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Analytical Methods

Colilert method for total coliforms and Escherichia coli. Updated January 2007

Colilert is a commercially available enzyme-substrate liquid-broth medium (IDEXX Laboratories, Inc., Westbrook, Maine) that allows the simultaneous detection of total coliforms and Escherichia coli (E. coli).  It is available in the most-probable number (MPN) or the presence/absence (PA) format. The MPN method is facilitated by use of a specially designed disposable incubation tray called the Quanti-Tray®.  The PA method can be done using any sterile bottle of the appropriate size.  Both methods can be done in the field or laboratory.

Two enzyme substrates are included in Colilert—a chromogen that reacts with the enzyme found in total coliforms (galactosidase), and a fluorogen that reacts with an enzyme found in E. coli (glucuronidase).  After 24 hours incubation at 35oC, a total- coliform-positive reaction turns the medium yellow; an E. coli-positive reaction causes the medium to fluoresce under a long-wave ultraviolet light (366 nm).

The Colilert method is approved for use with drinking water.   It can be used to monitor all types of water, including water that is high in suspended sediment; it can also be used to monitor sediments.  The PA format is suitable for use in drinking-water or ground-water studies, when only a presence or absence result is needed.  No special equipment is required to use the PA format.  The MPN format can be used with all waters; however, it requires the use of a special machine that seals the incubation tray.  Quanti-Tray provides counts from 1 to 200 MPN/100 mL.  Quanti-Tray/2000 provides counts from 1 to 2,400 MPN/100 mL.  Both tests can provide a wider range by diluting the sample 1:10 or 1:100 with sterile distilled or deionized water.

See Colilert Quanti-Tray directions for water (Appendix D) and sediment (Appendix D1).

See Colilert presence/absence direction (Appendix E).

The Colilert substrate: Colilert, Cat WP020 (20 tests) or Cat WP200 (200 tests); OR Colilert-18, Cat WP020-18 (20 tests) or Cat WP200-18 (200 tests)

  • Incubation trays (Cat WQT100 (Quanti-Tray) or Cat WQT-2K (Quanti-Tray/2000)

  • The Quanti-Tray sealer (Cat WQTS2X-115) is purchased from the manufacturer (IDEXX, 800/321-0207).

Standard sample-collection bottles may be used. Because the Colilert substrate is already buffered, when preparing dilutions, use sterile distilled or deionized water.

American Public Health Association, American Water Works Association, and Water Pollution Control Federation, 2004, Standard methods for the analysis of water and wastewater (21st ed.): Washington, D.C. American Public Health Association, Section 9223.

Other references are available at

50569 Total coliforms, defined-substrate technology, most-probable number per 100 mL

50468 E. coli, defined-substrate technology, most-probable number per 100 mL

99595  Total coliforms, defined-substrate technology, presence or absence in 100 mL

99596, E. coli, defined-substrate technology, presence or absence in 100 mL


Coding results for PA tests are as follows:

  Value Remark
Present # M
Absent # U