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Short answer
The bile esculin agar test is a selective and differential culture-medium reaction. Bile inhibits most strains of streptococci and enterococci other than group D, and organisms that hydrolyze esculin turn the medium dark brown to black. Blackening is a presumptive finding only: the Remel instructions state that a Gram stain and additional biochemical or serological testing are required for definitive identification.
Definition and scope
Bile esculin agar (BEA) is a solid medium. Thermo Fisher’s Remel instructions for use describe it as recommended for “qualitative procedures for presumptive identification of group D streptococci and enterococci.” The same chemistry is also sold for other settings: Neogen’s technical specification sheet for its bile esculin agar describes the product as intended for “selective isolation and differentiation of group D streptococci from foods” and states that it is not intended for the diagnosis of disease in humans.
That second sheet also explains a naming point that confuses readers of older protocols: enterococci were previously described as group D streptococci and now sit in the genus Enterococcus. Texts that say “group D strep” and texts that say “enterococci” are therefore often discussing the same reaction. The terms “bile esculin agar”, “bile esculin test” and “esculin agar” are used interchangeably in search, but a plain esculin agar without bile would not carry the bile selection described below.
How the medium works
Two reactions are combined in one plate.
Bile selects
According to the Remel instructions, oxgall at a 4% concentration (equivalent to 40% bile) “inhibits most strains of streptococci and enterococci other than group D.” The organisms that tolerate bile are the ones the test is built to pick out.
Esculin and ferric citrate differentiate
Group D streptococci and enterococci “hydrolyze esculin in the presence of bile to form esculetin and dextrose.” Esculetin reacts with ferric ions supplied by ferric ammonium citrate to form brown-black colonies. The color is the visible readout of esculin hydrolysis, not of growth alone.
The instructions credit the approach to a line of published work: Rochaix on the value of esculin, Meyer and colleagues finding that 61 of 62 enterococci hydrolyzed esculin with bile, Swan correlating 40% bile tolerance with serological group D, and Facklam and Moody testing over 700 strains of streptococci and enterococci representing all known serological groups, using Swan’s formula, and finding all of them bile-resistant and esculin-positive.
Reading a result: what positive and negative mean
Remel describes a positive reaction as dark-brown to black color around colonies that diffuses into the medium, and a negative as no blackening in the medium. Incubation is aerobic at 33-37 °C, up to 72 hours for Remel’s product, with quality-control reading at 18-24 hours. Procedure details for a given lot belong in the manufacturer’s insert and the laboratory’s own standard operating procedure, not in a general explainer.
- Growth with blackening: consistent with bile tolerance plus esculin hydrolysis, the pattern the medium is designed to show.
- No blackening: negative for esculin hydrolysis under the conditions of the test.
- Growth without blackening: not a positive result, because the reaction depends on esculin hydrolysis and not on growth alone.
Formulations and controls differ by maker
The same named medium is not the same recipe everywhere. Remel’s instructions for use and Neogen’s specification sheet differ in bile source, bile amount, pH and quality-control conditions, so a laboratory should read the insert that ships with the lot it is using rather than a generic protocol.
| Item | Remel (Thermo Fisher) IFU 1190 | Neogen NCM0117 specification sheet |
|---|---|---|
| Stated intended use | Qualitative presumptive identification of group D streptococci and enterococci | Selective isolation and differentiation of group D streptococci from foods; not for diagnosing disease in humans |
| Bile ingredient (per litre, typical or classical formula) | Oxgall 40.0 g | Ox bile 2.0 g |
| Esculin / ferric ammonium citrate (per litre) | 1.0 g / 0.5 g | 1.0 g / 0.5 g |
| Agar (per litre) | 15.0 g | 15.0 g |
| Final pH | 6.8 ± 0.2 | 6.6 ± 0.2 |
| Incubation conditions given | Aerobic, 33-37 °C, up to 72 hours; QC read at 18-24 hours | QC at 35 ± 2 °C for 18-24 hours |
Both makers list quality-control organisms. The table below combines the two lists, and each row is attributed to the maker that lists it.
| Control organism | Expected result | Listed by |
|---|---|---|
| Enterococcus faecalis ATCC 29212 | Growth, blackening | Remel; Neogen |
| Streptococcus gallolyticus ATCC 9809 | Growth, blackening | Remel |
| Enterococcus faecalis ATCC 19433 and 33186 | Black colonies | Neogen |
| Streptococcus pyogenes ATCC 19615 | Partial to complete inhibition | Remel; Neogen |
| Streptococcus pyogenes ATCC 12344 | Complete inhibition | Neogen |
| E. aerogenes ATCC 13048 and E. coli ATCC 25922 | 70% recovery listed; no blackening reaction listed | Neogen |
Neogen states that the organisms it lists “are the minimum that should be used for quality control testing.” The Remel insert adds a reporting rule: if aberrant quality control results are noted, patient results should not be reported.
Where it matters for labs and supply decisions
- Match the product’s stated intended use to the work. One maker positions its plate for presumptive identification in clinical microbiology; another positions its plate for food testing and excludes human diagnosis. The chemistry is similar, the declared use is not.
- Treat the lot insert as the protocol of record. Bile amount, pH and incubation conditions differ between the two makers’ documents above, so a method copied from a different manufacturer’s sheet may not describe the plate on the bench.
- Run the listed controls. Both makers list control organisms that are expected to blacken and one that is expected to be inhibited, and Neogen calls its list the minimum. Controls are part of the quality-control routine, not an optional extra.
- Follow storage notes. Neogen states the dehydrated medium is stored at 2-30 °C and should be discarded if it is not free flowing or has changed color.
- Fit it into your safety framework. Culturing clinical or environmental isolates falls under the laboratory’s biosafety program; see BMBL and BSL-2 for the containment vocabulary.
For other plate-based reactions that technicians read alongside this one, see alpha hemolysis on a blood agar plate and MacConkey agar.
Common misunderstandings
“A black plate identifies Enterococcus.”
It does not. The Remel instructions state that organisms other than streptococci and enterococci can grow on this medium and hydrolyze esculin, and that a Gram stain and additional biochemical or serological testing are required for definitive identification.
“Any growth means a positive.”
Growth alone is not the reaction. Blackening from esculin hydrolysis is what separates a positive from a negative.
“All bile esculin agar is the same.”
The comparison table above shows otherwise: one maker lists 40.0 g/L oxgall, another 2.0 g/L ox bile. Do not quote a single universal bile concentration.
“Bile esculin agar is a diagnosis.”
It is a presumptive, qualitative medium. One of the two makers explicitly excludes use for diagnosing disease in humans.
FAQ
What is the bile esculin agar test used for?
Presumptive identification of group D streptococci and enterococci in the Remel instructions, and selective isolation and differentiation of group D streptococci from foods in the Neogen specification sheet.
What does a black reaction on bile esculin agar mean?
The organism tolerated the bile and hydrolyzed esculin, and the resulting esculetin reacted with ferric ions to form the brown-black color. It is presumptive, not a final identification.
Is bile esculin agar the same as esculin agar?
The searches overlap, but the bile is what makes the medium selective for bile-tolerant group D streptococci and enterococci. Both manufacturer documents cited on this page describe the bile-containing medium.
Why do incubation times differ between sources?
Manufacturers specify their own conditions. Remel gives 33-37 °C for up to 72 hours with a quality-control reading at 18-24 hours; Neogen gives 35 ± 2 °C for 18-24 hours for quality control. Use the insert for the lot in hand.
Sources
- Thermo Fisher Scientific, Remel Bile Esculin Agar, instructions for use IFU 1190 (rev. 12 May 2010), https://documents.thermofisher.com/TFS-Assets/LSG/manuals/IFU1190.pdf. Date checked: 2026-10-29.
- Neogen, Bile Esculin Agar (NCM0117), Technical Specification Sheet FS-TSS-0272 rev 1.0, effective 2023-11-27, https://media.neogen.com/m/4af19e76e2b07924. Date checked: 2026-10-29.
