[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"$fxLN3MUwXCdr5RPjwZYIDpOj8CHyjOmngWTgoKXPtZbg":3,"$fUkLn83-eybcOX5ft71H8igT4aOL4tZ0m0lkks5-xxs4":32,"$f3Ft0rKFJHppdzE-vuveecxx1BUcg9iOlMLtyzf_MJDg":153},[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":36,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":39,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"body":43,"faq":44,"tags":45,"related":47},"diarrheagenic-escherichia-coli","Diarrheagenic Escherichia coli",null,"Acharya Tankeshwar","2013-02-12","2026-07-18",false,"bacteriology","*Escherichia coli (E. coli)* is a bacterium commonly found in the gut of humans and warm-blooded animals. Most *E. coli* are normal flora or commensals found in the intestinal tract. It is a gram-negative short rod, a member of the family *Enterobacteriaceae*.\n\nPathogenic strains of *E. coli* are distinguished from normal flora by possessing virulence factors such as exotoxins. Diarrheagenic strains of *Escherichia coli* are Shiga toxin-producing *E. coli* **(STEC or EHEC), enterotoxigenic *E. coli* (ETEC),** enteropathogenic *E. coli* **(EPEC),** enteroinvasive *E. coli* **(EIEC)**, and enteroaggregative *E. coli* (EAEC). The role of STEC and ETEC in disease has been firmly established. However, the roles of the other *E. coli* pathotypes, EIEC, EPEC, and EAEC, are less clear.\n\n![Escherichia coli in EMB agar (EHEC) - Escherichia coliin EMB](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FEscherichia-coli-in-EMB-Agar.jpg)Figure: *Escherichia coli* in EMB\n\n## Enterotoxigenic E. coli (ETEC)\n\nDiarrhea caused by ETEC is usually watery, self-limited, and of short duration (1-3 days) which is frequently associated with travel and is also known as traveler’s diarrhea. It is transmitted by contaminated food and water.\n\n| **Strains of E. coli** | **Virulence Factors** | **Disease** |\n| --- | --- | --- |\n| Enterotoxigenic E. coli (ETEC) | Pili helps in colonization in GI tract. Heat-labile (LT) and heat-stable (ST) enterotoxins mediate the secretion of water and electrolytes into the bowel lumen. | Travelers and childhood diarrhea characterized by profuse, watery stools. |\n| Enteroinvasive E. coli (EIEC) | Virulence factors are uncertain, but the organism invades enterocytes lining the large intestine in a manner nearly identical to Shigella spp . | Dysentery; is usually seen in young children living in areas of poor sanitation. |\n| Enteropathogenic E. coli (EPEC) | Bundle-forming pilus, intimin, and other factors mediate the organism’s attachment to mucosal cells of the small bowel, resulting in changes in cell surface (i.e., loss of microvilli) | Diarrhea in infants in developing\u002Funderdeveloped countries; can cause chronic diarrhea. |\n| Enterohemorrhagic E. coli (STEC) | Toxin similar to Shiga toxin produced by Shigella dysenteriae . Most frequently associated with certain serotypes such as E. coli O157: H7 | Hemorrhagic colitis can also lead to hemolytic uremic syndrome (HUS). |\n| Enteroaggregative E. coli (EAEC) | Probably involves binding by pili, ST-like, and hemolysis-like toxins; actual pathogenesis unknown. | Watery diarrhea |\n\n## Enteropathogenic E. coli (EPEC)\n\nEnteropathogenic *Escherichia coli* (EPEC) is one of the diarrheagenic *E. coli* pathotypes. It is one of the leading causes of persistent diarrhea in children. The ability to produce attaching and effacing (A\u002FE) lesions is a hallmark phenotype of EPEC. EPECs are identified based on the presence of specific virulence genes.\n\n## Enteroinvasive E. coli (EIEC)\n\nEnteroinvasive *E. coli* (EIEC), one of the significant pathotypes of *E. coli*, are a group of intracellular pathogens able to enter epithelial cells of the colon, multiply within them, and move between adjacent cells with a mechanism similar to *Shigella*. EIEC are etiological agents of [bacillary dysentery](\u002Fdifference-amoebic-dysentry-bacillary-dysentery\u002F) in humans, particularly in low-income countries.\n\n## Enteroaggregative E. coli (EAEC)\n\nEnteroaggregative *E. coli* (EAEC) is a heterogeneous group of *E. coli* strains that can cause acute and persistent diarrhea in children, especially <2 yr old and malnourished children. EAEC infections are caused by exposure to food or water contaminated with human or animal waste. EAEC causes diarrhea in children worldwide and adults who travel to developing countries.\n\n## Enterohemorrhagic E. coli (EHEC)\n\nEnterohemorrhagic *E. coli* (EHEC) **are also referred to as verocytotoxin-producing *E. coli*** (VTEC) or Shiga-like toxin-producing *E. coli* **(STEC).** EHEC are verotoxigenic *E. coli* (as they produce one or both verocytoxins vt1 and vt2.\n\nInfection with EHEC results in a dysentery-like syndrome characterized by bloody diarrhea, abdominal cramping, and fever similar to that caused by *Shigella.*\n\n### Carriers\u002FReservoirs of infections\n\nThe reservoirs for EHEC O157:H7 are ruminants, particularly cattle and sheep, which are infected asymptomatically and shed the organism in feces. Other animals such as rabbits and pigs can also carry this organism.\n\n### Mode of transmission\n\nHumans acquire EHEC O157:H7 by direct contact with animal carriers, their feces, and contaminated soil or water, or by ingesting contaminated food (such as raw or undercooked meat products, unpasteurized milk & dairy products) and contaminated raw vegetables). **The infectious dose is very low, which increases the risk of disease.**\n\n### Disease\n\nEHEC can cause severe foodborne disease, diarrhea, or hemorrhagic colitis (*bloody diarrhea without pus cells*) in humans. Hemorrhagic colitis occasionally progresses to **hemolytic uremic syndrome (HUS)**, a significant cause of acute renal failure in children and morbidity and mortality in adults.\n\n### Laboratory Diagnosis\n\nWhen infection with EHEC O157:H7 is suspected (hemorrhagic colitis), a presumptive diagnosis can be made by **isolating sorbitol non-fermenting** *E. coli* on sorbitol **MacConkey agar**. The colonies can be identified as *E. coli* O157 by testing with specific O157 antiserum, available as a latex agglutination test.\n\n**References**\n\n1. Gomes, T. A., Elias, W. P., Scaletsky, I. C., Guth, B. E., Rodrigues, J. F., Piazza, R. M., Ferreira, L. C., & Martinez, M. B. (2016). Diarrheagenic Escherichia coli. *Brazilian journal of microbiology : \\[publication of the Brazilian Society for Microbiology\\]*, *47 Suppl 1*(Suppl 1), 3–30. \u003Chttps:\u002F\u002Fdoi.org\u002F10.1016\u002Fj.bjm.2016.10.015>\n2. Nguyen, T. V., Le Van, P., Le Huy, C., Gia, K. N., & Weintraub, A. (2005). Detection and characterization of diarrheagenic Escherichia coli from young children in Hanoi, Vietnam. *Journal of clinical microbiology*, *43*(2), 755–760. \u003Chttps:\u002F\u002Fdoi.org\u002F10.1128\u002FJCM.43.2.755-760.2005>",[],[46],"enterobacteriaceae",[48,73,80,87,104,110,140,146],{"slug":49,"title":50,"description":51,"seoTitle":37,"seoDescription":37,"author":52,"createdDate":53,"lastUpdatedDate":54,"draft":41,"category":55,"image":37,"faq":56,"tags":72},"mixed-acid-fermentation","Mixed Acid Fermentation: The Pathway That Makes E. coli Methyl Red-Positive","Mixed acid fermentation splits glucose into a mixture of strong acids, lactate, acetate, formate, succinate, plus ethanol, in variable proportions. That flood of acid drops the pH below 4.4, which is exactly what the methyl red test detects. Here is the pathway, the enzymes, why \"mixed\" is the whole point, and how it differs from the 2,3-butanediol route that VP detects.","Ashma Shrestha","2023-08-21","2026-07-15","biochemical-tests",[57,60,63,66,69],{"question":58,"answer":59},"Why is E. coli methyl red-positive?","Because E. coli carries out mixed acid fermentation. When it ferments glucose anaerobically, it produces a mixture of strong acids, lactic, acetic, formic, and succinic, along with ethanol, all at once. These acids accumulate and drive the pH of the medium below 4.4. The methyl red test detects exactly this: methyl red turns red at pH 4.4 or below, so a mixed-acid fermenter like E. coli gives a positive (red) methyl red result. The test is essentially a readout of whether the organism ran mixed acid fermentation.",{"question":61,"answer":62},"What makes mixed acid fermentation different from other fermentations?","It produces several end products simultaneously in variable proportions, rather than one product in a fixed amount. Homolactic fermentation makes only lactate; alcoholic fermentation makes ethanol and CO2. Mixed acid fermentation makes a mixture, lactic, acetic, formic, and succinic acids plus ethanol and gas, and the exact proportions vary with the organism and conditions. This variable mixture is what the name refers to and is why it drives the pH low enough to be detected by the methyl red test.",{"question":64,"answer":65},"What is the difference between mixed acid fermentation and the 2,3-butanediol pathway?","They are the two roads enteric bacteria take from pyruvate. Mixed acid fermentation produces strong acids that lower the pH and is detected by the methyl red test; E. coli, Salmonella, Shigella, and Proteus take this road. The 2,3-butanediol pathway produces mostly neutral products (acetoin and 2,3-butanediol), spares the pH, and is detected by the Voges-Proskauer test; Klebsiella, Enterobacter, and Serratia take this road. The two are largely mutually exclusive, which is why the methyl red and VP tests usually give opposite results.",{"question":67,"answer":68},"Why do some mixed acid fermenters produce gas and others do not?","Gas production depends on a specific enzyme, formate hydrogen-lyase, which splits the formic acid made during mixed acid fermentation into carbon dioxide and hydrogen. A mixed-acid fermenter that has this enzyme produces gas, seen as a bubble in a Durham tube. One that lacks or has limited formate hydrogen-lyase, such as Shigella or Salmonella Typhi, still ferments glucose to acid but produces no gas, making it anaerogenic. So acid production and gas production are separate features.",{"question":70,"answer":71},"Why is mixed acid fermentation important in biotechnology?","Because its end products, ethanol, succinate, lactate, and acetate, are commercially valuable, and the pathway's flexibility makes it tunable. Since E. coli can be directed toward one product or another, strains have been metabolically engineered to over-produce specific end products, for example ethanol as a biofuel or succinate as a chemical feedstock. The same feature that makes the pathway produce a variable mixture makes it a useful target for metabolic engineering, with redox balance being the main constraint.",[46],{"slug":74,"title":75,"description":75,"seoTitle":37,"seoDescription":37,"author":52,"createdDate":76,"lastUpdatedDate":77,"draft":41,"category":42,"image":37,"faq":78,"tags":79},"klebsiella-oxytoca-properties-and-pathogenesis","Klebsiella oxytoca: Properties and Pathogenesis","2022-07-12","2026-07-05",[],[46],{"slug":81,"title":82,"description":82,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":83,"lastUpdatedDate":84,"draft":41,"category":42,"image":37,"faq":85,"tags":86},"yersinia-pestis-properties-disease-diagnosis","Yersinia pestis: Properties, Disease, Lab Diagnosis","2020-04-24","2026-07-19",[],[46],{"slug":88,"title":89,"description":90,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":91,"lastUpdatedDate":92,"draft":41,"category":55,"image":37,"faq":93,"tags":103},"kliglers-iron-agar-kia-principle-procedure-and-results","Kligler’s Iron Agar (KIA): Principle, Procedure, Results","A faint black line at the slant-butt junction is easy to overlook, and it's exactly the reaction that can point toward Salmonella Typhi. Full KIA principle, tube-reading rules, and the KIA\u002FTSI distinction explained.","2019-04-30","2026-07-12",[94,97,100],{"question":95,"answer":96},"Why does KIA give an alkaline slant\u002Facid butt (K\u002FA) result for glucose-only fermenters?","KIA contains glucose (0.1%) and lactose (1.0%) at a 10:1 concentration ratio. A glucose-only fermenter initially produces acid throughout the tube, turning both slant and butt yellow. However, glucose is quickly exhausted. The organism then shifts to aerobic oxidative metabolism on the slant surface, oxidatively deaminating peptone to produce alkaline amine compounds — the slant reverts to red\u002Falkaline. The anaerobic butt retains the acid from initial glucose fermentation. Result: K\u002FA. Organisms that ferment lactose (10x more concentrated) maintain sustained acid production throughout the tube, giving A\u002FA.",{"question":98,"answer":99},"What is the critical difference between Vibrio cholerae results on KIA vs TSI?","Vibrio cholerae ferments glucose and sucrose but NOT lactose. On KIA (which contains only glucose and lactose): the organism ferments glucose only, giving a K\u002FA result — identical to Salmonella or Shigella. On TSI (which contains glucose, lactose, AND sucrose): V. cholerae also ferments sucrose, giving an A\u002FA result. This distinction is diagnostically critical — in a cholera outbreak setting, a K\u002FA result on KIA alone could lead to V. cholerae being worked up as Salmonella. Always use TSI rather than KIA alone when V. cholerae is clinically suspected, and confirm with oxidase test, string test, and serology.",{"question":101,"answer":102},"If a KIA tube shows heavy black precipitate throughout, how should the butt colour be interpreted?","A heavily H₂S-positive KIA tube where black iron sulphide precipitate obscures the butt colour should always be interpreted as acid (A) in the butt — regardless of whether yellow colour is visible. H₂S is only produced in an acidic environment, so the presence of H₂S itself confirms the butt is acid. Never report an H₂S-positive tube as K\u002FK. The blackening in the butt indicates glucose fermentation (acid) plus H₂S production, making the full reading: K\u002FA + H₂S positive — consistent with Salmonella (non-Typhi) or Citrobacter.",[46],{"slug":105,"title":106,"description":106,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":107,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":108,"tags":109},"klebsiella-pneumoniae-properties-virulence-diseases-diagnosis","Klebsiella pneumoniae: Properties, Diseases, Lab Diagnosis","2019-03-26",[],[46],{"slug":111,"title":112,"description":113,"seoTitle":114,"seoDescription":115,"author":38,"createdDate":116,"lastUpdatedDate":40,"draft":41,"category":55,"image":37,"faq":117,"tags":139},"api-20e-test-system-introduction-procedure-results-interpretations","API 20E Test: Procedure, Reading the 21 Reactions, and the 7-Digit Profile Code","How to set up, incubate, and read the API 20E strip: which wells need oil, which need reagents, how to run the 21st test (oxidase), and how to build the 7-digit profile number for identification.","API 20E: Inoculation, Reading, Profile Number, and Identification","Prepare and inoculate an API 20E strip, add reagents, read biochemical reactions, calculate the profile number, and interpret organism identification.","2015-05-06",[118,121,124,127,130,133,136],{"question":119,"answer":120},"How many tests are in the API 20E, 20 or 21?","The strip has 20 wells, but a complete identification uses 21 reactions. The oxidase test is performed separately, off the strip, and fills the last position in the profile code.",{"question":122,"answer":123},"Which API 20E wells need a mineral oil overlay?","Five: ADH, LDC, ODC, URE, and H₂S. The oil creates the anaerobic conditions these reactions need. Without it, they read falsely.",{"question":125,"answer":126},"Which wells need reagents added after incubation?","Three: TDA (ferric chloride), IND (Kovács' reagent), and VP (KOH followed by α-naphthol). Add these only after reading every self-developing well.",{"question":128,"answer":129},"Why does the VP well take longer to read?","The pink-red color from acetoin detection can take up to 10 minutes to develop. Do not call VP negative before then. TDA and IND, by contrast, are read almost immediately.",{"question":131,"answer":132},"How is the 7-digit profile number generated?","The 21 reactions are grouped into seven triplets. Within each triplet the wells score 1, 2, and 4 from top to bottom; you add up only the positives, giving a digit from 0 to 7. The seven digits form the profile, which you look up in apiweb or the API catalog.",{"question":134,"answer":135},"What do I do if the profile gives a doubtful or low-confidence identification?","apiweb reports a %ID and a T-value; a low or non-discriminating result means you need supplementary tests (such as oxidase, nitrate reduction, or motility) or a repeat run, rather than accepting the closest match.",{"question":137,"answer":138},"Can API 20E identify organisms other than Enterobacteriaceae?","It is designed for Enterobacteriaceae and other non-fastidious Gram-negative rods. It is not suitable for fastidious organisms or non-fermenters outside its database scope, which need different panels.",[46],{"slug":141,"title":142,"description":142,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":143,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":144,"tags":145},"shigella-disease-properties-pathogenesis-and-laboratory-diagnosis","Shigella: Disease, Properties, Pathogenesis, Lab Diagnosis","2013-05-18",[],[46],{"slug":147,"title":148,"description":148,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":149,"lastUpdatedDate":77,"draft":41,"category":150,"image":37,"faq":151,"tags":152},"e-coli-only-bacteria-that-wins-record-number-of-nobel-prizes","E. coli: Only bacterium that wins records number of Nobel Prizes","2013-05-04","general-microbiology",[],[46],[154,160,166,171,175,179,184,189,193,197],{"slug":155,"name":38,"description":156,"image":157,"body":158,"postCount":159},"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":161,"name":52,"description":162,"image":163,"body":164,"postCount":165},"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":167,"name":168,"description":169,"image":37,"body":37,"postCount":170},"sushmita-baniya","Sushmita Baniya","Author \u002F Contributor",32,{"slug":172,"name":173,"description":169,"image":37,"body":37,"postCount":174},"samikshya-acharya","Samikshya Acharya",20,{"slug":176,"name":177,"description":169,"image":37,"body":37,"postCount":178},"alisha-tripathi","Alisha Tripathi",6,{"slug":180,"name":181,"description":182,"image":37,"body":37,"postCount":183},"aastha-shrestha","Aastha Shrestha"," Author \u002F Contributor",10,{"slug":185,"name":186,"description":187,"image":37,"body":37,"postCount":188},"guest-author","Guest Author","Guest Author \u002F Contributor",2,{"slug":190,"name":191,"description":169,"image":37,"body":37,"postCount":192},"srijana-khanal","Srijana Khanal",18,{"slug":194,"name":195,"description":187,"image":37,"body":37,"postCount":196},"dr-poonam-acharya","Dr. Poonam Acharya",1,{"slug":198,"name":199,"description":169,"image":37,"body":200,"postCount":201},"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]