Performance, Design, and Analysis in Microbial Source Tracking Studies

Abstract

Microbial source tracking (MST) includes a group of methodologies that are aimed at identifying, and in some cases quantifying, the dominant source(s) of fecal contamination in resource waters, including drinking, ground, recreational, and wildlife habitat waters. MST methods can be grouped into two major types. Library-dependent methods are culture based and rely on isolate-by-isolate typing of bacteria cultured from various fecal sources and from water samples These isolates are matched to their corresponding source categories by direct subtype matching (41, 70) or by statistical means (23, 37, 40, 41, 80, 83, 102). In contrast, library-independent methods frequently are based on sample-level detection of a specific, host-associated genetic marker in a DNA extract by PCR (6, 11, 26, 88). Analyses of certain chemicals associated with sewage, including fecal sterols (29, 30, 47), optical brighteners (29, 30, 68), and host mitochondrial DNA (67), have also been utilized for what can be more broadly termed fecal source tracking; however, in this review we compare the performance of only fecal source tracking studies in which the target(s) is microbial. Mounting pressure to determine the origin of nonpoint source fecal pollution, as exemplified by the U.S. Environmental Protection Agency's total maximum daily load program, has led to a steady increase in manuscript submissions and grant applications that include MST approaches. At the same time, resource managers concerned with water quality and regulatory pressures struggle with the choice of methodology in the face of requirements for immediate application. Although there has been significant progress in the MST field over the last 10 years, variability among performance measurements and validation approaches in laboratory and field studies has led to a body of literature that is very difficult to interpret, both for scientists and for end users (99). In this review, we first consider the development and validation of MST methods in a historical context to describe the lessons learned in early studies. Next, uniform performance characteristics are introduced to allow comparison of method performance across MST studies (Tables ​(Tables11 and ​and2),2), and this is followed by a discussion of considerations for field study design and implementation. TABLE 1. Performance statistics for tests in which results were based on isolate-by-isolate classification into the various known-source categoriesa TABLE 2. Performance statistics for tests in which MST methods were tested with reference samples to determine the ability or failure to detect the sole source of fecal contamination