Ohio District's Microbiology Laboratory

Total coliforms and Escherichia coli, using the Colilert system

Colilert is a commercially available liquid medium (Idexx Corporation, 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 Quantitray.  The PA method can be done using any sterile bottle of the appropriate size.  Both methods are best done in the 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 of 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.  The PA format is suitable for use in ground-water studies, as it can be modified for larger sample volumes (such as 1 L).  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 Quantitray.

See Colilert Quantitray directions (Appendix D).

See Colilert presence/absence direction (Appendix E).

MEDIA:  The Colilert substrate, Quantitray, and Quantitray sealer are purchased from the manufacturer.  Standard sample-collection bottles may be used. 

REFERENCES: 

American Public Health Association, American Water Works Association, and Water Pollution Control Federation, 1998, Standard methods for the analysis of water and wastewater (20^th ed.): Washington, D.C. American Public Health Association, Section 9223.

NWIS PARAMETER CODES:

50569 Total coliforms, Colilert Quantitray, most-probable number per 100 mL

50468 E. coli, Colilert Quantitray, most-probable number per 100 mL

84138  Total coliforms, presence (1) or absence (2)

code pending,  E. coli, presence (1) or absence (2)