Purpose: Our objective was to compare the results from two different FC/EC enumeration methods and determine their ability to predict the presence of verotoxigenic E. coli (VTEC), Salmonella, and Listeria monocytogenes in irrigation water.
Methods: Water was collected bi-monthly over a 15-month period from seven irrigation ditches within two distinct watersheds in British Columbia, Canada. FC and EC were enumerated by 1) a 1 ml aliquot on 3M™ Petrifilm™ E. coli/Coliform count plates, and 2) a 25 ml membrane filtration with growth on m-FC agar with 0.01% rosalic acid, followed by transfer to nutrient agar containing 4-methylumbelliferyl-β-D-glucuronide. The samples were tested for the presence of VTEC using a verotoxin colony immunoblot (VT-IB), and for Salmonella and L. monocytogenes using Health Canada methods.
Results: The two methods were found to be significantly correlated for observed counts of both FC (r = 0.60; p < 0.001) and EC (r = 0.77; p < 0.001); however, the Petrifilm™ method gave higher numbers in general for both indicators. Enumeration by membrane filtration significantly correlated with pathogen presence for both FC (r = 0.431; p < 0.001) and EC (r = 0.408; p < 0.001), but the Petrifilm™ method only correlated significantly for FC (r = 0.234; p < 0.001). Regression tree analysis showed a split of an 83.3% chance of pathogen occurrence in samples with ≥ 7.54 CFU/ml FC when measured by membrane filtration.
Significance: This research shows that while FC and EC may correlate with the presence of pathogens in water, the enumeration method and volume tested may play a role in their accuracy.