Colilert 250

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  • Simultaneously detects both total coliforms and Escherichia coli in a 250 mL format.
  • The Colilert 250 Test provides coliform/E. coli detection with confirmed results in 24 hours in a 250-mL format.
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  • 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 glassware cleaning or colony counting.
  • Suppresses up to 5 million per 250 mL.
  • Eliminates the subjective interpretation found in traditional methods.
  • Detects a single viable coliform or E. coli per sample.
  • Minimises evening and weekend work.
  • Up to 12-month shelf life at room temperature.


The Colilert 250 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 the Colilert 250 Test and can be metabolised by the coliform enzyme β-galactosidase and the E. coli enzyme β-glucuronidase, respectively.

As coliforms grow in the Colilert 250 Test, 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 250 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—which may inadvertently suppress target organisms and give further false negatives.

How to Use

Learn how to use the Colilert 250 Test

File Icon View/Print Colilert 250 Test Procedure

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

Step-by-step guides to using the Colilert 250 test


Step 1

Add reagent to sample and incubate at 36°C ± 2°C for 24 hours.

Step 2

Read results:
Colourless = negative
Yellow = total coliforms
Yellow/fluorescent = E. coli


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 fluorescence 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 250 reagent powder be?
The Colilert 250 reagent powder should be should be white to off-white in colour; it should be dry and free-flowing. Any concerns about the colour or integrity of the powder should be directed to IDEXX Technical Support.

Can Colilert 250 samples be read after 24 hours?
Yes, if they are negative. If an inoculated Colilert 250 Test 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 Colilert 250 samples be read before 24 hours?
Yes, if they are positive. An inoculated Colilert 250 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 read window 24–28 hours?
The reagent system was designed to suppress noncoliform 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 250 suppression system and react with ONPG or MUG. When incubation exceeds 28 hours, only negative results are valid. Positive results must be confirmed or revalidated and resampled.

Can I quantify my Colilert 250 test results?
No, the Colilert 250 Test works only with 250 mL of sample, so it cannot be used with the Quanti-Tray or Quanti-Tray/2000, which require 100 mL of sample.

What are the disposal requirements for a Colilert 250 sample?
Check with your local authority for proper disposal of bacteriological biohazard materials at your facility.

What is the detection limit of the Colilert 250 test?
The Colilert 250 test detects total coliforms and E. coli at 1 organism/250 mL.

What is the shelf life of the Colilert 250 test?
The shelf life of the Colilert 250 test is up to 12 months from the date of manufacture.

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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 250 Test (20-test pack)
Catalogue Number: WP250-20I

Colilert 250 Test (100-test pack)
Catalogue Number: WP250-100I

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