Bile Esculin Agar

Bile Esculin Agar

Selective & Differential Medium for Presumptive Identification of Enterococci

Catalog Number: AS-1140

Overview

Bile Esculin Agar is a selective and differential solid culture medium widely used for the isolation and presumptive identification of enterococci (Enterococcus faecalis, E. faecium, E. durans, E. gallinarum, etc.) and Group D streptococci from clinical, food, water, and environmental samples.

Selectivity is achieved through bile (oxgall), which inhibits most Gram-positive bacteria other than enterococci. Differential identification is based on esculin hydrolysis; in the presence of ferric citrate, hydrolysed esculin forms a black iron complex, producing characteristic blackening of the medium around positive colonies.

Bile Esculin Agar is referenced in CLSI M100, ISO 7899-2, and FDA-BAM and remains one of the most reliable media for enterococcal screening worldwide.

Key Features & Benefits

• Selective for enterococci and Group D streptococci

• Esculin hydrolysis produces clear black halo reactions

• Rapid presumptive identification within 24–48 hours

• Does not contain sodium azide, allowing improved growth of some strains

• Widely referenced in clinical, food, and water microbiology standards

Typical Applications

Clinical Microbiology

• Presumptive identification of enterococci from urine, blood, wound, and other specimens

• Supportive screening in antimicrobial resistance workflows

Food & Water Microbiology

• Isolation of enterococci from foods, drinking water, and environmental samples

• Indicator organism testing in hygiene and quality-control programs

Research & Reference Laboratories

• Confirmation testing and method validation

• Teaching and training in selective/differential media

Principle of the Medium

• Bile (oxgall) inhibits most Gram-positive bacteria except enterococci

• Esculin is hydrolysed by enterococci to esculetin

• Ferric citrate reacts with esculetin to form a black iron complex

A positive reaction is indicated by blackening of the medium or a black halo (≥5 mm) around colonies.

Composition (Typical – per litre)

• Tryptone / peptone: 17.0 g

• Meat (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 at 25 °C: 7.1 ± 0.2

Preparation (Typical)

• Suspend 51.5–56.5 g of dehydrated medium in 1 L purified water

• Heat with frequent agitation and boil for 1 minute until fully dissolved

• Do not autoclave (esculin is heat-sensitive)

• Cool to 45–50 °C, mix well, and pour into sterile Petri dishes

Incubation

• 35–37 °C for 24–48 hours

• Examine at both 24 h and 48 h

Interpretation

• Enterococci / Group D streptococci:
  Growth with black or dark-brown colonies and surrounding black halo

• Non-enterococci:
  No growth or growth without blackening

Storage & Stability

Dehydrated medium

• Store below 30 °C in a cool, dry place

Prepared plates

• Store at 2–8 °C, protected from light

• Use within 4 weeks

Intended Use

For clinical, food, water, environmental, and research microbiology use only.

Not for therapeutic use.

Quality & References

• Referenced in CLSI M100, ISO 7899-2, and FDA-BAM

• Based on methods described by Facklam & Moody

• Manufactured under controlled conditions for batch consistency

• Certificate of Analysis (COA) available

• Safety Data Sheet (SDS) available upon request