title: Enterococcus and Escherichia coli fecal source apportionment with microbial source tracking genetic markers--is it feasible?
abstract: Fecal pollution is measured in surface waters using culture-based measurements of enterococci and Escherichia coli bacteria. Source apportionment of these two fecal indicator bacteria is an urgent need for prioritizing remediation efforts and quantifying health risks associated with source-specific pathogens. There are a number of quantitative real-time PCR (QPCR) assays that estimate concentrations of source-associated genetic markers; however, their concentrations are not necessarily amenable to source apportionment because the markers may differ in prevalence across sources. Here we mathematically derive and test, under ideal conditions, a method that utilizes the ratios of fecal source-associated genetic markers and culture and molecular measurements of general fecal indicators to apportion enterococci and E. coli. The source contribution is approximately equal to the ratio of the source-associated and the general fecal indicator concentrations in a water sample divided by their ratio in the source material, so long as cross-reactivity is negligible. We illustrate the utility of the ratio method using samples consisting of mixtures of various fecal pollution sources. The results from the ratio method correlated well with the actual source apportionment in artificial samples. However, aging of contamination can confound source allocation predictions. In particular, culturable enterococci and E. coli, the organisms presently regulated in the United States and much of the world, decay at different rates compared to source-associated markers and as a result cannot be apportioned using this method. However, limited data suggest a similar decay rate between source-associated and QPCR-measured Enterococcus and E. coli genetic markers, indicating that apportionment may be possible for these organisms; however further work is needed to confirm.
authors.affiliation: Environmental and Water Studies, Dept. Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA.
authors.forename: Dan
authors.initials: D
authors.lastname: Wang
authors.forename: Andreas H
authors.initials: AH
authors.lastname: Farnleitner
authors.forename: Katharine G
authors.initials: KG
authors.lastname: Field
authors.forename: Hyatt C
authors.initials: HC
authors.lastname: Green
authors.forename: Orin C
authors.initials: OC
authors.lastname: Shanks
authors.forename: Alexandria B
authors.initials: AB
authors.lastname: Boehm
mesh_terms: D016983:Enterococcus / Q000145:classification* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D004784:Environmental Monitoring / Q000379:methods*; D004926:Escherichia coli / Q000235:genetics / Q000302:isolation & purification* / Q000378:metabolism; D005243:Feces / Q000382:microbiology; D005819:Genetic Markers; D008954:Models, Biological; D060888:Real-Time Polymerase Chain Reaction / Q000379:methods*; D014871:Water Microbiology; D014876:Water Pollution / Q000032:analysis*
chemical_list: D005819:Genetic Markers
keywords: Fecal pollution; Microbial source tracking; Water quality