Correlative assessment of fecal indicators using human mitochondrial DNA as a direct marker

Identifying the source of surface water fecal contamination is paramount to mitigating pollution and risk to human health. Fecal bacteria such as E. coli have been staple indicator organisms for over a century, however there remains uncertainty with E. coli-based metrics since these bacteria are abundant in the environment. The relationships between the presence of direct indicator of human waste (human mitochondrial DNA), human-specific Bacteroidales, and E. coli were studied for water samples taken from an urban creek system (Duck Creek Watershed, Cincinnati, OH) impacted by combined sewer overflows. Logistic regression analysis shows that human-specific Bacteroidales correlates much more closely to human mitochondrial DNA (R = 0.62) relative to E. coli (R = 0.33). We also examine the speciation of Bacteroidales within the Duck Creek Watershed using next-generation sequencing technology (Ion Torrent) and show the most numerous populations to be associated with sewage. Here we demonstrate that human-specific Bacteroidales closely follow the dynamics of human mitochondrial DNA concentration changes, indicating that these obligate anaerobes are more accurate than E. coli for fecal source tracking, lending further support to risk overestimation using coliforms, especially fecal coliforms and E. coli.