Multi-tiered approach utilizing microbial source tracking and human associated-IMS/ATP for surveillance of human fecal contamination in Baja California, Mexico

Highlights: • Sources of fecal pollution to surface waters included partially treated sewage from inadequate treatment. • The Inv-IMS/ATP assay can yield measurements of viable B. theta that are comparable to qPCR. • A strong association was observed between Inv-IMS/ATP assay and HF183/B. theta marker levels. • The Inv-IMS/ATP assay is an important tool that could be used by local water agencies/citizen scientists. • Inv-IMS/ATP samples scoring above a threshold could trigger additional testing of supernatant via qPCR. As both the need for reuse of reclaimed wastewater and the burden placed on existing wastewater treatment plants increase, so does the need for methods that can reliably, rapidly and economically identify human-associated contamination. A survey of surface water quality was conducted in Baja California, Mexico where inadequate infrastructure or its inefficient operation leads to poor water quality. The HF183 and Bacteroides thetaiotaomicron (B. theta) human-associated gene markers were detected in 84% and 82% of samples collected during dry weather, illustrating evidence of widespread human fecal contamination. In addition, an inversely-coupled (Inv-IMS/ATP) viability-based assay for detection of B. theta was developed and applied for rapid detection and screening of human-associated fecal contamination. The Inv-IMS/ATP assay was able to effectively differentiate between surface waters impacted with human fecal contamination, and B. theta levels measured by Inv-IMS/ATP were highly correlated with HF183 and B. theta human marker measurements (r = 0.76; r = 0.82) in complex surface water samples. In areas with widespread human fecal contamination and limited access to more expensive methods, a multi-pronged approach utilizing a combination of methods including the Inv-IMS/ATP assay for rapid evaluation and screening of surface water quality alongside human-associated genetic markers may improve risk assessment and surveillance capabilities.