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 41C (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 35C (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). MEDIA PREPARATION
This medium is normally not prepared from scratch. Instead, use manufactured mEI agar. The ingredients for basal medium are included below for information only.
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
Add 72 g of pre-mixed dehydrated basal medium to 1 L of reagent water in a large flask.
Heat to dissolve (to boiling).
Autoclave the flask of media and 10 empty dilution bottles with caps for 15 minutes.
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.
Add 0.02 g triphenyl tetrazolium chloride to the medium and mix.
Pour medium into the sterilized 100-mL dilution bottles.
Store at 4C for up to 6 months.
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.
Pour the plates after the agar is tempered (50-60C).
Store the plates at 4C 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 http://pubs.water.usgs.gov/twri9A/.
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 http://www.epa.gov/waterscience/methods/method/biological/ NWIS PARAMETER CODES:
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 41C (22–24hr): ____________
Filter (0.45 m) lot number: ____________ Time out of incubator: ____________
Read by: ____________ Date plates read: ____________
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