M-EI Chromogenic Agar Base

The medium was developed as a single-step procedure that does not require the transfer of the membrane filter to another substrate. Observation of blue color colonies confirms the presence of enterococci.

A wide range of sample volumes or dilutions can be tested by this single-step membrane filtration procedure for the detection and enumeration of enterococci in potable, fresh, estuarine, marine and shellfish-growing waters.

Peptone provides nitrogen, vitamins, minerals and amino acids essential for growth. Yeast extract provides trace elements, vitamins and amino acids. Esculin is hydrolyzed by enterococci to form esculetin and dextrose. Cycloheximide inhibits most fungi, and the sodium azide inhibits Gram-negative bacteria. X-Glucoside is the substrate of the glucosidase-positive enterococci and the agar is added into the medium as a solidifying agent.

Typical formula in g/L, adjusted and/or supplemented as required to meet performance criteria.

Preparation

Suspend 71.95 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Sterilize in autoclave at 121 °C for 15 minutes. Cool to 50 °C, mix well and dispense into plates. For a more selective medium, prepare a solution of 0.24 grams of nalidixic acid in 5 mL of sterile distilled water with a few drops of sodium hydroxide 0.1 N (for a better dissolution), and aseptically add to one liter of medium. If desired, 2 mL per liter of a 1% TTC solution can be added.

Instructions for use

Inoculate and incubate at 41 ± 0.5 °C and observe after 18-24 hours. Enterococcus species will grow as blue colonies.

• Levin, Fischer and Cabelli. 1975. Appl. Microbiol. 30.66.
• U.S. Environmental Protection Agency. 2002. Method 1600: Enterococci in water by membrane filtration using membrane enterococcus indoxyl-beta-D-glucoside agar (mEI). Publication EPA-821-R-02-022. USEPA Office of Water, Office of Science and Technology, USEPA, Washington, DC.