Appendix c7 Ohio Water Microbiology Laboratory mei method for enterococci

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Ohio Water Microbiology Laboratory

mEI method for enterococci
U.S. Environmental Protection Agency Method 1600

Updated March 2013

The mEI agar method is a one-step membrane-filtration method that allows the detection of enterococci in 24 hours with an incubation at 41C (U.S. Environmental Protection Agency, 2006a). It is recommended for use in place of the old enterococci method, the mE-EIA method, which is a two-step method that takes 48 hours to complete with an incubation at 35C (U.S. Environmental Protection Agency, 2006b). The mEI method can be done in the field or laboratory.
THEORY: The mEI medium is similar to the mE medium except that it contains a reduced amount of triphenyltetrazaolium chloride (TTC) and contains a substrate, indoxyl -D-glucoside, that turns blue when cleaved by an enzyme present in enterococci (-glucosidase). All colonies with any blue halo are recorded as enterococci, regardless of colony color. Magnification with a dissecting microscope is used for counting to give maximum visibility of colonies.

USE: The mEI test for enterococci can be applied to potable, fresh, estuarine, marine, and shellfish-growing waters.
MEDIA SOURCES: The mEI agar medium is available from Government Scientific Source, Inc. (800/248-8030, Cat 214885 (100 g) or Cat 214881 (500 g)) or from Fisher Scientific (800/766-7000, Cat B14885 (100 g) or Cat B14881 (500 g)). Nalidixic acid, and 1% presterilized TTC must also be purchased if the medium is made from dehydrated ingredients. Nalidixic acid can be purchased from Hach (800/227-4224, Cat 2407124) and needs to be dissolved in 10 N NaOH that can be purchased from Fisher Scientific (800/766-7000, Cat SS255-1 (1 L)). The 1% presterilized TTC can be purchased from Hach (Cat 2406042).
Pre-poured plates can be purchased in lots of 20 plates from Fisher Scientific (Cat B15045) or VWR (Cat 90006-316). Other quantities and plate sizes are available from this manufacturer.
Use phosphate buffered dilution water and 0.45 m membrane filters. Buffer can be purchased from Hardy Diagnostics (800/266-2222, Cat D699 (99mL) or Cat U193 (500mL)). See buffer preparation (Appendix A2).

This medium is normally not prepared from scratch. Instead, use manufactured mEI agar. The ingredients for basal medium are included below for information only.

Basal medium

Peptone 10.0 g/L

Sodium chloride 15.0 g/L

Yeast extract 30.0 g/L

Esculin 1.0 g/L

Actidione (cycloheximide) 0.05 g/L

Sodium azide 0.15 g/L

Indoxyl--D glucoside 0.75 g/L

Agar 15.0 g/L

  1. Add 72 g of pre-mixed dehydrated basal medium to 1 L of reagent water in a large flask.

  2. Heat to dissolve (to boiling).

  3. Autoclave the flask of media and 10 empty dilution bottles with caps for 15 minutes.

  4. Mix 0.24 g nalidixic acid in 5 mL sterile reagent water (add a few drops of 0.1N NaOH to dissolve if needed). Add this solution to the tempered medium.

  5. Add 0.02 g triphenyl tetrazolium chloride to the medium and mix.

  6. Pour medium into the sterilized 100-mL dilution bottles.

  7. Store at 4C for up to 6 months.

  8. Melt the basal medium using a beaker with water on a hot plate. DO NOT autoclave to melt the agar because antibiotics have been added and may deteriorate in the autoclave.

  9. Pour the plates after the agar is tempered (50-60C).

  10. Store the plates at 4C for up to 2 weeks in a tightly sealed container.


Refer to the USGS National Field Manual (Myers and others, 2007) for membrane filtration procedures. Record results on a bench sheet form (an example is attached).


Myers, D.N., Stoeckel, D.M., Bushon, R.N., Francy, D.S., and Brady, A.M.G., 2007, Fecal indicator bacteria (ver. 2.0): U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chap. A7, section 7.1.3.C. accessed December 2012 from

U.S. Environmental Protection Agency, 2006a, Method 1600—Enterococci in water by membrane filtration using membrane-Enterococcus Indoxyl--D-Glucoside agar (mEI): Washington, D.C., EPA/821/R-06/009, 42 p.
U.S. Environmental Protection Agency, 2006b, Method 1106.1—Enterococci in water by membrane filtration using membrane-Enterococcus Esculin Iron Agar (mE-EIA): Washington, D.C., EPA 821/R-06-008, 42 p.
These documents can be obtained at
Enterococci, mEI (preferred) membrane-filtration method, colony-forming units per 100 mL

Parameter code: 90909

Method code: BAC08
Enterococci, mE (acceptable) membrane-filtration method, colony-forming units per 100 mL

Parameter code: 31649

Method code: BAC07

Enterococci on mEI – Water

To be filled out at time of collection:

Site Name: _________________________ Date: ____/____/­­___ Time: _________

MM / DD /YY (military)

To be filled out by laboratory analyst:

Analyzed by (initials): _____________ Analysis Date: ­­­­­­­­­­­­____________

Date mEI plates poured: ____________ Time in 41C (22–24hr): ____________

Filter (0.45 m) lot number: ____________ Time out of incubator: ____________

Read by: ____________ Date plates read: ____________

Colony counts:

Suggested sample volumes: 100, 30, 10, 3, 1, 0.3 (30mL of 10-2 dilution), and 0.1 (10mL of 10-2 dilution)

[Positive Enterococci colony: any colony with a blue halo, regardless of colony color.]

NWIS parameter code for enterococci in water on mEI: 90909; Method code: BAC08

Sample size

(volume - mL)

Colony count

Filter Blank

(before plating series)

Process Blank

(after plating series)


Enterococci colonies/100 mL


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