Comparison of Fecal Indicator Bacteria Densities in Marine Recreational Waters by QPCR

The US EPA is currently investigating the use of quantitative PCR (qPCR) analysis techniques to estimate densities of fecal indicator bacteria (FIB) in recreational waters. Present water quality guidelines, based on culturable FIB, prevent same day water quality determination, whereas results from qPCR-based approaches are available within several hours. Epidemiological studies at Publicly-Owned Treatment Works (POTW)-impacted freshwater beaches have also indicated correlations between qPCR determined Enterococcus densities and swimming-related illness rates. Similar qPCR assays are now available for several other accepted or emerging FIB groups. This study provides an initial assessment of qPCR estimated Enterococcus, Bacteroidales, E. coli and Clostridium spp. densities in marine water and sand samples collected over one summer from two POTW-impacted recreational beaches. Relative target sequence densities of these organisms in the samples did not correspond with their relative estimated cell densities. These observations were attributable to differences in target sequences recovered from the calibrator cells of the different types of organisms. Comparative cycle threshold (CT) qPCR analyses of whole cell calibrator samples provide a simple and standardizable approach for estimating both total cell and target sequence densities of different types of FIB in water. Cell density estimates obtained by this approach are subject to uncertainty due to potential variability in absolute numbers of target sequences in the target organisms under different physiological or environmental conditions, but still may allow for informative comparisons with the target sequence estimates.