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Interpreting Coliform Bacteria Test Results


The Total Coliform Bacteria test is the standard microbiological test of the sanitary quality of drinking water. The EPA has stated that good drinking water should not contain any Coliform bacteria.    

There are primarily 18 different bacteria which make up the group known as “Coliforms”. In most cases, Coliform bacteria are not harmful. However, if these bacteria are found in your water supply, this indicates that other disease causing bacteria may enter through the same pathway and be present in your drinking water.    

If Coliform bacteria are found, the water supply is considered a potential health hazard and is classified as “UNSAFE” for human consumption. This test DOES NOT indicate if the water is chemically safe to drink.    

E. COLI – One of the Coliform Bacteria    

E. Coli is one of the approximate 18 members of Coliform group. The standard Coliform Bacteria test tells the difference between coliforms of fecal origin, specifically E. coli (from the intestines of warm-blooded animals) and coliforms from other sources. Drinking water contaminated with E. coli is considered an “EXTREME HEALTH HAZARD”.    

The test results are reported as “PRESENT” or “ABSENT”    

“ABSENT” – Good news, it means that Coliform bacteria were NOT in the test sample. In other words it “PASSED” the test.    

“PRESENT” – Means that Coliform Bacteria were found in the sample and it “FAILED” the test. If Coliforms were found in the sample then E.coli (as a member of the Coliform Group) is also reported as “Present” or “Absent”.    

Coliforms “Present” and E. coli “Absent” – Very Unsafe – Potential Health Hazard.    

Coliforms “Present” and E. coli “Present” – “EXTREME HEALTH HAZARD”.    

If your water system fails, the first step is usually to perform a “batch chlorination” on your drinking water supply. NRC will send written instructions for this procedure if your test results indicate a failure. After the chlorination procedure has been performed, use your water supply for a minimum of 5-7 days and retest. Until you have a test which shows that the bacteria are ABSENT, exercise caution with this system. It is recommended that you test for Coliform bacteria at least every six months.