In general, bacteria are single-celled microorganisms with no membrane around the nucleus and a single strand of DNA. Coliform bacteria are a group of related species sharing several common characteristics. Coliform bacteria are rod-shaped and utilize lactose for food. Coliform bacteria are found throughout the environment in different ecological niches. Coliform bacteria can occur naturally in soil and water. They can also be found in human and animal waste.

Fecal coliform is a type of coliform bacteria most commonly found in animal intestines and feces. Therefore, the presence of these bacteria may indicate fecal contamination. However, fecal coliform bacteria are not necessarily associated with feces. Some are associated with textile, pulp and paper mill wastes (e.g. klebsiella). Escherichia Coli (E. coli) is a type of fecal coliform bacteria that occurs only in the feces of warm-blooded animals and is highly correlated with the presence of human pathogens. Historically, bacteria monitors have tested for both fecal coliform and E. coli bacteria.

In freshwater E. coli is now favored for detection because it is only found in the digestive tract of warm-blooded animals, whereas, fecal coliform can be found in non-fecal material. Human sewage and agricultural waste are the two primary contributors of fecal coliform and E. coli bacteria. Human sewage enters the water from poorly drained or poorly maintained sanitary sewer systems and wastewater treatment plants. Human sewage can also contaminate water through leaky septic systems and “straight pipes.” There are various avenues for agricultural waste to enter the water. One of the main pathways is through “direct deposit” by farm animals. Other pathways include improper manure application and poorly constructed and/or maintained waste lagoons.

Many people wonder: Why not monitor for pathogens directly rather than bacteria? It is impractical to monitor for pathogens because there are too many pathogens. In addition, it is labor intensive, expensive and time consuming.

Instead, bacteria monitoring is substituted and is used as an indicator of harmful pathogens.

Enterococcus is another group of bacteria, though not a coliform, is also found in the intestinal tract of warm-blooded animals. Unlike E. coli, these bacteria are salt tolerant and are used as an indicator of human pathogens in salt water. Enterococcus is used as the best indicator of health risk for humans engaged in

marine recreational activities.

It is important to remember that not all bacteria are bad, and many have important roles in the aquatic ecosystem. Bacteria assist in decomposition (break down of plant and animal remains which releases nutrients back into the food web) and are a source of food for other organisms.

All the above information was quoted from the Alliance for the Cheaspeake Bay's "RiverTrends Volunteer Water Quality Monitoring Program Manual" ©2007. 
http://allianceforthebay.org/wordpress/wp-content/uploads/2010/08/RiverTrends-Volunteer_Water_Quality_Monitoring_Manual.pdf

Map of Bacteria Impairments in Virginia, 2008

FOR's 2010 Hazel Run E. coli Study Report Card