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BBL™ mEI Agar is a selective culture medium used for the chromogenic detection and enumeration of enterococci in water by the single-step membrane filtration technique. It conforms with the U.S. Environmental Protection Agency (USEPA) Approved Method 1600: Enterococci in Water by Membrane Filtration Using membrane-Enterococcus Indoxyl-ß-D-Glucoside Agar (mEI).

Enterococci are found in the feces of humans and other warm-blooded animals. Although some strains are ubiquitous and are not related to fecal pollution, the presence of enterococci in water is an indication of fecal pollution and the possible presence of enteric pathogens.¹ In epidemiological studies conducted by the USEPA, it was found that the presence of enterococci had a higher correlation with swimming-associated gastroenteritis in fresh and marine water environments than fecal coliforms.² In 1986, the USEPA recommended that both Escherichia coli and enterococci be used as bacterial water quality indicators to monitor recreational waters.³

A two-step membrane filtration (MF) method⁴ was developed by Levin et al. to measure enterococci in fresh and marine recreational waters. Using mE agar, the method required a 48-hour incubation and a transfer of the membrane to another substrate medium, Esculin Iron Agar, to differentiate enterococci.

In 1997, the USEPA improved on the mE agar formulation by reducing the triphenyltetrazolium chloride component and adding the chromogen indoxyl ß-D-glucoside. The new medium, mEI Agar,¹ was developed as a single-step procedure that does not require the transfer of the membrane filter to another substrate. Observation of a blue halo around colonies in 24 hours is confirmatory for the presence of enterococci. A wide range of sample volumes or dilutions can be tested by this single-step MF procedure for the detection and enumeration of enterococci in potable, fresh, estuarine, marine and shellfish-growing waters. The USEPA published false-positive rate is 6.0% and false-negative rate is 6.5%.¹

BBL mEI Agar conforms to the 1986 revisions to the bacteriological ambient water quality criteria, that included the indicator bacteria E. coli and enterococci, which provide better correlation with swimming-associated gastrointestinal illness. In response to this health risk, the USEPA established the Beaches Environmental Assessment Closure and Health (BEACH) Program. This method is published for use in the BEACH Program.¹

Colonies having a blue halo can be verified as enterococci by appropriate biochemical procedures in instances where required in evidence gathering or for performing quality control for the initial use of the test.¹

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 41 ± 0.5°C for 24 ± 2 h. Count all colonies with blue halos.

3. Determine the pH potentiometrically at room temperature for adherence to the specification of 7.1 ± 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.