E. coli Frequently Asked Questions

What is E. coli? 

Escherichia coli (E. coli) is a bacteria that is commonly found in the gastrointestinal tract and feces of warm-blooded animals and humans. According to the U.S. EPA, E. coli is the best indicator of the presence of pathogens in surface waters and its presence provides direct evidence of fecal contamination of the water. 


How are the E. coli results interpreted to determine if they are acceptable? 

EPA recommends an E. coli recreational safety level for primary contact of a geometric average of 126 cfu/100ml or less. Because it is common to find high bacteria counts in urban areas, Georgia Adopt-A-Stream advises that counts that exceed a 1000 cfu/100 ml threshold may warrant special attention. NWW results that show a “high” bacterial count may be a one-time event or occurrence or may be a part of a chronic problem. This information is useful, but before taking further action additional sampling is necessary to document bacterial levels over a long period to determine seasonal fluctuations and in response to rain events. This long term information is vital in assessing the overall health of the stream and determining if special attention is warranted to investigate potential pollution sources.

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What do the E. coli results tell us about safety? 

The most recent recommendations developed by the US EPA for recreational primary contact state that surface waters should not exceed an E. coli level of 126 cfu/100mL on an average. Primary contact recreation includes swimming, bathing, diving, surfing, water skiing, tubing, water play by children, and similar water contact activities where a high degree of bodily contact with the water and immersion and ingestion are likely. Studies conducted by the EPA indicate that for surface waters with an average E. coli level of 126 cfu/100mL, the estimated rate of illness is 36/1000 for people having primary contact.

Bacteria are a natural component of all rivers, lakes and streams and most bacteria are harmless, however, certain species of bacteria in groups such as Campylobacter, Salmonella, and Shigella can cause sickness in humans. The most common side effects of these sicknesses may include a short period of vomiting and/or acute diarrhea but can also cause much more serious complications in rarer circumstances. Those most at risk for the serious side effects associated with these waterborne diseases are the very young, old, and immuno-compromised. 


What is an indicator organism and why do we measure indicators instead of pathogens? 

A microbial indicator organism is a single celled organism that is usually harmless, found in high numbers, and originates from the same sources as the pathogens whose numbers you are interested in obtaining. Monitoring for indicator organisms is usually much easier and more cost effective than actually monitoring for the disease causing organisms. Pathogens are disease-causing organisms such as some types of bacteria, viruses, and protozoans and typically occur in very small concentrations, so routinely monitoring for these pathogens is comparable to looking for a needle in a haystack. Field sampling and laboratory methods for measuring pathogens are very time consuming, expensive, and sometimes put the analyst at more risk for being infected by the pathogen.

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How do waters become contaminated? 

Many of the rivers, streams, and lakes throughout the Chattahoochee Watershed are vulnerable to increased bacterial contamination especially during heavy rains which can convey bacteria from various sources to susceptible waterways. Common sources of bacteria and other pollutants include cracked and overflowing septic systems, leaking septic systems, failing infrastructure, agriculture, wildlife, and pet waste.

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What makes bacteria levels go up and down? 

Levels of bacteria in a river or stream can be influenced by many factors. The most common causes for elevated levels of bacteria are due to runoff associated with storms, however, leaking sewage lines or spills from treatment plants can also cause levels to increase. E. coli concentrations are typically higher (1) during and after rain events, (2) on tributary streams than in the Chattahoochee River, (3) during the summer season than during the winter months, and (4) at night rather than during daylight hours.

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How is E. coli measured? 

There are several ways of measuring numbers of E. coli bacteria in streams. The method used to determine the numbers reported on CRK’s web site involves a relatively new method recently approved by the EPA called the IDEXX Colilert-18 system. It involves the collection of water from a stream or river using sterile equipment and adding a chemical called MUG. This mixture is then poured into a container that separates the sample water into equal-sized compartments. The container is sealed and incubated for 18-22 hours. The sample water in the compartments containing coliform bacteria and E. coli will change color. The number of compartments which turn yellow indicates the most probable number (MPN, which is a similar value to CFU) of total coliform bacteria in the stream while the number compartments which fluoresce under ultraviolet light indicates the number of E. coli present in the stream at the time that it was sampled.

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Are the fish safe to eat? 

The presence of E. coli in waters does not directly affect fish consumption, however, when fishing it is often difficult to avoid some contact with the water. Any primary contact with water that has elevated E. coli levels increases the likelihood that a person may be exposed to or accidentally ingest water from the river or stream. This is especially true if the person has open cuts or wounds or if eating or drinking is involved. Fish consumption advisories are issued by the Georgia Department of Natural Resources (GA DNR) and are based on an analysis of chemical and metal concentrations in fish tissue.  Click here for the most updated guidelines for eating fish from Georgia Waters (2015).

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