Updated May 2005

The MI method is a membrane-filtration method that allows the simultaneous detection of total coliforms and Escherichia coli (E. coli) on one medium. This method can be done in the field or laboratory.

THEORY:  Two enzyme substrates are included in MI medium—a fluorogen reacts with the enzyme found in total coliforms (galactosidase), and a chromogen reacts with an enzyme found in E. coli (glucuronidase).  Cefsulodin, an antibiotic, is added to MI medium to inhibit non-target growth.  After 24 hours of incubation at 35°C, total coliform colonies glow under a black light, and E. coli colonies appear blue under natural light.

USE:  The MI method has been validated for use with drinking water.   This method is especially suitable for enumerating bacteria in ground water because in ground-water studies, it is often desirable to enumerate both total coliforms and E. coli.  The method can also be used to enumerate E. coli in surface waters; however, plates may be difficult to read because of non-target growth in moderately- or highly-polluted waters. 

MEDIA:  The MI medium is commercially available in a dehydrated format from Becton Dickinson and Company (BD).  This can be ordered from Government Scientific Source, Inc. ( Vienna , VA , 800/248-8030, Cat 214882).  The field crew must add freshly prepared cefsulodin solution to the tempered agar medium before pouring the plates. It can be purchased from Aldrich (800/325-3010, Cat C-8145).   See preparation instructions (Appendix C). 

Prepoured plates can be purchased from Microbiology International (800/EZ-MICRO). Cefsulodin is incorporated in the prepoured plates and does not have to be added.

Use phosphate buffered dilution water and 0.45 mm membrane filters.  Buffer can be purchased from Hardy Diagnostics (800/346-2766, Cat D699 (99mL) or Cat U193 (500mL)).  See buffer preparation (Appendix M).

NOTE:  Bright green fluorescent colonies are NOT to be counted as total coliforms (see photo above).  These are Pseudomonas species and may indicate the breakdown of cefsulodin in the agar.  Total coliform colonies are bright blue.

REFERENCES: 

U.S. Environmental Protection Agency, 2002, Method 1604—Total coliforms and Escherichia coli in water by membrane filtration using a simultaneous detection technique (MI medium): Washington D.C., EPA 821-R-02-024, 14 p.

Can be obtained at http://www.epa.gov/nerlcwww/MI_emmc.pdf

NWIS PARAMETER CODES:

90900 Total coliforms on MI, colonies per 100 mL

90901 E. coli on MI, colonies per 100 mL