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How to Check for Coliform Bacteria

Most coliform bacteria is not harmful to humans. Water samples are tested for coliforms because the presence of high levels of coliforms indicate presence of other harmful water-borne pathogens. All U.S. public water supplies require regular testing for coliform bacteria as proof of safe water conditions. Water systems failing coliform bacteria tests trigger mandatory public safety actions such as issuing "boil water" notices, as required by the federal Safe Drinking Water Act (SDWA). Coliform tests consist of adding a nutrient mix to a water sample and incubating to bring coliform bacteria to a visible quantity or density.

Things You'll Need

  • Coliform Detection Kit (for this article, we'll use the IDEXX Colilert Test Kit)
  • 100 mL sample of water for test (collected in a sterile container)
  • Sterile, clear non-fluorescing test vessel with secure cap
  • 6 Watt, 365 nm Ultra-Violet light
  • Oven or incubator capable of holding temperature set point +/- 0.5C
  • Clock
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Instructions

    • 1

      Pour a 100 mL sample of water to be tested into the sterile test vessel.

    • 2

      Add the contents of one pack of test nutrient to the sample. Cap it securely and shake to mix the nutrient into the sample.

    • 3

      Put the test vessel into the incubator with the temperature set for 35 +/- 0.5 degrees C. Incubate for 24 hours.

    • 4

      After 24-hour incubation, remove the test vessel and compare the sample color with the sample color chart included in the Colilert Kit. If the sample is less yellow than the comparison chart, the test is negative for total coliforms. If the sample's yellow color is equal to or greater than the comparison chart, the test is positive for total coliforms.

    • 5

      Turn on the Ultra-Violet lamp and hold within 5 inches of the sample vessel. Compare any fluorescence in the sample to the comparison chart, yellow color and fluorescence equal to or greater than the chart is a Positive for E. Coli as well.

    • 6

      Clean and sterilize the sample collection bottle and test vessel before further use.

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