[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"$fxLN3MUwXCdr5RPjwZYIDpOj8CHyjOmngWTgoKXPtZbg":3,"$fJePQ4B-IWWsagumMRw78_AO281-UvH5zRsgH-nEHFGY":32,"$f3Ft0rKFJHppdzE-vuveecxx1BUcg9iOlMLtyzf_MJDg":60},[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":58,"related":59},"indole-test-principle-procedure-results","Indole Test: Principle, Procedure, Interpretation & Indole-Positive Organisms","Indole test principle, spot vs. tube procedure, and result interpretation — plus the \"OK VIP\" trick to remember indole-positive Enterobacteriaceae for your practicals.",null,"Acharya Tankeshwar","2012-04-04","2026-07-11",false,"biochemical-tests","Indole test is used to determine the ability of an organism to split the amino acid tryptophan to form the compound indole. It's one of the four [IMViC tests](https:\u002F\u002Fmicrobeonline.com\u002Fimvic-tests-principle-procedure-and-results\u002F) used to differentiate members of the family *Enterobacteriaceae* — and it's usually the fastest of the four to read.\n\n## Why It Matters\n\nPicture a urine culture growing a lactose-fermenting Gram-negative rod on MacConkey agar. Full identification and sensitivity will take another 24–48 hours — but a clinician needs an empiric antibiotic choice today. A 20-second spot indole test, combined with the colony's oxidase result, can presumptively call it *E. coli* right then: indole-positive, oxidase-negative, pink lactose-fermenter on MacConkey. That presumptive call is often enough to start a sensible empiric regimen while the formal culture finishes.\n\nThis is also why indole is one of the oldest tests in clinical bacteriology — it doesn't need expensive reagents or incubators beyond what every lab already has, and it still separates indole-positive *E. coli* from indole-negative *Enterobacter* and *Klebsiella pneumoniae* faster than almost any other single test.\n\n## Principle\n\nTryptophan is hydrolyzed by the enzyme **tryptophanase** into three end products: indole, pyruvate, and ammonia. Indole, when present, combines with the aldehyde in the test reagent to form a colored compound:\n\n- **Pink to red-violet** (quinoidal compound) — if a benzaldehyde reagent (Kovac's or Ehrlich's) is used\n- **Blue to green** — if a cinnamaldehyde reagent (DMACA) is used\n\nNo tryptophanase → no indole → no color change (indole-negative).\n\n![](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FPrinciple-of-Indole-Test.png)*Figure: Indole Test Reaction*\n\n## Procedure of Indole Test\n\nThere are two ways to run this test, and **which one you choose matters**.\n\n### 1. Spot (Rapid) Indole Test\n\nBest for: quick presumptive ID directly from an isolated colony.\n\n- Saturate a filter paper with reagent, then rub a colony sample onto it — **or**\n- Swab the colony and add a drop of reagent to the swab — **or**\n- Add reagent directly onto the colony on the agar plate\n\nRead the color change within **20 seconds**.\n\n### 2. Conventional Tube Method\n\nBest for: confirming a negative spot test, or testing weak indole producers that the rapid method misses.\n\n1. Inoculate tryptophan broth with the test organism.\n2. Incubate at 37°C for 24–48 hours.\n3. **Kovac's method:** Add 0.5 mL Kovac's reagent down the side of the tube; read the color at the meniscus.\n4. **Ehrlich's method** (for anaerobes\u002Ffastidious organisms): Add 0.5 mL xylene, mix and let settle, then add 6 drops of Ehrlich's reagent down the side; read the color below the xylene layer.\n\n![](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002Findole-test-results.jpg)Figure: Indole Test Results: Positive (Right Tube)-development of Red-ring\n\n### Where students actually get confused\n\n- **Reagent choice isn't optional.** Ehrlich's (or DMACA\u002Fcinnamaldehyde) is more sensitive and is the only reagent suitable for anaerobes and fastidious organisms like *Cardiobacterium hominis* — Kovac's alone can miss weak producers in this group.\n- **Don't inoculate from MacConkey or EMB agar.** The dyes in these media interfere with color interpretation. Use a non-selective medium for the colony you're testing.\n- **Don't use Mueller-Hinton agar colonies either** — tryptophan is destroyed during the acid hydrolysis used to make the medium, which can produce a false negative.\n- **A negative spot test isn't the final answer.** If it's negative, run the tube method — some organisms are weak producers that only the more sensitive tube test will catch.\n- **Watch pigmented colonies on a spot test.** A naturally pigmented organism (for example the blue-green of *Pseudomonas aeruginosa*) can tint the filter paper and be misread. Judge the color of the reagent reaction, not the colony's own color, and confirm in a tube if unsure.\n- **Avoid plates with a nitrate disk nearby.** Nitrate can cause a false-negative spot indole reaction.\n\n## Result Interpretation\n\n| Reaction | Reagent | Result |\n| --- | --- | --- |\n| Positive | Benzaldehyde (Kovac's\u002FEhrlich's) | Brown-red to purple-red color within 20 seconds |\n| Positive | Cinnamaldehyde (DMACA) | Blue to blue-green color |\n| Negative | Either | Colorless or pale yellow |\n\n**Reporting:**\n\n- *E. coli*, most *Enterobacteriaceae*, *Vibrio*, *Aeromonas*, *Plesiomonas*, and *Pasteurella* species — indole positive\n- Most strains of *P. vulgaris*, *M. morganii*, and *Providencia* — indole positive\n- Fastidious Gram-negatives like *Cardiobacterium hominis* and *Pasteurella bettyae* — indole positive\n- *Propionibacterium acnes* — indole positive\n\n## The \"OK VIP\" Trick\n\nThree closely related genus-pairs trip students up every exam season because one species is indole-positive and its close relative is indole-negative:\n\n| Genus | Indole-positive species | Indole-negative species |\n| --- | --- | --- |\n| *Klebsiella* | *K. oxytoca* | *K. pneumoniae* |\n| *Citrobacter* | *C. koseri* | *C. freundii* |\n| *Proteus* | *P. vulgaris* | *P. mirabilis* |\n\nRemember it as **\"OK VIP\"**: **O**xytoca (Klebsiella), **K**oseri (Citrobacter), **V**ulgaris (Proteus) → **I**ndole **P**ositive. Everything else in that trio defaults to negative.\n\nThe *Klebsiella* split is worth more than an exam mark. *K. oxytoca*, the indole-positive one, is the species behind **antibiotic-associated hemorrhagic colitis**: it produces a toxin (tilivalline) that damages colonic mucosa, and it classically emerges when penicillins suppress competing flora. So \"the indole-positive *Klebsiella*\" is not just a practical-exam distinction; it names the *Klebsiella* you think of when a patient develops bloody diarrhea after starting an antibiotic. A mnemonic you can attach to a real clinical picture is one you keep.\n\n## Multitest Media That Include Indole\n\nIn diagnostic labs, indole is rarely tested alone — it's usually read off a combination medium:\n\n| Medium | Tests for |\n| --- | --- |\n| [SIM](https:\u002F\u002Fmicrobeonline.com\u002Fsulfide-indole-motility-sim-medium) | Indole + Motility + H₂S |\n| MIU | Indole + Motility + Urease |\n| MIO | Indole + Motility + Ornithine decarboxylase |\n\n## Quality Control\n\n- Discard benzaldehyde reagents (Kovac's\u002FEhrlich's) if the color isn't pale yellow before use.\n- QC every new reagent lot before use; QC in-house reagents weekly — they degrade, especially if not kept at 4°C.\n\n| Organism | Expected result |\n| --- | --- |\n| *E. coli* ATCC 25922 | Positive |\n| *Pseudomonas aeruginosa* ATCC 27853 | Negative |\n| *Porphyromonas asaccharolytica* ATCC 25260 (Ehrlich's, anaerobes) | Positive |\n| *Bacteroides fragilis* ATCC 25285 (Ehrlich's, anaerobes) | Negative |\n\n## Limitations\n\n1. The growth medium must contain adequate tryptophan — avoid [Mueller-Hinton agar](https:\u002F\u002Fmicrobeonline.com\u002Fmueller-hinton-agar\u002F) colonies.\n2. Avoid plates with a nitrate disk nearby (false negatives).\n3. Only cinnamaldehyde reagent is validated for spot-testing anaerobes — it's more sensitive but less stable than benzaldehyde reagents.\n4. A negative spot test should be confirmed with the tube method before reporting indole-negative.\n5. Fastidious Gram-negative rods (e.g., *C. hominis*) may need a heavier inoculum and extraction step.\n\n**References and further readings**\n\n1. MacWilliams MP. Indole test protocol. American Society for Microbiology, 2009.\n2. Leber AL, editor. *Clinical Microbiology Procedures Handbook.* 4th ed. Washington, DC: ASM Press; 2016. doi:10.1128\u002F9781555818814\n3. Tille PM. *Bailey and Scott's Diagnostic Microbiology.* 15th ed. St. Louis: Elsevier; 2022.",[46,49,52,55],{"question":47,"answer":48},"What's the difference between Kovac's and Ehrlich's reagent?","Both are benzaldehyde-based and give the same pink-to-red color, but Ehrlich's uses a higher concentration of reagent and an extraction step with xylene, making it more sensitive for anaerobes and fastidious organisms that Kovac's can miss.",{"question":50,"answer":51},"Why is my indole test negative when I expected E. coli to be positive?","Check three things first: was the colony picked from MacConkey\u002FEMB\u002FMueller-Hinton agar (all can interfere)? Was a nitrate disk nearby on the plate? And was the spot test read within 20 seconds? If all three are ruled out, confirm with the tube method before reporting negative.",{"question":53,"answer":54},"Is the indole test enough to identify E. coli on its own?","No — it's a presumptive test. Combine it with oxidase and colony morphology on MacConkey for a same-day presumptive call, but confirm with full biochemical\u002Fserological identification before finalizing a report.",{"question":56,"answer":57},"Why does the Klebsiella oxytoca versus Klebsiella pneumoniae indole result matter clinically?","Because K. oxytoca, the indole-positive species, is the cause of antibiotic-associated hemorrhagic colitis. It produces the toxin tilivalline and tends to emerge when antibiotics suppress competing gut flora, presenting as bloody diarrhea after starting an antibiotic. K. pneumoniae, the indole-negative species, is not associated with this syndrome. So the indole split that students learn as OK VIP also separates two clinically different organisms.",[],[],[61,67,74,79,83,87,92,97,101,105],{"slug":62,"name":39,"description":63,"image":64,"body":65,"postCount":66},"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":68,"name":69,"description":70,"image":71,"body":72,"postCount":73},"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":75,"name":76,"description":77,"image":38,"body":38,"postCount":78},"sushmita-baniya","Sushmita Baniya","Author \u002F Contributor",32,{"slug":80,"name":81,"description":77,"image":38,"body":38,"postCount":82},"samikshya-acharya","Samikshya Acharya",20,{"slug":84,"name":85,"description":77,"image":38,"body":38,"postCount":86},"alisha-tripathi","Alisha Tripathi",6,{"slug":88,"name":89,"description":90,"image":38,"body":38,"postCount":91},"aastha-shrestha","Aastha Shrestha"," Author \u002F Contributor",10,{"slug":93,"name":94,"description":95,"image":38,"body":38,"postCount":96},"guest-author","Guest Author","Guest Author \u002F Contributor",2,{"slug":98,"name":99,"description":77,"image":38,"body":38,"postCount":100},"srijana-khanal","Srijana Khanal",18,{"slug":102,"name":103,"description":95,"image":38,"body":38,"postCount":104},"dr-poonam-acharya","Dr. Poonam Acharya",1,{"slug":106,"name":107,"description":77,"image":38,"body":108,"postCount":109},"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]