Understanding E. coli: What It Is and Why It Matters 


When you hear the term E. coli, if you’re like most people, your mind probably goes to thoughts of food poisoning and contaminated lettuce. But did you know this tiny bacterium also plays a significant role in the health of our rivers and streams?

Let’s dive into what E. coli is, how it impacts water quality, and learn more about these tiny rod-shaped bacteria that make a big splash!

What is E. coli?

E. coli, short for Escherichia coli, is a type of bacteria that naturally lives in the intestines of humans and animals. While most strains are harmless and even help keep our guts healthy, some can cause serious illnesses when we come into direct contact with these organisms, typically through contaminated food or water.  

Escherichia coli has dimensions like those of a cylinder. It’s typically around 1.0-2.0 micrometers long, with a radius about 0.5 micrometers.

E. coli in Rivers and Streams

E. coli can make its way into rivers and streams through various routes. When we have heavy rainfall, stormwater runoff from agricultural lands or sewage overflows can introduce these bacteria into our rivers and streams. Even droppings from wildlife (or pets) can help add to dangerous E. coli levels! Not picking up after your pets can play a major role in how E. coli gets into our rivers.

Did you know that a single gram of pet waste contains an average of 23 million fecal coliform bacteria, including E.coli? This waste is often picked up by stormwater runoff and carried into drains that lead to rivers and streams.

E. coli is known as an indicator species. It typically doesn’t grow and reproduce in the environment, so its presence indicates that there is contamination from fecal sources. When E. coli is found in rivers and streams, it signals potential health risks for anyone who comes into contact with the water. Its presence suggests that other harmful bacteria, viruses, and pollutants may also be present.

Factors Influencing E. coli Levels

Several factors can affect E. coli levels in water: 

  • Rainfall and Runoff: Heavy rains can wash fecal matter from land into water bodies from animal waste and even fertilizers.
  • Sewage Overflows: Malfunctioning or overloaded sewage systems can spill untreated waste into rivers and streams.
  • Agricultural Practices: Runoff from farms, especially those using manure, can carry E. coli into nearby water
  • Wildlife and Domestic Animals: Animal feces, whether from wildlife or livestock, can contribute to E. coli levels, especially in rural areas.
  • Leaking Septic Systems: Septic systems that are not regularly cleaned out and maintained properly can result in fecal matter and bacteria seeping into nearby waterways. 

The Impact of E. coli on Water Quality

Elevated E. coli levels indicate poor water quality for our rivers and streams. Because the E. coli is an indication of other pollutants and pathogens in the water, it affects both humans and wildlife who rely on these streams for recreation and life-sustaining resources.  

Poor water quality and contamination can lead to gastrointestinal illnesses for people who swim or play in the water, causing symptoms like diarrhea, stomach cramps, and vomiting. Similarly, pets can become ill, as they swallow the water from the streams while playing or licking their fur after getting wet. 

What do the TN Water Watch status colors and E. coli counts mean for my health?

When using TN Water Watch, you’ll notice the access points are color-coded. The status colors (green, yellow, orange, and red) incorporated in our TN Water Watch tool correspond to “risk”.

The U.S. Environmental Protection Agency recommends a Recreational Water Quality Criteria of 235 colony forming units (CFUs)/100 mL. At this water quality criteria, the EPA estimates an illness rate of 36 per 1,000 people that come into direct contact (swimming, boating, wading, etc.) with that body of water. 

  • Green represents values of E. coli from 0 – 234 CFUs/100 mL, corresponding to all locations with less E. coli than the EPA recommendation.  
  • Yellow represents values of E. coli from 235 – 350 CFUs/100 mL, slightly above the EPA recommended “safe” value.  
  • Orange represents a more risky values of E. coli from 351 – 750 CFUs/100 mL  
  • Red represents values of E. coli greater than 751 CFUs/100 mL.  

*Note: In certain circumstances, E. coli values can be as high as several thousand CFUs/100 mL. Our current scale does not differentiate between 800 CFUs/100 mL and 2,000 CFUs/100 mL as both results would be “red” however, the location with 2,000 CFUs/100 mL would be more risky due to the higher levels of E. coli.

Feel free to check the actual predicted values of E. coli by clicking on your location of choice. These values will offer more information about risk than the status colors. Status colors should be used as a “quick glance” metric.