[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"$fxLN3MUwXCdr5RPjwZYIDpOj8CHyjOmngWTgoKXPtZbg":3,"$fB-70zFY9bcuVm8w-e9uZtTzxjdvc2svMAWeny3OoLpc":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},"methyl-red-mr-test-principle-procedure-results","Methyl Red (MR) Test: Principle, Procedure, Results & MR-Positive Organisms","MR test principle, procedure, and the mixed-acid vs. butanediol fermentation logic that tells you which organisms will turn red — without memorizing two separate lists.",null,"Acharya Tankeshwar","2014-01-24","2026-07-11",false,"biochemical-tests","The Methyl Red (MR) test determines whether an organism performs **mixed acid fermentation** and produces enough stable acid end products to drop the broth's pH to 4.4 or below. It's the second test in the [IMViC panel](https:\u002F\u002Fmicrobeonline.com\u002Fimvic-tests-principle-procedure-and-results\u002F), and it's always run alongside its mirror-image partner, the [Voges-Proskauer test](https:\u002F\u002Fmicrobeonline.com\u002Fvoges-proskauer-test-principle-procedure-results\u002F) — same broth, opposite logic.\n\n## Why It Matters\n\nEvery Enterobacteriaceae member ferments glucose to pyruvic acid the same way. What happens *next* to that pyruvate is where the family splits into two camps, and that split is exactly what MR and VP detect:\n\n- **Mixed acid pathway** → large amounts of stable acid (lactic, acetic, succinic) → pH drops below 4.4 → **MR positive**\n- **2,3-Butanediol pathway** → acid is converted into a neutral compound (acetoin) → pH stays near neutral → **MR negative, VP positive**\n\nThis single fork is why *E. coli* is MR+\u002FVP– and *Klebsiella*\u002F*Enterobacter* are MR–\u002FVP+ — and why, historically, this reciprocal pattern was used in public health labs to distinguish fecal coliforms (*E. coli*, implying fecal contamination of water) from environmental coliforms that don't carry the same contamination significance. The two tests were never meant to be memorized as separate organism lists — they're reading the same metabolic decision from two different angles.\n\n## Principle\n\nIn mixed acid fermentation, the organism produces enough acetic, lactic, and succinic acid to drop the medium's pH below 4.4. Methyl red (a pH indicator) is **red at pH ≤ 4.4** and **yellow at pH ≥ 5.8**, with an intermediate orange in between.\n\n![Methyl Red (MR) Test Reaction](https:\u002F\u002Fmicrobeonline.com\u002Fblogs\u002FMethyl-Red-Reaction.jpg)*Figure: Methyl Red (MR) Test Reaction*\n\nBecause methyl red only flips color at a notably low pH, the organism has to genuinely overpower the broth's buffering capacity — a weak acid producer simply won't trigger it. That's the whole point: this test only flags strong, stable acid producers.\n\n### Why MR and VP are opposite sides of one fork\n\nMR and VP are run from the same MR-VP broth because they read the two possible fates of pyruvate, and most Enterobacteriaceae commit to one or the other. After glucose is fermented to pyruvate, an organism takes one of two roads. The **mixed-acid** road produces stable acids (acetic, lactic, formic, succinic) and drives the pH below 4.4, which MR detects. The **butanediol** road produces neutral acetoin and 2,3-butanediol, spares the pH, and consumes acids that were already made, which VP detects.\n\nBecause the two roads are largely mutually exclusive, MR and VP usually give opposite results in the same organism. *E. coli* takes the mixed-acid road: MR positive, VP negative. *Klebsiella* and *Enterobacter* take the butanediol road: MR negative, VP positive. This is why the tests share a broth and why a result on one usually predicts the other. The exceptions, *Hafnia alvei* and *Proteus mirabilis*, which can be positive for both, are worth remembering precisely because they break the reciprocal rule.\n\n## Procedure\n\nMR-VP broth is shared between this test and the VP test — only the reagent added at the end differs.\n\n1. Bring the MR-VP broth to room temperature.\n2. Inoculate **two tubes** of MR-VP broth with a pure culture of the test organism.\n3. Incubate at 35°C for **at least 48 hours**.\n4. Add 5 drops of methyl red indicator to the first tube. (The second tube is reserved for the VP test, using Barritt's reagent instead.)\n5. Read the color.\n\n### Where students actually get confused\n\n- **48 hours is a minimum, not a suggestion.** Reading the tube too early is the single most common reason for a false-negative or unreliable MR result — the organism may not have produced enough acid yet to drop the pH past the threshold.\n- **Orange is not positive.** An intermediate orange color means some acid was produced, but not enough to count. Only a stable, distinct red counts as positive — if you're unsure, this usually means under-incubation, not a \"weak positive.\"\n- **Over-inoculation skews the result.** Too heavy an inoculum (above \\~10⁹ viable cells\u002FmL) inhibits growth and can produce a misleading reaction — use a standard light inoculum, not \"more is better.\"\n- **MR and VP should normally be reciprocal — but not always.** *Hafnia alvei* and *Proteus mirabilis* are the classic exceptions: both can give a positive MR *and* a positive VP (the VP reaction is often delayed). If your \"reciprocal\" expectation breaks, check whether you're looking at one of these two before assuming a technical error.\n\n## Quality Control\n\nExamine each new lot of broth for contamination, dehydration, or deterioration before use. Run one known-positive and one known-negative control organism with every new lot.\n\n| Organism | Expected result |\n| --- | --- |\n| *Escherichia coli* ATCC 25922 | MR positive (red) |\n| *Klebsiella pneumoniae* ATCC 13883 | MR negative (yellow) |\n\n## Result Interpretation\n\n| Color | Interpretation |\n| --- | --- |\n| Stable red | MR positive — pH ≤ 4.4, strong mixed-acid fermentation |\n| Yellow | MR negative — pH stayed near neutral |\n| Orange | Not a valid positive — usually under-incubation |\n\n![](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FMethyl-Red-Test-229x300.jpg)Figure: Methyl Red Test Left: Negative Right: Positive\n\n### MR-positive organisms\n\n*E. coli*, *Shigella* species, *Salmonella* species, *Citrobacter* species, *Proteus* species, *Yersinia* species\n\n### MR-negative organisms\n\n*Enterobacter* species, *Hafnia* species, *Serratia marcescens*, *Klebsiella pneumoniae*\n\n> Instead of memorizing both lists separately: if it's a strong acid producer (mixed-acid pathway), it's almost always also **VP-negative**. Learn the pathway, and both lists fall out of it — with *Hafnia alvei* and *P. mirabilis* as the two named exceptions worth remembering by name.\n\n## Limitations\n\n1. Avoid over-inoculation — bacterial growth is inhibited above \\~10⁹ viable cells\u002FmL, which can distort the reaction.\n2. Incubating for less than 48 hours risks a false-negative or unreliable intermediate color.\n\n### How to remember\n\n**MR is the acid test, and E. coli is its poster child.** Mixed-acid fermenters hold a low pH against the buffer, and methyl red only turns red at a genuinely low pH (4.4). So MR positive means the organism made a *lot* of stable acid and kept it. *E. coli*, *Shigella*, *Salmonella*, *Proteus*, *Yersinia*: MR positive. The *Klebsiella-Enterobacter* group takes the neutral road and stays yellow.\n\n**MR and VP are a see-saw.** Same broth, opposite answers. If you know one, you usually know the other. When both come back positive, suspect the two rule-breakers, *Hafnia alvei* and *Proteus mirabilis*.\n\n**Orange is not a pass.** An intermediate orange is not a weak positive. It means the acid did not reach the threshold. Reincubate and retest rather than call it. Read no earlier than 48 hours, because an under-incubated tube can mislead in both directions.\n\n**References and further readings**\n\n1. Leber AL, editor. *Clinical Microbiology Procedures Handbook.* 4th ed. Washington, DC: ASM Press; 2016. doi:10.1128\u002F9781555818814\n2. Procop GW, Church DL, Hall GS, Janda WM, Koneman EW, Schreckenberger PC, Woods GL. *Koneman's Color Atlas and Textbook of Diagnostic Microbiology.* 7th ed. Philadelphia: Wolters Kluwer; 2017.\n3. MacFaddin JF. *Biochemical Tests for Identification of Medical Bacteria.* 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 2000.\n4. Tille PM. *Bailey and Scott's Diagnostic Microbiology.* 15th ed. St. Louis: Elsevier; 2022.",[46,49,52,55,58],{"question":47,"answer":48},"Why is my MR test giving an orange color instead of a clear red or yellow?","Orange means some acid was produced but not enough to cross the pH 4.4 threshold — it isn't a \"weak positive.\" It's almost always a sign the tube needs more incubation time; re-check at the full 48 hours before reporting.",{"question":50,"answer":51},"Can an organism be both MR-positive and VP-positive?","Yes, though it's the exception rather than the rule. Hafnia alvei and Proteus mirabilis can show both reactions positive, with the VP reaction sometimes delayed. Most Enterobacteriaceae give reciprocal MR\u002FVP results.",{"question":53,"answer":54},"Why do the methyl red and Voges-Proskauer tests use the same broth?","Because they read the two possible fates of pyruvate after glucose fermentation, and most Enterobacteriaceae take one road or the other. The mixed-acid road makes stable acids and drops the pH below 4.4, which methyl red detects. The butanediol road makes neutral acetoin, sparing the pH, which the VP test detects. Since the two roads are largely mutually exclusive, the same MR-VP broth can be split and tested both ways, and the two tests usually give opposite results in the same organism.",{"question":56,"answer":57},"Why must the methyl red test be incubated for at least 48 hours?","Because methyl red only changes color at a genuinely low pH of 4.4 or below, and it takes time for a mixed-acid fermenter to produce and accumulate enough stable acid to reach that threshold. Reading earlier can give a false negative on a true positive, since the acid has not yet built up, and in some cases a misleading early color. Read no earlier than 48 hours, and extend incubation if the result is inconclusive.",{"question":59,"answer":60},"Why is over-inoculation a problem in the methyl red test?","Because too heavy an inoculum can inhibit bacterial growth once the number of viable cells exceeds about 10^9 per milliliter, which interferes with the fermentation the test depends on. Use a light, pure inoculum so the organisms grow and ferment glucose normally over the incubation period.",[],[],[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.*",432,{"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]