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  • The Colilert test simultaneously detects or quantifies both total coliforms and Escherichia coli, with results in 24 hours.
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  • Approved for use in the United States and approved or accepted in more than 20 countries around the world.
  • 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.
  • 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.


The Colilert Test uses proprietary 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 metabolised by the coliform enzyme β-galactosidase and the E. coli enzyme β-glucuronidase, respectively.

As coliforms grow in the Colilert medium, they use β-galactosidase to metabolise ONPG and change it from colourless to yellow.

E. coli use β-glucuronidase to metabolise 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 the Colilert Test'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.

Only IDEXX reagents have been validated and approved for use with Quanti-Tray and Quanti-Tray/2000.

How to Use

Learn how to use the Colilert test

File Icon View/Print Colilert Test Procedure

Learn how to use Colilert
Learn how to use the Colilert test

Step-by-step guides to using the Colilert test


Step 1

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

Step 2

Read results:

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

Step 1

Add reagent to sample.

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°C ± 0.5°C incubator for 24 hours (in other countries, the temperature requirement may be different per regulatory requirements).

Step 4: Quanti-Tray

—Read results:

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

Step 4: Quanti-Tray/2000

—Read results:

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

Q & A

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 colour 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 colour or fluorescence equal to or greater than the comparator is considered a positive test result.

What should the colour and consistency of the Colilert reagent powder be?
The Colilert reagent powder should be white to off-white in colour; it should be dry and free-flowing. Any concerns about the colour or integrity of the reagent powder should be directed to IDEXX Technical Services.

Can the 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 colour is a valid negative test; a yellow colour after the 28 hours is not valid and should be repeated or verified.

Can the 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 test 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 test results?
You can quantify the Colilert test 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.

What is the detection limit of the Colilert test?
The Colilert test detects total coliforms and E. coli at 1 organism/100 mL.

What is the shelf life of the Colilert test?
The shelf life of the Colilert test is up to 12 months from the date of manufacture for the snap pack formats (100 mL).

Test & Accessories

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Resources & Product Information


IDEXX Water has reference materials and approval documents to support the many products in our water portfolio.

Product Information

Colilert Test (20-test pack)
Catalogue Number: WP020I

Colilert Test (100-test pack)
Catalogue Number: WP100I

Colilert Test (200-test pack)
Catalogue Number: WP200I

10mL Colilert Test (100-test pack)
Catalogue Number: W100I

10mL Colilert Test (200-test pack)
Catalogue Number: W200I

IDEXX Water Customer Support

UK Headquarters

IDEXX Technologies Ltd Units 1B & 1C, Newmarket Business Park, Studlands Park Avenue, Newmarket, Suffolk, CB87ER

Tel: 01638 676800

ISO Certifications

File Icon  ISO 9001:2008

File Icon  ISO 17025:2005

File Icon  ISO 14001:2004