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BBL™ MI Agar is a chromogenic/fluorogenic medium used to detect and enumerate Escherichia coli and total coliforms in drinking water by the membrane filtration technique. It conforms with the U.S. Environmental Protection Agency (USEPA) Approved Method 1604: Total Coliforms and Escherichia coli in Water by Membrane Filtration Using a Simultaneous Detection Technique (MI Medium).

Coliform bacteria are species that inhabit the intestines of warm-blooded animals or occur naturally in soil, vegetation and water. They are usually found in fecally-polluted water and are often associated with disease outbreaks. Although these bacteria are not usually pathogenic themselves, their presence in drinking water indicates the possible presence of other pathogens. E. coli is one species in this group of coliform bacteria. Since it is always found in feces, it is a more direct indicator of fecal contamination and the possible presence of enteric pathogens.

Chromogens or fluorogens have been used for many years to detect and identify total coliforms (TC) and E. coli. Some methods use liquid media in a multiple-tube-fermentation (MTF) test, a presence-absence (PA) format or other tube tests. Agar media are also used for direct plating or membrane filtration (MF) technology. However, standard MF technology for the detection of TC and fecal coliforms requires the use of several different types of media and two different incubation temperatures.¹

The newest technology developed by the USEPA for testing drinking water is a single membrane filtration technique where no membrane filter transfers are required.^1-4 The medium is named after the two enzyme substrates included in the formulation: a fluorogen, 4-Methylumbelliferyl-ß-D-galactopyranoside (MUGal) and a chromogen, Indoxyl-ß-D-glucuronide (IBDG). MI Agar can simultaneously detect and enumerate both TC and E. coli in water samples in 24 hours or less based on their specific enzyme activities. MI Agar detects the presence of the bacterial enzymes ß-galactosidase and ß-glucuronidase produced by TC and E. coli, respectively.

MI Agar is approved for use by certified drinking water laboratories for microbial analysis of potable water. Other uses include recreational, surface or marine water, bottled water, groundwater, well water, treatment plant effluents, water from drinking water distribution lines, drinking water source water and possibly foods.⁵

As referenced in USEPA method 1604, this method has a detection limit of one E. coli and/or one total coliform per sample volume or dilution tested.⁵ The false-positive and false-negative rates for E. coli are both 4.3%.⁵ Specificity for E. coli is 95.7% and for total coliforms is 93.1%.⁵ The single lab recovery of E. coli is 97.9% of the heterotrophic plate count (pour plate) and 115% of the R2A spread plate count.⁵ For Klebsiella pneumoniae and Enterobacter aerogenes, recoveries are 87.5% and 85.7% of the heterotrophic plate count and 89.3% and 85.8% of the R2A spread plate method, respectively.⁵

1. Examine plates for signs of deterioration.

2. Check performance by inoculating a representative sample of plates with pure cultures of stable control organisms that give known, desired reactions. Inoculate and incubate the plates at 35°C ± 0.5 for 20-24 hours. Count all blue or indigo colonies under ambient light. Expose MI Agar plates to long-wave ultraviolet light (366 nm) and count all fluorescent colonies.

3. Determine the pH potentiometrically at room temperature for adherence to the specification of 6.95 ± 0.2.

4. Incubate uninoculated representative plates at 35 ± 2°C for 72 h and examine for microbial contamination.

For Laboratory Use.

Observe aseptic techniques and established precautions against microbiological hazards throughout all procedures. After use, prepared plates and other contaminated materials must be sterilized by autoclaving.