m-EI Chromogenic Agar Base, Modified

This medium is a modification of the m-EI Chromogenic Agar Base, where another chromogenic substrate is added. This addition allows the differentiation of Enterococcus faecium and Enterococcus faecalis.

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. Chromogenic Mix is added to differentiate Enterococcus faecium from Enterococcus faecalis. Bacteriological agar is the solidifying agent.

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

Preparation

Suspend 71.48 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.

Caution: This medium contains sodium azide and cycloheximide and it is very toxic if swallowed, inhaled or comes into contact with skin. Wear gloves and eye/face protection.

Instructions for use

Inoculate and incubate at 41 ± 0.5 °C and observe after 18-24 hours.

• 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.