Comparison and Verification of Bacterial Water Quality Indicator Measurement Methods Using Ambient Coastal Water Samples

Abstract

More than 30 laboratories routinely monitor water along southern California's beaches for bacterial indicators of fecal contamination. Data from these efforts frequently are combined and compared even though three different methods (membrane filtration (MF), multiple tube fermentation (MTF), and chromogenic substrate (CS) methods) are used. To assess data comparability and quantify variability within method and across laboratories, 26 laboratories participated in an intercalibration exercise. Each laboratory processed three replicates from eight ambient water samples employing the method or methods they routinely use for water quality monitoring. Verification analyses also were conducted on a subset of wells from the CS analysis to confirm or exclude the presence of the target organism. Enterococci results were generally comparable across methods. Confirmation revealed a 9% false positive rate and a 4% false negative rate in the CS method for enterococci, though these errors were small in the context of within- and among-laboratory variability. Fecal coliforms also were comparable across all methods, though CS underestimated the other methods by about 10%, probably because it measures only E. coli, rather than the larger fecal coliform group measured by MF and MTF. CS overestimated total coliforms relative to the other methods by several fold and was found to have a 40% false positive rate in verification. Across-laboratory variability was small relative to within- and among-method variability, but only after data entry errors were corrected. One fifth of the laboratories committed data entry errors that were much larger than any method-related errors. These errors are particularly significant because these data were submitted in a test situation where laboratories were aware they would be under increased scrutiny. Under normal circumstances, it is unlikely that these errors would have been detected and managers would have been obliged to issue beach water quality warnings.

Enterococci Concentrations in Diverse Coastal Environments Exhibit Extreme Variability

“…Colilert has been shown to perform well in California marine waters for E. coli enumeration (23,28). Conclusions regarding variability in ENT apply to these bacteria as well.…”

Enterococci in the Environment

“…In addition to standard methods using membrane filtration, numerous monitoring laboratories have also relied on alternative culturing techniques. A comparison of membrane filtration with multiple-tube fermentation and chromogenic substrate methods, i.e., Enterolert (34), showed that results did not vary significantly by method (132) and were being used interchangeably to manage beaches across a large portion of Southern California. With the increasing frequency and number of beaches being monitored for enterococci since the passage of the BEACH Act (22), many locations are using either chromogenic substrate or membrane filtration analytical techniques, with results being used interchangeably across jurisdictions for beach management (246).…”

Comparison ofEnterococcusSpecies Diversity in Marine Water and Wastewater Using Enterolert and EPA Method 1600

“…The methods have been used interchangeably for (1) regulatory monitoring to detect possible fecal contamination of water; (2) epidemiology studies to correlate swimmer's illness rates with densities of Enterococci in water, and (3) microbial source tracking studies to reduce fecal inputs to protect public health. A number of studies have found that these two methods generally produce comparable results [2–5]. However, several authors have found that the results may be markedly different [6–9].…”