Distribution of Human-Specific Bacteroidales and Fecal Indicator Bacteria in an Urban Watershed Impacted by Sewage Pollution, Determined Using RNA- and DNA-Based Quantitative PCR Assays

Abstract

The identification of fecal pollution sources is commonly carried out using DNA-based methods. However, there is evidence that DNA can be associated with dead cells or present as "naked DNA" in the environment. Furthermore, it has been shown that rRNA-targeted reverse transcription-quantitative PCR (RT-qPCR) assays can be more sensitive than rRNA gene-based qPCR assays since metabolically active cells usually contain higher numbers of ribosomes than quiescent cells. To this end, we compared the detection frequency of host-specific markers and fecal bacteria using RNA-based RT-qPCR and DNA-based qPCR methods for water samples collected in sites impacted by combined sewer overflows. As a group, fecal bacteria were more frequently detected in most sites using RNA-based methods. Specifically, 8, 87, and 85% of the samples positive for general enterococci, Enterococcus faecalis, and Enterococcus faecium markers, respectively, were detected using RT-qPCR, but not with the qPCR assay counterpart. On average, two human-specific Bacteroidales markers were not detected when using DNA in 12% of the samples, while they were positive for all samples when using RNA (cDNA) as the template. Moreover, signal intensity was up to three orders of magnitude higher in RT-qPCR assays than in qPCR assays. The human-specific Bacteroidales markers exhibited moderate correlation with conventional fecal indicators using RT-qPCR results, suggesting the persistence of nonhuman sources of fecal pollution or the presence of false-positive signals. In general, the results from this study suggest that RNA-based assays can increase the detection sensitivity of fecal bacteria in urban watersheds impacted with human fecal sources. Sewage overflows and stormwater runoff introduce high levels of fecal bacteria into surface waters and are considered the primary cause of water quality impairments in urban watersheds, particularly those affected by combined sewer overflows (CSOs) (1, 2). Sewage contamination of surface waters poses a serious risk to human and environmental health via waterborne disease outbreaks (3-5), deterioration of recreational and drinking water quality (6, 7), and degradation of aquatic ecology (8, 9). Hence, identifying the primary source(s) of fecal contamination is imperative to enable best management practices for mitigating pollution and public health risks.Microbial source tracking (MST) methods targeting fecal bacteria have recently been used to identify the sources of fecal contamination impacting water systems (10, 11). Many of these MST methods are based on quantitative PCR (qPCR) assays targeting the bacterial rRNA genes present within water DNA extracts (12, 13). However, the value of DNA-based monitoring in microbial ecology studies is limited by the possibility of DNA being associated with dead cells or the extent to which "naked DNA" may survive in the environment once bacteria are lysed (14-16). These facts pose a significant challenge to the environmental fate and transport of fecal bacteria which are often assess...

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