Synergy between quantitative microbial source tracking (qMST) and quantitative microbial risk assessment (QMRA): A review and prospectus.

The use of microbial source tracking (MST) marker genes has grown in recent years due to the need to attribute point and non-point fecal contamination to specific sources. Quantitative microbial risk assessment (QMRA) is a modeling approach used to estimate health risks from exposure to feces-contaminated water and associated pathogens. A combination of these approaches [quantitative MST (qMST) and QMRA] can provide additional pathogen-related information for prioritizing and addressing health risks, compared to reliance on conventional fecal indicator bacteria (FIB). To inform expansion of this approach, a review of published qMST-QMRA studies was conducted to summarize the state of the science and to identify research needs. The reviewed studies primarily aimed to identify what levels of MST marker genes in hypothetical recreational waterbodies would exceed the United States Environmental Protection Agency (USEPA) risk benchmarks for primary contact recreators. The QMRA models calculated relationships between MST marker gene(s) and reference pathogens based on published data in the literature. The development of a robust, accurate relationship was identified as an urgent research gap for qMST-QMRA. This metric requires additional knowledge to quantify the relationship between MST marker genes and the degree of variability in decay of pathogens as a dynamic function of environmental conditions and combinations of fecal sources at multiple spatial and temporal scales. Improved characterization of host shedding rates of host-associated microorganisms (i.e., MST marker genes), as well as fate and transport of these microorganisms and their nucleic acids, would facilitate expansion of this approach to other exposure pathways. Incorporation of information regarding the recovery efficiency, and host-specificity of MST marker genes into QMRA model parameters, and the sensitivity analysis, would greatly improve risk management and site-specific water monitoring criteria.

Evaluation of the novel crAssphage marker for sewage pollution tracking in storm drain outfalls in Tampa, Florida.

CrAssphage are recently-discovered DNA bacteriophages that are prevalent and abundant in human feces and sewage. We assessed the performance characteristics of a crAssphage quantitative PCR (qPCR) assay for quantifying sewage impacts in stormwater and surface water in subtropical Tampa, Florida. The mean concentrations of crAssphage in untreated sewage ranged from 9.08 to 9.98 log10 gene copies/L. Specificity was 0.927 against 83 non-human fecal reference samples and the sensitivity was 1.0. Cross-reactivity was observed in DNA extracted from soiled poultry litter but the concentrations were substantially lower than untreated sewage. The presence of the crAssphage marker was monitored in water samples from storm drain outfalls during dry and wet weather conditions in Tampa, Florida. In dry weather conditions, 41.6% of storm drain outfalls samples were positive for the crAssphage marker and the concentrations ranged from 3.60 to 4.65 log10 gene copies/L of water. After a significant rain event, 66.6% of stormwater outlet samples were positive for the crAssphage marker and the concentration ranged from 3.62 to 4.91 log10 gene copies/L of water. The presence of the most commonly used Bacteroides HF183 marker in storm drain outfalls was also tested along with the crAssphage. Thirteen samples (55%) were either positive (i.e., both markers were present) or negative (i.e., both markers were absent) for both the markers. Due to the observed cross-reactivity of this marker with DNA extracted from poultry litter samples, it is recommended that this marker should be used in conjunction with additional markers such as HF183. Our data indicate that the crAssphage marker is highly sensitive to sewage, is adequately specific, and will be a valuable addition to the MST toolbox.