A comparison study of Colilert and qPCR methods at Pere Marquette Beach, Muskegon County, MI

Pere Marquette Beach serves as the primary attraction for tourism and coastal recreation in Muskegon, MI. Because beaches attract many people daily, it is important to monitor beach water quality for pathogens that may cause waterborne disease. Molecular-based methods are emerging as replacements for culture-based techniques for monitoring beaches. Culture-based methods require 18-hour incubation while Quantitative Polymerase Chain Reaction (qPCR) can yield results in two hours. My research examines the correlation between the culture-based Colilert-18® method and qPCR measurements of E. coli at a Lake Michigan beach in Muskegon County, MI. While Colilert 18 is a defined substrate method and measures culturable cells, the qPCR method quantifies both living and nonliving DNA. Regression analysis (R2) was used to correlate analytical results and the Index of Agreement (IA) was employed to evaluate method comparability. This research demonstrates the equivalency of both methods for E. coli measurements at Pere Marquette beach (R2 = 0.8012; IA = 0.71). The significant positive difference between the methods suggests that current guidelines for beach warnings and closures need to be revised to reflect the presence of nonviable DNA/cells in beach water. This study was important for assessing the applicability of qPCR for providing same-day results for pathogens at local beaches