Comparison of Defined Chromogenic Substrate Assay and Method 1600 for Monitoring Enterococci in Tidal Rivers

Background: Defined substrate technology (Enterolert, IDEXX Laboratories, Westbrook, ME) is accepted by the USEPA as an alternative to the membrane filtration method (Method 1600) for detection of fecal pollution at marine beaches. Studies have found that these two methods generally produce comparable results in marine environment. In this method, instead of culturing the bacteria on agar, multiple tube technique is used and positive Enterolert results are indicated by a fluorescent metabolite. In Georgia, all beaches, regardless of whether they are influenced by marine or freshwater, are monitored using Method 1600. The purpose of this study is to compare the reliability and performance of Enterolert with Method 1600 as an alternative for monitoring Enterococci in tidal rivers.

Materials: Samples were collected monthly from 14 sites along a 2.6 mile section of the Ogeechee River that included a beach with persistent high concentrations of Enterococci. Samples were transferred to the laboratory within six hours of collection and analyzed immediately. Enterolert and EPA Method 1600 were used to enumerate Enterococci. In situ environmental conditions (temperature, salinity, turbidity, pH) were recorded. Statistical analysis was conducted using IBM SPSS version 21.

Results: According to USEPA, Beach Action Values (BAV) for Enterococci should not exceed 60 CFU/ 100 mL in marine recreational water. According to the first three months of data, 67% and 81% of the 42 samples tested higher than BAV when tested by Method 1600 and Enterolert, respectively. The highest Enterococci detected in December with both methods were 361 MPN/100mL by Enterolert method and 507 CFU/100mL by Method 1600. Enterolert and Method 1600 means were analyzed by using a nonparametric Mann-Whitney U test. There was a significant difference between two methods (Z=2.315, p=0.021).

Conclusion: According to the preliminary study, there was a significant difference between these two methods within the winter samples. Further analyses using more data from the upcoming sampling events will provide more data to validate the Enterolert method for these points. GA has 3400 miles of tidal shoreline. Therefore using the most reliable and user friendly method to monitor these beaches has critical outcomes in terms of improved public health.