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Enterococcus and Escherichia coli fecal source apportionment with microbial source tracking genetic markers - is it feasible?
Wang, D., A. H. Farnleitner, K. G. Field, H. C. Green, O. C. Shanks, AND A. B. Boehm. Enterococcus and Escherichia coli fecal source apportionment with microbial source tracking genetic markers - is it feasible? WATER RESEARCH. Elsevier Science Ltd, New York, NY, 47(18):6849-6861, (2013).
To inform the public on matters involving the use of genetic markers to determine sources of fecal pollution in environmental waters.
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 host-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 a method that utilizes the ratios of fecal host-associated genetic markers and culture and molecular measurements of general fecal indicators to apportion enterococci, E. coli, and Bacteroidales. The source contribution is approximately equal to the ratio of the host-associated and the general fecal indicator concentrations in a water sample divided by the 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. Although source apportionment works well with these artificial samples, application to aged samples 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 host-associated markers and as a result cannot be apportioned using this method. However, limited data from a previous study suggest a similar decay rate between host-associated and QPCR-measured Enterococcus, E. coli and Bacteroidales genetic markers suggesting that apportionment may be possible for these organisms, however further work is needed to confirm.