Bacterial Source Tracking
Bacterial Source Tracking (BST), also referred to as microbial source tracking, is a method that determines the sources of bacteria in a waterbody. For instance, BST may be able to tell if bacterial contamination is from human sources (septic system or sewer issues) vs. water fowl vs. cows on a nearby farm. In BST, bacteria in water samples are compared to bacteria from known sources, often forming a host origin library. There are two types of BST methods, phenotypic and genotypic. Phenotypic methods are based on characteristics expressed by fecal bacteria and genotypic methods are based on DNA sequences.
While fecal bacteria itself rarely causes human illness, it is used to indicate that
fecal contamination may have occurred and pathogens may be present (See the
URI Watershed Watch Fact Sheet: Bacterial Monitoring
for more information). When fecal bacteria counts in water samples reach a certain level
(determined by the state), the water body is considered impaired and a TMDL
(Total Maximum Daily Load)
may be initiated. One of the current difficulties with the TMDL process is uncertainty in
identifying specific sources of fecal bacteria in water bodies. BST methodology has been
described as having the ability to turn nonpoint sources into point sources to the degree
that it is possible to more accurately assign waste load allocations and develop a
scientifically-defensible TMDL.
BST techniques include
ribotyping
,
antibiotic resistance analysis, pulsed-field gel electrophoresis of DNA, cell wall fatty
acid analysis, and O-serogrouping. The use of BST techniques for fecal contamination source
location is currently developing and it is likely that several of the techniques would be
used during the course of a TMDL for screening and confirmation of source determination.
The links below provide further information on BST techniques, including E. coli ribotyping:
How Maine Sea Grant is using BST to investigate
non-point pollution in Webhannet and Little River Estuary watersheds
A summary of research being conducted at Cornell University using
anitobiotic resistant gene analysis to improve BST techniques
A Sea Grant newsletter
article
about
an EPA New England BST project on Mallets Bay and Lower Winooski River in Connecticut
Virginia Cooperative Extension: Microbial Source Tracking and the TMDL Process
An example
of how ribotyping is being used in TMDL development in Virginia.