How to Interpret the Presence of Coliform Bacteria

Coliform bacteria are naturally present in the digestive tracts of people and animals, according to the New York State Department of Health website. These bacteria are found in waste and are found naturally in soil and on plants. Not all coliforms are dangerous and not all live strictly in fecal matter. But some bacteria like Escherichia coli, or E. coli, live only in feces and are indicators of fecal pollution. To interpret the presence of Coliform bacteria, you'll need to perform simple water tests and understand the results by identifying bacteria colonies.

Things You'll Need

• Sterile calibrated dropper
• Liquid Coliscan medium
• Ice
• Petri dish
• Magnifying glass

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Instructions

Test Water For Coliform Bacteria

• 1

  Coliform bacteria is found naturally on plants and in water.

  Collect a water sample using a sterile vessel, to prevent cross-contamination. The amount of water you need varies, depending on the use of the water you're testing. According to a testing guide produced by Indiana University, you should test 100 milliliters of drinking water, five milliliters of river water and one milliliter or less of raw sewage.

• 2

  Transfer water, in 1 mL doses, immediately to a petri dish containing Coliscan medium. If you're testing outside of a lab, mix the 1 ml water and Coliscan medium, and then put it on ice until you return to the lab.

• 3

  Incubate your sealed petri dish for 24 to 48 hours in a warm place. The temperature should stay around 90 degrees.

Interpreting Test Results

• 4

  Count bacteria colonies using a magnifying glass; note their color and dispersal. Colonies that are unevenly dispersed in a dish may mean improper sealing. Those dishes are unusable. Your results should read like: Dish A: Total 21, 6 Red, 5 Purple,10 Grey.

• 5

  Find the TC (total coliform) and FC (fecal coliform) of each petri dish. According to the Indiana University testing guide, red and purple bacteria colonies in your petri dish are coliform bacteria. The number of red and purple colonies combined is your TC. Purple colonies are specifically fecal-dwelling bacteria, or FC. Other colors, such as grey, are not coliform bacteria. Disregard them.

• 6

  Pools can contain enough fecal coliform bacteria to grow 200 colonies out of one mL sample of water.

  Compare your results to coliform standards. According the Indiana University guide, the standard allowable colonies of coliform bacteria are:

  Drinking Water: 1 TC

  Total Body Contact Water (such as in a swimming pool): 200 FC

  Partial Body Contact Water (such as a river used for canoeing): 1000 FC

  Treated Sewage: 200 FC

• 7

  Understand your results. If more than 1 colony grows in your drinking water sample, you are at increased risk for disease. If colonies growing in your drinking water are red, your drinking water is has certainly been contaminated with feces. However, if 950 colonies are growing in a dish of water from a river where you go boating, you're still safe.