[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"$fxLN3MUwXCdr5RPjwZYIDpOj8CHyjOmngWTgoKXPtZbg":3,"$fPW-0zwFoIq2JK0y88DKtQDMZyXe77k8cBr5CrB2sOA8":32,"$f3Ft0rKFJHppdzE-vuveecxx1BUcg9iOlMLtyzf_MJDg":63},[4,8,12,16,20,24,28],{"title":5,"slug":6,"path":7},"About Microbeonline.com","about-microbeonline-com","\u002Fabout-microbeonline-com\u002F",{"title":9,"slug":10,"path":11},"About Me","about-me","\u002Fabout-microbeonline-com\u002Fabout-me\u002F",{"title":13,"slug":14,"path":15},"Advertise with Us","advertise-us","\u002Fadvertise-us\u002F",{"title":17,"slug":18,"path":19},"Privacy Policy","privacy-policy","\u002Fprivacy-policy\u002F",{"title":21,"slug":22,"path":23},"Abbreviations","abbreviations","\u002Fabbreviations\u002F",{"title":25,"slug":26,"path":27},"Microbes","microbes","\u002Fmicrobes\u002F",{"title":29,"slug":30,"path":31},"Books","recommended-books","\u002Frecommended-books\u002F",{"type":33,"data":34},"blog",{"slug":35,"title":36,"description":37,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":40,"lastUpdatedDate":41,"draft":42,"category":43,"image":38,"body":44,"faq":45,"tags":61,"related":62},"bile-esculin-test-enterococcus-species-principle-procedure-results","Bile-Esculin test for Enterococcus species","Bile esculin test principle, procedure, and interpretation for identifying Group D streptococci and Enterococcus species using esculin hydrolysis.",null,"Acharya Tankeshwar","2014-01-21","2026-07-12",false,"biochemical-tests","Bile-esculin test is widely used to differentiate Enterococci and non-enterococcus group D streptococci, which are bile tolerant and can hydrolyze esculin to esculetin, **from** non-group D viridans group streptococci, which grow poorly on bile. It is a low-cost, rapid test with good sensitivity and specificity  (>90%).\n\n![Bile Esculin Test Results: Control, Positive and Negative  - Bile esculin test results: Control, Positive and Negative](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FBile-Esculin-Test-Result-200x300.jpg)Figure: Bile esculin test results: Control, Positive and Negative\n\n## Principle of Bile-esculin test\n\nBile-esculin test is based on the ability of certain bacteria, notably the group D streptococci and *Enterococcus* species, to hydrolyze esculin in the presence of bile (4% bile salts or 40% bile).\n\nNote: *Many bacteria can hydrolyze esculin, but few can do so in the presence of bile.*\n\nEsculin is a glycosidic coumarin derivative (6-beta-glucoside-7-hydroxy-coumarin). The two moieties of the molecule (glucose and 7-hydroxycoumarin) are linked together by an ester bond through oxygen. For this test, esculin is incorporated into a medium containing 4% bile salts.\n\nBacteria that are bile-esculin positive are, first of all, able to grow in the presence of bile salts. Bile salts are detergent-like and damage bacterial cell membranes, most Gram-positive organisms simply can't survive the 4% concentration used here. Enterococcus and Group D streptococci carry dedicated stress-response systems that detect bile exposure and trigger protective changes to the cell envelope, which is what lets them survive long enough to also hydrolyze the esculin. Hydrolysis of the esculin in the medium results in the formation of **glucose** and a compound called **esculetin.**\n\n![Esculin Hydrolysis; Chemical reaction - Esculin hydrolysis test](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FESCULIN-HYDROLYSIS.jpg)Figure: Esculin hydrolysis test\n\nEsculetin, in turn, reacts with ferric ions (supplied by the inorganic medium component ferric citrate) to **form a black diffusible complex**. Group D streptococci and enterococci include opportunistic pathogens such as *Enterococcus faecalis*, *Enterococcus faecium*, and *Streptococcus bovis*.\n\n![Fig: Chemical Reaction of the Bile Esculin Test  - Fig: Chemical Reaction of the Bile Esculin Test](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FChemical-reaction-of-Bile-esculin-test-300x138.jpg)Figure: Chemical Reaction of the Bile Esculin Test\n\n## Materials\n\nBile-esculin agar medium is prepared as agar slants or plates. The constituents of bile-esculin agar medium are peptone, beef extract, oxgall (bile), esculin, ferric citrate, and agar. Bile esculin medium contains esculin and peptone for nutrition and bile to inhibit Gram-positive bacteria other than Group D streptococci and enterococci. Ferric citrate is added as a color indicator.\n\n## Procedure\n\n1. With an [inoculating wire or loop](\u002Finoculating-loop-types-and-uses\u002F), touch two or three morphologically similar streptococcal colonies and inoculate the slant of the bile esculin medium with an S-shaped motion, or streak the surface of a bile esculin plate for isolation. (*Note: There is no need to stab the medium.)*\n2. Incubate the inoculated tube at 35-37°C for 24 hours and then observe the results.\n\n***Note:*** *The published optimum for this test is a bile concentration of 40% (4% oxgall), a standardized inoculum, and a reading at 24 hours, which gives greater than 99% sensitivity and 97% specificity (Rosa et al., 1998). Departing from these, especially a heavier inoculum or a lower bile concentration, is what lets non-group D viridans streptococci give a false positive.*\n\n## Results and Interpretation\n\nDiffuse blackening of more than half of the slant within 24-48 hours indicates esculin hydrolysis. On plates, black haloes will be observed around isolated colonies and any blackening is considered positive. All group D streptococci will be bile-esculin positive within 48 hours.\n\n## Where bile esculin fits: the group D differentiation at a glance\n\nBile esculin is the first step, not the whole answer. A positive result tells you the organism is **either** a group D *Streptococcus* (the *S. gallolyticus* \u002F *S. bovis* group) **or** an *Enterococcus*, because both are bile-tolerant esculin hydrolyzers. It does not separate the two. Two more tests do that, and together the three form the standard presumptive workup:\n\n| Test | *Enterococcus* | *S. gallolyticus* group (group D strep) | Non-group D viridans strep |\n| --- | --- | --- | --- |\n| **Bile esculin** | Positive | Positive | Negative |\n| **6.5% NaCl** | Positive | Negative | Negative |\n| **PYR** | Positive | Negative | Negative |\n\nRead the sequence like this:\n\n- **Bile esculin negative** → not group D; think non-group D viridans streptococci. Stop here.\n- **Bile esculin positive, 6.5% NaCl positive** → *Enterococcus* (confirmed by PYR positive). The salt tolerance is what separates enterococci from the group D streptococci.\n- **Bile esculin positive, 6.5% NaCl negative** → *S. gallolyticus* group (non-enterococcal group D). PYR is negative here, and this is the organism whose presence in a blood culture prompts a colonoscopy (see the starch hydrolysis and *S. gallolyticus* articles).\n\nCheck the  respective articles for the  full methods for [salt tolerance](https:\u002F\u002Fmicrobeonline.com\u002Fsalt-tolerance-test-enterococcus-species-principle-procedure-results) and [PYR](https:\u002F\u002Fmicrobeonline.com\u002Fpyrrolidonyl-arylamidase-pyr-test-principle-procedure-results\u002F).\n\n## Quality Control\n\n1. Positive control: *Enterococcus* species (e.g. *E. faecalis*)\n2. Negative control: Viridans streptococcus, not group D\n\n### Where students actually get confused\n\n- **Esculin hydrolysis alone doesn't mean anything here, it has to happen in the presence of bile.** Many bacteria can hydrolyze esculin under ordinary conditions. The bile is what makes this test selective, almost nothing survives the bile concentration except Group D strep and Enterococcus, so only survivors get the chance to show a positive esculin result.\n- **A positive result doesn't separate Enterococcus from non-enterococcal Group D strep (like *S. bovis*).** Both give a positive bile-esculin result. Salt tolerance is the test that splits them apart, Enterococcus grows in 6.5% NaCl, *S. bovis* doesn't.\n- **About 3% of viridans streptococci can also hydrolyze esculin in bile.** Rare, but real, this is why a positive bile-esculin result on its own should still be interpreted alongside colony morphology and other tests, not treated as an automatic Group D call.\n- **\"Any blackening\" counts, on plates,** but on slants, the rule is more specific: diffuse blackening of more than half the slant within 24-48 hours. Don't apply the plate-reading rule to a slant or vice versa.\n\n### Key exam facts in one table\n\n| Feature | Detail | Memory hook |\n| --- | --- | --- |\n| Detects | Esculin hydrolysis in the presence of 4% bile | Bile is the selectivity filter |\n| Chemistry | Esculin → glucose + esculetin → black ferric complex |  |\n| Positive (slant) | Blackening of >50% of slant within 24-48h |  |\n| Positive (plate) | Black halo around colonies, any blackening | Different rule than slant |\n| Mechanism | Cell-envelope stress response to bile exposure | Same toughness story as salt tolerance |\n| Confirms | Group D strep (Enterococcus + non-enterococcus) | Doesn't separate the two |\n| Pairs with | Salt Tolerance Test | Splits Enterococcus from non-enterococcal Group D |\n| Rare exception | \\~3% of viridans streptococci | Don't read in isolation |\n\n**References**\n\n1. Chuard, C., & Reller, L. B. (1998). Bile-esculin test for presumptive identification of enterococci and streptococci: effects of bile concentration, inoculation technique, and incubation time. *Journal of clinical microbiology*, *36*(4), 1135–1136. \u003Chttps:\u002F\u002Fdoi.org\u002F10.1128\u002FJCM.36.4.1135-1136.1998>\n2. Facklam, R. R., & Moody, M. D. (1970). Presumptive identification of group D streptococci: the bile-esculin test. *Applied microbiology*, *20*(2), 245–250. \u003Chttps:\u002F\u002Fdoi.org\u002F10.1128\u002Fam.20.2.245-250.1970>\n3. Forbes, S., Sahm, D. F., & Weissfeld, A. S. (2002). [Bailey & Scott’s Diagnostic Microbiology](https:\u002F\u002Famzn.to\u002F2WZxPHL). Mosby.\n4. Rosa TT, et al. Bile-esculin test for presumptive identification of enterococci and streptococci: effects of bile concentration, inoculation technique, and incubation time. *J Clin Microbiol.* 1998;36(4):1135-1136. doi:10.1128\u002FJCM.36.4.1135-1136.1998",[46,49,52,55,58],{"question":47,"answer":48},"Why does esculin hydrolysis only count in this test when it happens in the presence of bile?","Many bacteria can hydrolyze esculin under ordinary conditions, so that alone isn't selective. Bile salts are detergent-like and damage most Gram-positive cell membranes, almost nothing survives the 4% bile concentration except Group D streptococci and Enterococcus. Requiring hydrolysis to happen in the presence of bile is what makes the test specific to these organisms.",{"question":50,"answer":51},"Does a positive bile-esculin test confirm Enterococcus specifically?","No. Both Enterococcus and non-enterococcal Group D streptococci (such as Streptococcus bovis) give a positive bile-esculin result. The salt tolerance test is needed to separate them, Enterococcus grows in 6.5% NaCl broth, while non-enterococcal Group D strep does not.",{"question":53,"answer":54},"Can organisms other than Group D streptococci and Enterococcus give a positive bile-esculin test?","Rarely. Approximately 3% of viridans streptococci can also hydrolyze esculin in the presence of bile. A positive result should be interpreted alongside colony morphology and other tests rather than treated as an automatic confirmation.",{"question":56,"answer":57},"The organism grew on the slant but there is no black color. Is that positive?","No. Growth alone is not a positive result. Bile esculin positivity requires both bile tolerance (growth) and esculin hydrolysis (blackening of more than half the slant). An organism that grows but produces no black color is bile-tolerant but does not hydrolyze esculin, so it is bile esculin negative and not a group D streptococcus or Enterococcus by this test.",{"question":59,"answer":60},"Is Listeria bile esculin positive?","Yes. Listeria monocytogenes is bile esculin positive, which can cause confusion because it is a Gram-positive rod that shares this reaction with the group D cocci. The distinction is made on Gram stain and morphology: bile esculin is used for presumptive identification within the catalase-negative Gram-positive cocci, so Listeria, a Gram-positive rod, is separated by its appearance and other tests before bile esculin is interpreted.",[],[],[64,70,77,82,86,90,95,100,104,108],{"slug":65,"name":39,"description":66,"image":67,"body":68,"postCount":69},"acharya-tankeshwar","Editor-in-chief","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Ftankeshwar-acharya-author-microbeonline.jpg","***Tankeshwar Acharya, MSc (Medical Microbiology)***\n\n*Tankeshwar Acharya is an Assistant Professor in the Department of Microbiology at Patan Academy of Health Sciences (PAHS), Nepal, where he has been teaching and practicing clinical microbiology for over 14 years. He is the founder of Microbe Online, one of the leading free microbiology education resources on the web, covering bacteriology, mycology, parasitology, immunology, and clinical laboratory diagnostics written from direct experience in both the classroom and the diagnostic laboratory.*",433,{"slug":71,"name":72,"description":73,"image":74,"body":75,"postCount":76},"ashma-shrestha","Ashma Shrestha","SEO Copywriter and Science Communicator\nKathmandu, Nepal","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Fashma-shrestha.png","Ashma Shrestha holds a Master of Science in Medical Microbiology from the Institute of Science and Technology (IOST), Tribhuvan University, Nepal, where she developed a strong foundation in virology, molecular biology, and diagnostic microbiology.\n\nShe now works as an SEO Copywriter at Resolution Digital, where she combines her scientific training with research-driven content strategy. She is certified in Google Analytics and Google Business Profile (GBP), and brings a data-informed approach to science communication writing content that is not only accurate but structured to reach and serve the students who need it most.\n\nAt microbeonline, Ashma contributes articles primarily in virology and molecular biology, areas she finds most compelling for their mechanistic depth and their growing clinical relevance. Her writing reflects the same standard the site is built on: factual rigor, clear explanation of the *why* behind microbiology concepts, and content that helps students move from memorization to genuine understanding.\n\nShe is passionate about making complex microbiological concepts accessible without sacrificing accuracy; a skill that sits at the intersection of her scientific training and her professional work in content and SEO.",81,{"slug":78,"name":79,"description":80,"image":38,"body":38,"postCount":81},"sushmita-baniya","Sushmita Baniya","Author \u002F Contributor",32,{"slug":83,"name":84,"description":80,"image":38,"body":38,"postCount":85},"samikshya-acharya","Samikshya Acharya",20,{"slug":87,"name":88,"description":80,"image":38,"body":38,"postCount":89},"alisha-tripathi","Alisha Tripathi",6,{"slug":91,"name":92,"description":93,"image":38,"body":38,"postCount":94},"aastha-shrestha","Aastha Shrestha"," Author \u002F Contributor",10,{"slug":96,"name":97,"description":98,"image":38,"body":38,"postCount":99},"guest-author","Guest Author","Guest Author \u002F Contributor",2,{"slug":101,"name":102,"description":80,"image":38,"body":38,"postCount":103},"srijana-khanal","Srijana Khanal",18,{"slug":105,"name":106,"description":98,"image":38,"body":38,"postCount":107},"dr-poonam-acharya","Dr. Poonam Acharya",1,{"slug":109,"name":110,"description":80,"image":38,"body":111,"postCount":112},"nisha-rijal","Nisha Rijal","**Nisha Rijal** is a microbiologist and quality assurance specialist. She served for nearly 12 years as a microbiologist at the National Public Health Laboratory (NPHL), Nepal's national reference laboratory, and continues to work as a consultant microbiologist in international public health organization. ",51]