The relationship between Enterolert and ddPCR concentrations of enterococci in summer samples from beaches in Coronado, California

Enterococci, widely used as a fecal indicator in coastal recreational waters, can be quantified using different methods, including the culture-based Enterolert method and the newer, more rapid droplet digital polymerase chain reaction (ddPCR) method. Previous studies on the correlation between culture-based and PCR-based methods have shown mixed results. To address this knowledge gap, data from a study conducted at three beaches in the city of Coronado, California during the summer of 2023 were analyzed to compare ddPCR-based concentrations of Enterococcus with Enterolert-based concentrations of enterococci in samples collected daily for approximately two months. The concentrations using both methods were consistent with the lognormal distribution. A linear regression between ddPCR and Enterolert concentrations was statistically significant with an R 2 value of 0.41, but the slope was significantly lower than slopes reported in previous studies. The index of agreement between the two methods was 0.25, but it increased to 0.47 when the ddPCR concentrations were scaled to Enterolert equivalents using the regression. When comparing beach action values based on Enterolert and ddPCR, the false positive rate associated with the use of ddPCR was 56.3 %. Overall, the results of this study showed a discordance between the Enterolert and ddPCR methods at Coronado beaches during the summer of 2023, indicating that the relationship may be influenced by spatial and temporal factors.