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.

THEORY:
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 35^oC, 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).

USE: 
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 (Appendix D).

See Colilert presence/absence direction (Appendix E).

MEDIA: 
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.

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

Other references are available at http://www.idexx.com/water/colilert/moreinfo.jsp.

NWIS PARAMETER CODES:
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: