This monograph has been reviewed by members of the IUMS–ICFMH Working Party on Culture Media and given ‘Proposed’ status.

This medium is based on the aesculin–bile salt–azide medium (ABA; Swan, 1954; Isenberg et al., 1970; Reuter, 1985) and has been used for the detection of vancomycin-resistant or susceptible enterococci in foods (Klein et al., 1998) and clinical (faecal) samples (Swenson et al., 1995; Ieven et al., 1999). The medium relies on the inhibitory properties of bile salts against most of the Gram-positive microorganisms, whereas enterococci are not inhibited. The growth of Gram-negative species is reduced by sodium azide. As an indicator system, aesculin and ferric iron differentiate group D streptococci (enterococci) from other organisms. Enterococci are able to hydrolyse aesculin which, in the presence of iron ions, results in the formation of black iron phenolic compounds. Enterococci produce brown-black zones beneath and around brown-black colonies. In order to select glycopeptide-resistant enterococci (GRE) from a mixed population of enterococci, vancomycin may be added in concentrations varying from 4 or 6 to 32 or even 64 mg L⁻¹. Lower concentrations are often used in liquid enrichment media (broth), higher concentrations in plates.