Coliform/E. coli results in 24 Hours

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Simultaneously detects or quantifies both total coliforms and Escherichia coli. Results in 24 hours. 


Which IDEXX coliform/E.coli test is right for you? View a comparison.



Call 1-800-321-0207 or email

Colilert: The standard for confident outcomes




  • U.S. EPA-approved, 24-hour test for drinking and source waters.
  • International approvals include AOAC, IBWA and EBWA.
  • Millions of tests performed worldwide every year.


  • Ease of use simplifies training.
  • Unit-dosed packaging eliminates media preparation.
  • No repeat testing due to clogged filters or heterotrophic interference.
  • Quality Control (QC) procedure can be done in 15 minutes.


  • Less than 1 minute of hands-on time.
  • Detects coliforms and E. coli simultaneously in 24 hours or less.
  • No confirmations needed.
  • No glassware cleaning or colony counting.


  • Identifies E. coli specifically, eliminating unnecessary public notification due to non-target organisms.
  • Suppresses up to 2 million heterotrophs per 100 mL and 5 million per 250 mL.
  • Eliminates the subjective interpretation found in traditional methods.
  • Detects a single viable coliform or E. coli per sample.


  • Minimizes evening and weekend work.
  • Up to 12-month shelf life at room temperature.


Resource List
Product Information

Colilert® (20-test pack)
Catalog Number: WP020I

Colilert® (100-test pack)
Catalog Number: WP100I

Colilert® (200-test pack)
Catalog Number: WP200I

Predispensed Colilert® (100-test pack)
Catalog Number: W100I

Predispensed Colilert® (200-test pack)
Catalog Number: W200I


Combo packs:

Snap packs and Quanti-Trays together.

How Colilert® works


Colilert uses the patented Defined Substrate Technology® (DST®) to simultaneously detect total coliforms and E. coli. Two nutrient-indicators, ONPG and MUG, are the major sources of carbon in Colilert and can be metabolized by the coliform enzyme β-galactosidase and the E. coli enzyme β-glucuronidase, respectively.


As coliforms grow in Colilert, they use β-galactosidase to metabolize ONPG and change it from colorless to yellow.



E. coli use β-glucuronidase to metabolize MUG and create fluorescence. Since most non-coliforms do not have these enzymes, they are unable to grow and interfere.



The few non-coliforms that do have these enzymes are selectively suppressed by Colilert's specifically formulated matrix.


This approach is different from traditional media, which provide a nutrient-rich environment that supports the growth of both target organisms and nontargets. When nontargets grow and mimic target organisms, false positives occur. Growth of nontargets can also suppress target organisms and give false negatives in traditional media. To suppress nontargets, traditional media often include high levels of salts, detergents, or other selective agents that may inadvertently suppress target organisms and give further false negatives.


All IDEXX DST reagents can be used in a presence/absence (P/A) vessel, Quanti-Tray®, or Quanti-Tray®/2000. Only IDEXX reagents are approved for use with Quanti-Tray and Quanti-Tray/2000.


View/Print Colilert Procedure 

Learn how to use Colilert




A step-by-step guide to using Colilert

Presence / Absence (P/A)

Step 1

Add reagent to sample and incubate at 35±0.5°C for 24 hours.


Step 1

Add reagent to sample.


Step 2

Read results:

  • Colorless = negative
  • Yellow = total coliforms
  • Yellow/fluorescent = E. coli

Step 2

Pour into Quanti-Tray® (counts from 1–200) or Quanti-Tray®/2000 (counts from 1–2,419)


Step 3

Seal in Quanti-Tray® Sealer and place in 35±0.5°C incubator for 24 hours (in other countries, the temperature requirement may be different per regulatory requirements).

View the fecal coliform procedure.


Step 4

Quanti-Tray—Read results:

  • Yellow wells = total coliforms
  • Yellow/fluorescent wells = E. coli
  • Count positive wells and refer to MPN table

Quanti-Tray/2000—Read results:

  • Yellow wells = total coliforms
  • Yellow/fluorescent wells = E. coli
  • Count positive wells and refer to MPN table

Frequently asked questions and answers


What are the recommended storage conditions?

Store at 2-30°C and away from light.


What is the Colilert comparator?

The Colilert comparator is a liquid color and fluourescent reference. Its purpose is to assist in distinguishing a minimal positive from a negative test result.


How do you use the Colilert comparator?

At 24–28 hours, any yellow color or fluorescence equal to or greater than the comparator is considered a positive test result.


What should the color and consistency of the Colilert powder be?

The Colilert reagent powder should be white to off-white in color; it should be dry and free-flowing. Any concerns about the color or integrity of the reagent powder should be directed to IDEXX Technical Support at +1 800 321 0207 or via email at


Can Colilert samples be read after 24 hours?

Yes, if they are negative. If an inoculated Colilert sample is inadvertently incubated over 28 hours, lack of yellow color is a valid negative test; a yellow color after the 28 hours is not valid and should be repeated or verified.


Can Colilert samples be read before 24 hours?

Yes, if they are positive. An inoculated Colilert sample that is positive for coliforms and E. coli before 24 hours is a valid positive test for both coliforms and E. coli. If the sample is positive before 24 hours for coliforms only, the sample is a valid positive test for coliforms. However, the sample should be incubated for a longer period (up to 24 hours, but not greater than 28 hours total time) to determine whether E. coli are present.


Why is the test limited to a 24- to 28-hour incubation?

The reagent system was designed to suppress non-coliform ONPG or MUG positive bacteria for at least 28 hours. If incubation extends past 28 hours, it is possible that such bacteria will overcome the Colilert suppression system and react with ONPG or MUG. When incubation exceeds 28 hours, only negative results are valid. Positive results must be confirmed by classical methods.


Can I quantify my Colilert results?

You can quantify Colilert with Quanti-Tray®, Quanti-Tray®/2000 or any multiple-tube format.


What are the disposal requirements for a Colilert sample?

Check with your local, state and/or federal authorities for proper disposal of bacteriological biohazard materials at your facility.


Is Colilert approved by the U.S. EPA?

Yes. Colilert is U.S. Environmental Protection Agency (EPA)-approved as a method for total coliform and E. coli detection in drinking water and for E. coli detection in the following types of waters:

  • Ambient waters
  • Surface water
  • Ground water
  • Wastewater


What is the detection limit of Colilert?

Colilert detects total coliforms and E. coli at 1 organism/100 mL.


What is the shelf life of Colilert?

Colilert shelf life is up to 12 months from the date of manufacture for the snap pack formats (100 mL) and up to 15 months from the date of manufacture for glass-tube formats (10 mL).


Colilert: Everything you need for confident outcomes


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