Overview:
Bile Esculin Agar is a selective and differential solid medium for the isolation and presumptive identification of enterococci (Enterococcus faecalis, E. faecium, E. durans, E. gallinarum, etc.) and group D streptococci. It is one of the most widely used and reliable media worldwide. Bile (oxgall) inhibits most Gram-positive bacteria except enterococci, while esculin hydrolysis in the presence of ferric citrate produces black halos around colonies.
Applications:
?? Presumptive identification of enterococci from clinical specimens (urine, blood, wounds)
?? Isolation of enterococci from foods, water, and environmental samples
?? Confirmation test for vancomycin-resistant enterococci (VRE) screening
?? Standard medium in CLSI M100, ISO 7899-2, and FDA-BAM
Key Features:
4 % bile ? inhibits streptococci and most other Gram-positives
Esculin + ferric citrate ? blackening of medium by enterococci within 24�48 h
Positive reaction = black halo ? 5 mm around colonies
Does NOT contain azide (unlike Bile Esculin Azide Agar) ? better growth of some strains
Composition (per liter):
Tryptone/Peptone 17.0 g
Meat Extract/Beef Extract 3.0 g
Yeast Extract 5.0 g
Oxgall (bile) 10.0�15.0 g
Esculin 1.0 g
Ferric Citrate 0.5 g
Sodium Chloride 5.0 g
Agar 15.0 g
Final pH: 7.1 � 0.2 at 25�C
Preparation:
Suspend 51.5�56.5 g dehydrated medium in 1 L purified water.
Heat with frequent agitation and boil for 1 minute.
DO NOT AUTOCLAVE (esculin is heat-sensitive).
Cool to 45�50�C, mix well, and pour plates.
Incubation:
35�37�C for 24�48 h (read at 24 h and 48 h).
Interpretation:
Enterococci/Group D streptococci: black or dark-brown colonies with black halo
Non-enterococci: no growth or growth without blackening
Storage:
Dehydrated medium: cool, dry place below 30�C
Prepared plates: 2�8�C, protected from light, use within 4 weeks
For clinical, food, and environmental microbiology use only.
Reference: Facklam & Moody (1970), CLSI, ISO 7899-2, FDA-BAM.