glucuronidase (GLU) substrates to detect E. coli as blue/purple colonies and coliform as red colonies. The method has been validated in various food matrices. The 47 mm test area of the test and the absence of lid contact with the test medium allows the use of membrane-filters to concentrate liquid/water samples and apply the filter as test samples.

Purpose: The purpose of the study was to evaluate the new method in comparison to reference methods using mixed cellulose acetate filters of water and produce rinse samples.

Methods: Bottled water, irrigation water, produce flume, and diluted-cooling-water (5% propylene glycol) samples were co-inoculated with different coliform and E. coli strains at low, medium and high concentrations.  One hundred ml samples of bottled water, produce rinse (mixed greens) chlorine neutralized with thiosulfate, and the diluted cooling water were filtered through 0.45 µm mixed cellulose filters and plated in comparison m-Endo LES agar method and confirmed by BGLB/EC-MUG broth.  Irrigation water was filtered, 100 ml, and tested in comparison to EPA method 1604.  

Results: In the method comparisons, results were evaluated for repeatability (S_r) and by paired t-test for statistical difference, > 0.5 log.  The Peel Plate EC was not significantly different from the reference methods in the detection of coliform or E. coli.  

Significance: Peel Plate EC detected E. coli and coliform at 35°C in 24 h similarly to reference methods in water and rinse samples and is a testing alternative for detecting fecal indicators in food production environments.