[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"$fxLN3MUwXCdr5RPjwZYIDpOj8CHyjOmngWTgoKXPtZbg":3,"$fYi7CDZUIEzhdQz_zmiwQCakPdslrK0BA53vx5k4BJkE":36,"$f3Ft0rKFJHppdzE-vuveecxx1BUcg9iOlMLtyzf_MJDg":269,"$fucxFBm2ZjZfGSdmdRaSNGBI_F0jJme4f0GTvzUhQfL8":334},[4,8,12,16,20,24,28,32],{"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},"Authors","authors","\u002Fauthors\u002F",{"title":25,"slug":26,"path":27},"Microbes","microbes","\u002Fmicrobes\u002F",{"title":29,"slug":30,"path":31},"Books","recommended-books","\u002Frecommended-books\u002F",{"title":33,"slug":34,"path":35},"Tags","tags","\u002Ftags\u002F",{"type":37,"data":38},"blog",{"slug":39,"title":40,"description":41,"seoTitle":42,"seoDescription":42,"author":43,"createdDate":44,"lastUpdatedDate":45,"draft":46,"category":47,"image":42,"body":48,"faq":49,"commentsClosed":46,"tags":74,"related":76,"comments":265},"salmonella-disease-properties-pathogenesis-and-laboratory-diagnosis","Salmonella: Properties, Enteric Fever, and Laboratory Diagnosis","\u003Cp>\u003Cem>Salmonella\u003C\u002Fem> classification: two species, 2,500+ serotypes, and the key split into typhoidal (S. Typhi, enteric fever, blood culture) and non-typhoidal (gastroenteritis). Pathogenesis, TSI\u002FH₂S identification, and the Widal test.\u003C\u002Fp>",null,"Acharya Tankeshwar","2013-04-27","2026-09-04",false,"bacteriology","A young adult comes in with a fever that has climbed higher each day for a week, a headache, and a dull ache in the abdomen. There is no cough and no burning micturition. In many parts of the world, this slow, step-by-step fever has one classic cause: *Salmonella* Typhi, the organism behind typhoid fever.\n\n## Overview\n\n*Salmonella* is one of the most common causes of foodborne infection worldwide. Some strains cause a brief bout of diarrhea and vomiting. One strain, *S.* Typhi, does something more dangerous: it enters the bloodstream and causes enteric fever, an illness that can last weeks and turn fatal if missed. This page is about how *Salmonella* causes disease and how the laboratory identifies it, starting with the test that matters most, blood culture.\n\n*Salmonella* is a Gram-negative, motile rod of the [family Enterobacteriaceae](https:\u002F\u002Fmicrobeonline.com\u002Fenterobacteriaceae\u002F). It swims using flagella spread over its whole surface (peritrichous). The genus has two species, *Salmonella enterica* and *Salmonella bongori*, and almost all human disease is caused by *S. enterica*. Within that species, there are more than 2,500 serotypes (serovars), named by their surface antigens, for example *S.* Typhi and *S.* Typhimurium.\n\n## Classification of *Salmonella*\n\nThe naming of *Salmonella* confuses almost everyone at first, because the everyday names (*Salmonella* Typhi, *Salmonella* Typhimurium) are not species names. Getting the levels straight makes the rest of the topic easier.\n\n**Two species.** The genus *Salmonella* has only two species: *Salmonella enterica* and *Salmonella bongori*. Almost all human disease is caused by *S. enterica*.\n\n**Six subspecies.** *S. enterica* is divided into six subspecies, of which subspecies *enterica* (subspecies I) contains nearly all the serotypes that infect humans.\n\n**More than 2,500 serotypes (serovars).** Within the species, organisms are divided into over 2,500 serotypes based on their surface antigens (the O and H antigens). The familiar names are serotypes, not species: *Salmonella* Typhi, *Salmonella* Typhimurium, and *Salmonella* Enteritidis are all serotypes of *S. enterica*.\n\nThis is why the correct written form is *Salmonella* Typhi (or *S.* Typhi), with the genus italicized and the serotype name in roman type with a capital letter, not *Salmonella typhi* in italics as though \"typhi\" were a species. The serotype is not a species, so it is not written like one.\n\n| Level | Example | How it is written |\n| --- | --- | --- |\n| Genus | *Salmonella* | Italic |\n| Species | *Salmonella enterica*, *Salmonella bongori* | Italic |\n| Subspecies | *S. enterica* subsp. *enterica* | Italic |\n| Serotype (serovar) | *Salmonella* Typhi, *Salmonella* Typhimurium | Genus italic, serotype roman and capitalized |\n\nFor everyday purposes, the level that matters clinically is the serotype, because the serotype is what determines whether the organism causes enteric fever or gastroenteritis, which is the split covered next.\n\n## Typhoidal vs non-typhoidal Salmonella\n\n*Salmonella* serotypes split into two clinically different groups. Getting this split clear is the single most useful thing on this page, because it decides what illness the organism causes and how you approach it.\n\n**Typhoidal Salmonella** (*S.* Typhi and *S.* Paratyphi A, B, C). These are adapted to humans only. They do not stay in the gut; they invade through the intestinal wall, enter the bloodstream, and cause **enteric fever** (typhoid and paratyphoid), a systemic illness lasting weeks. Blood culture, not stool culture, is how you catch them.\n\n![Classification of Salmonella](\u002Fblogs\u002FClassification-of-Salmonella.jpg)Fig: *Salmonella* classification based on their serotype\n\n**Non-typhoidal Salmonella** (NTS, such as *S.* Typhimurium and *S.* Enteritidis). These come from animals and food. In most healthy people they cause **gastroenteritis**: diarrhea, vomiting, and fever that resolves on its own in a few days. Stool is the specimen. But in infants, the elderly, and immunocompromised people (including those with HIV or sickle cell disease), NTS can invade the blood and cause **septicemia** and seed distant sites, causing **osteomyelitis**, especially in sickle cell disease.\n\nSo the same genus causes two very different pictures: a prolonged systemic fever with few gut symptoms (typhoidal), and an acute self-limited diarrhea (non-typhoidal) that occasionally turns invasive in a vulnerable host.\n\n| Feature | Typhoidal (*S.* Typhi, *S.* Paratyphi) | Non-typhoidal (*S.* Typhimurium, *S.* Enteritidis) |\n| --- | --- | --- |\n| Reservoir | Humans only | Animals and food |\n| Disease | Enteric fever (systemic, weeks) | Gastroenteritis (self-limited diarrhea) |\n| Main specimen | Blood culture | Stool culture |\n| Gut symptoms early | Few | Prominent (diarrhea, vomiting) |\n| Invasive in healthy people | Yes (that is the disease) | Uncommon; invasive in infants, elderly, immunocompromised, sickle cell |\n| Vi antigen | Present on *S.* Typhi | Absent |\n\n**The main diseases at a glance**\n\n- **Enteric fever** (typhoid, paratyphoid): typhoidal *Salmonella*, systemic, blood culture.\n- **Gastroenteritis** (enterocolitis): non-typhoidal, self-limited diarrhea, stool culture.\n- **Septicemia**: invasive NTS in vulnerable hosts.\n- **Osteomyelitis**: a known complication, classically in sickle cell disease.\n\n*Salmonella* is one of the world's most common causes of foodborne infection. It lives in the intestines of many animals, including cattle, poultry, pigs, and reptiles. One serotype is different: *S.* Typhi lives only in humans. This single fact shapes typhoid control, because there is no animal reservoir to worry about; the source of a new infection is always another human, often a chronic carrier.\n\n**How people get infected:**\n\n- Eating contaminated meat, poultry, or eggs.\n- Drinking or eating food and water contaminated with human or animal feces.\n- Contact with infected animals or their environment (reptiles are a notable source).\n- Food handled by an infected person, including a chronic carrier who feels well.\n\n## What Salmonella looks like, and what that tells you\n\n*Salmonella* is a Gram-negative, motile, non-lactose-fermenting rod of the family Enterobacteriaceae. A few features do the identification work and each rules something in or out.\n\n**It does not ferment lactose, so it is pale on MacConkey agar.** This is the first sorting step in a stool culture. *Salmonella*, like *Shigella*, stays colorless while normal gut flora such as *E. coli* turn pink. So a pale colony is the one worth chasing.\n\n**It produces H₂S.** Most salmonellae make hydrogen sulfide gas from sulfur-containing amino acids. On selective stool media such as SS agar and Hektoen enteric agar, this shows as a **black center** in the colony. That black color is the *Salmonella* signature on a stool plate and is the main visible feature that separates it from *Shigella*, which does not make H₂S and stays pale. The culture media that show this are covered in the [media used for Salmonella isolation](https:\u002F\u002Fmicrobeonline.com\u002Fmedia-used-culture-identification-salmonella\u002F), [SS agar](https:\u002F\u002Fmicrobeonline.com\u002Fsalmonella-shigella-ss-agar-composition-principle-procedure-results\u002F), and [Hektoen enteric agar](https:\u002F\u002Fmicrobeonline.com\u002Fhektoen-enteric-agar-composition-principle-uses\u002F) articles.\n\n**It is motile.** Unlike *Shigella*, which is non-motile, *Salmonella* swims. Motility plus H₂S are the two features that separate the two pale stool pathogens.\n\n**Surface antigens**\n\nThree antigens define a *Salmonella* serotype and are the basis of both serotyping and the [Widal test](https:\u002F\u002Fmicrobeonline.com\u002Fwidal-test-principle-procedure-results\u002F):\n\n- **O antigen** (cell wall lipopolysaccharide): heat-stable.\n- **H antigen** (flagellar protein): heat-labile.\n- **Vi antigen** (capsular): found on *S.* Typhi. \"Vi\" means virulence. It coats the organism, can hide the O antigen from antibody, and helps the organism resist immune attack.\n\n## Virulence factors and how Salmonella causes disease\n\n*Salmonella* is built to survive being swallowed and then to invade. Each factor below is tied to what it does.\n\n**Acid tolerance.** To reach the gut, the organism must survive the stomach. *Salmonella* has acid-tolerance systems that let enough organisms pass the stomach alive. This is why a larger dose is more likely to cause disease, and why low-acid states (antacids, young age) increase risk.\n\n**Invasion of the gut wall (Type III secretion system).** *Salmonella* injects proteins into the intestinal lining cells using a needle-like Type III secretion system. These proteins force the cell to take the organism in. This is the step that separates *Salmonella* from organisms that stay on the surface: it gets *inside* the wall.\n\n**Survival inside macrophages.** Once through the wall, *Salmonella* is engulfed by macrophages, but instead of being killed, it survives and multiplies inside them. This is the key to enteric fever. The macrophage becomes a vehicle that carries the organism through the lymphatics and into the blood, spreading it to the liver, spleen, and bone marrow.\n\n**Vi capsule (S. Typhi).** The Vi antigen coats *S.* Typhi, helping it resist being killed by complement and antibody and reducing the immune response against it. This contributes to the organism's ability to cause a prolonged systemic infection.\n\n**Endotoxin (LPS).** As organisms grow and die in the blood, LPS drives the sustained fever and the systemic features of enteric fever.\n\n### Putting it together: how enteric fever develops\n\nThe sequence explains the illness. The organism is **swallowed** and survives the stomach. It **invades** the small intestine wall using its Type III secretion system. It is taken up by **macrophages but survives inside them**, and is carried through the lymphatics into the **blood** (the first bacteremia). This seeds the liver, spleen, and bone marrow, where it multiplies and re-enters the blood in a larger **second bacteremia**, which is when the patient becomes ill with sustained fever. Because the organism is inside cells and in the blood rather than in the gut lumen, the illness is a systemic fever with few diarrheal symptoms early on, and **blood culture, not stool, is what detects it**. This is the single fact that ties the mechanism to the diagnosis.\n\nNon-typhoidal strains, by contrast, mostly stay at the gut wall and trigger a brisk local inflammation, which is why they cause diarrhea rather than a prolonged fever, except in vulnerable hosts where they too invade the blood.\n\n## Laboratory diagnosis\n\n**Why blood culture, and when**\n\nFor enteric fever, **blood culture is the mainstay**. The reason follows from the pathogenesis above: the organism is in the blood and inside cells, not in the gut lumen, so stool culture is unreliable early on. A few practical points that change the yield:\n\n- **Take blood before antibiotics.** Even one dose lowers the chance of growing the organism.\n- **Volume matters.** More blood gives a higher yield, because the number of organisms per milliliter is low. Follow your blood culture system's recommended volume. Children need less blood than adults but carry more organisms per milliliter. The general principles are covered in the [blood culture article](https:\u002F\u002Fmicrobeonline.com\u002Fblood-culture-indications-timing-and-volume\u002F).\n- **Timing.** Yield is highest in the first week of fever.\n- **Bone marrow culture** is more sensitive than blood and can stay positive even after antibiotics have started, which is useful in a patient already treated.\n\nStool and duodenal aspirate culture are used in specific situations, and stool is the specimen for detecting **chronic carriers**, who continue to shed *S.* Typhi long after recovery.\n\n### **Identifying the isolate**\n\n**On the plate.**\n\n- **MacConkey agar:** pale (non-lactose-fermenting) colonies.\n- **Blood agar:** non-hemolytic, smooth white colonies.\n- **SS and Hektoen enteric agar:** colonies with black centers, from H₂S. See the [SS agar](https:\u002F\u002Fmicrobeonline.com\u002Fsalmonella-shigella-ss-agar-composition-principle-procedure-results\u002F) and [Hektoen enteric agar](https:\u002F\u002Fmicrobeonline.com\u002Fhektoen-enteric-agar-composition-principle-uses\u002F) articles.\n\n![Nlf in macconkey agar](https:\u002F\u002Fmicrobeonline.com\u002Fcdn-cgi\u002Fimage\u002Ff=webp\u002Fblogs\u002FNLF-In-MacConkey-Agar.jpg?width=640&height=588)**The identification panel.** Each result below states what it means. For the method of any one test, follow its link.\n\n| Test | Salmonella result | What it tells you |\n| --- | --- | --- |\n| [Oxidase](https:\u002F\u002Fmicrobeonline.com\u002Foxidase-test-principle-procedure-and-oxidase-positive-organisms\u002F) | Negative | Places it in the Enterobacteriaceae. |\n| [Catalase](https:\u002F\u002Fmicrobeonline.com\u002Fcatalase-test-principle-uses-procedure-results\u002F) | Positive | Consistent with the family. |\n| Lactose (MacConkey) | Non-fermenter (pale) | Separates it from *E. coli*; groups it with *Shigella*. |\n| [TSI](https:\u002F\u002Fmicrobeonline.com\u002Ftriple-sugar-iron-agar-tsi-principle-procedure-and-interpretation\u002F) | K\u002FA, H₂S (often), gas (variable) | Alkaline slant, acid butt: ferments glucose only, not lactose\u002Fsucrose. Black in the butt from H₂S. See the pattern below. |\n| H₂S | Positive (most), weak in *S.* Typhi | The black-center signature. Separates from H₂S-negative *Shigella*. |\n| [Motility](https:\u002F\u002Fmicrobeonline.com\u002Ftests-bacterial-motility-procedure-results\u002F) | Motile | Separates from non-motile *Shigella*. |\n| [Indole](https:\u002F\u002Fmicrobeonline.com\u002Findole-test-principle-procedure-results\u002F) | Negative | Helps separate from indole-positive organisms. |\n| [Urease](https:\u002F\u002Fmicrobeonline.com\u002Furease-test-principle-procedure-interpretation-and-urease-positive-organsims\u002F) | Negative | Separates from *Proteus*. |\n| [Citrate](https:\u002F\u002Fmicrobeonline.com\u002Fcitrate-utilization-test\u002F) | Variable (see below) | *S.* Typhi is citrate-negative; *S.* Paratyphi A is negative; many other salmonellae are positive. |\n\n**The TSI and screening pattern for typhoidal Salmonella**\n\nThis is the high-yield table for enteric fever. The key contrasts are H₂S and gas.\n\n| Organism | TSI slant | TSI butt | H₂S | Gas | Citrate |\n| --- | --- | --- | --- | --- | --- |\n| *S.* Typhi | Alkaline | Acid | Weak positive | Negative | Negative |\n| *S.* Paratyphi A | Alkaline | Acid | Negative | Positive | Negative |\n| Other *Salmonella* | Alkaline | Acid | Variable | Variable | Variable |\n\n**Two things students are tested on:** *S.* Typhi makes only a **little** H₂S and **no gas**, while *S.* Paratyphi A makes **no** H₂S but **does** produce gas. That single contrast (weak H₂S\u002Fno gas vs no H₂S\u002Fgas) separates the two most important typhoidal serotypes on a TSI tube.\n\n### **Serotyping**\n\nA confirmed isolate is serotyped by testing its O and H antigens with specific antisera. The typhoidal serotypes and their antigen formulas:\n\n| Serotype | O antigen group | Specific O antigen |\n| --- | --- | --- |\n| *S.* Typhi | Group D | O9 |\n| *S.* Paratyphi A | Group A | O2 |\n| *S.* Paratyphi B | Group B | O4 |\n| *S.* Paratyphi C | Group C | O6, O7 |\n\n### The Widal test\n\nThe Widal test measures agglutinating antibodies against the O and H antigens of *S.* Typhi. It is widely used where culture is not available, but it has real limits: it can be negative in culture-proven cases, and false positives occur in malaria, other infections, and past exposure or vaccination, because *S.* Typhi shares antigens with other organisms. A single Widal result should never be read as proof of typhoid on its own. The full principle, procedure, and interpretation are in the [Widal test article](https:\u002F\u002Fmicrobeonline.com\u002Fwidal-test-principle-procedure-results\u002F).\n\n### Treatment and the resistance problem\n\nEnteric fever needs antibiotics; untreated typhoid can be fatal. The choice of drug has been reshaped by resistance, which is why this is worth understanding rather than memorizing.\n\nHistorically, typhoid was treated with chloramphenicol, then with ampicillin and co-trimoxazole. Widespread resistance to all three produced **multidrug-resistant (MDR) typhoid**. Fluoroquinolones then became first-line, but **fluoroquinolone resistance** is now common in South Asia. Third-generation cephalosporins (such as ceftriaxone) and azithromycin are the mainstays where fluoroquinolone resistance is present. **Extensively drug-resistant (XDR) typhoid**, resistant to most oral options including fluoroquinolones and third-generation cephalosporins, has emerged and is a serious regional concern.\n\nThe practical point for a student: the right drug depends on local resistance patterns, and susceptibility testing matters. Doses and durations are clinical decisions and are not covered here. Two **typhoid vaccines** (Vi polysaccharide and Vi conjugate) exist and are used in high-burden areas; they reduce but do not eliminate risk.\n\n## How to remember\n\n**Black center means Salmonella.** On a stool plate (SS or Hektoen agar), *Salmonella* colonies have a black center from H₂S. Picture the black dot. Its H₂S-negative neighbor *Shigella* stays pale. Ask yourself: two pale non-lactose colonies on MacConkey, how do I tell them apart on selective media? The one with the black center and that moves is *Salmonella*; the pale, still one is *Shigella*.\n\n**Typhi is shy with gas, Paratyphi is not.** On TSI, *S.* Typhi makes weak H₂S and no gas; *S.* Paratyphi A makes no H₂S but does make gas. A memory hook: Typhi produces the \"smell\" (H₂S) but not the \"bubbles\" (gas); Paratyphi A is the reverse.\n\n**Why blood, not stool.** Enteric fever lives in the blood and inside cells, not in the gut lumen. That is why you culture blood. Tie it to the mechanism: the organism rides inside macrophages into the bloodstream, so that is where you look for it.\n\n**Vi means virulence.** The Vi antigen of *S.* Typhi is literally named for virulence. It hides the organism from antibody. Same three letters, same idea.\n\n## Key exam facts in one table\n\n| Fact | Detail |\n| --- | --- |\n| Gram reaction \u002F shape | Gram-negative motile rod, family Enterobacteriaceae |\n| Species | *S. enterica* (almost all human disease) and *S. bongori* |\n| MacConkey | Pale (non-lactose fermenter) |\n| SS \u002F Hektoen agar | Colonies with black centers (H₂S) |\n| Motility | Motile (separates from non-motile *Shigella*) |\n| H₂S | Positive in most; weak in *S.* Typhi; negative in *Shigella* |\n| TSI, *S.* Typhi | K\u002FA, weak H₂S, no gas |\n| TSI, *S.* Paratyphi A | K\u002FA, no H₂S, gas positive |\n| Key antigens | O, H, and Vi (Vi on *S.* Typhi) |\n| Typhoidal serotypes | *S.* Typhi, *S.* Paratyphi A\u002FB\u002FC; human-only; cause enteric fever |\n| Non-typhoidal (NTS) | *S.* Typhimurium, *S.* Enteritidis; animal source; gastroenteritis, invasive in vulnerable hosts |\n| Enteric fever diagnosis | Blood culture is the mainstay; bone marrow more sensitive; take before antibiotics |\n| Widal test | Supportive only; false positives (malaria, others); never definitive alone |\n| Osteomyelitis association | Non-typhoidal *Salmonella*, classically in sickle cell disease |\n| Key virulence factors | Type III secretion (invasion), survival in macrophages, Vi capsule, acid tolerance |\n| Resistance | MDR and fluoroquinolone-resistant typhoid common in South Asia; XDR emerging |\n\n## Where students get confused\n\n**Salmonella species vs serotype.** There are only two species (*S. enterica*, *S. bongori*), but over 2,500 serotypes. *S.* Typhi and *S.* Typhimurium are serotypes of *S. enterica*, not separate species. Write them as *S.* Typhi (serotype not italicized), not *Salmonella typhi*.\n\n**Typhoidal vs non-typhoidal.** These cause different diseases from different sources. Typhoidal (human-only) causes systemic enteric fever, diagnosed by blood culture. Non-typhoidal (animal source) causes self-limited diarrhea, diagnosed by stool. Mixing them up is the most common error.\n\n**Blood vs stool culture.** For enteric fever, blood is the specimen, not stool, because the organism is systemic. Students often assume a gut infection means a stool sample. Stool is for gastroenteritis and for detecting carriers.\n\n**S. Typhi H₂S is weak.** Most salmonellae are strong H₂S producers, but *S.* Typhi makes only a little. Reading a *S.* Typhi TSI and expecting a strongly black butt can mislead. Weak H₂S and no gas is the *S.* Typhi pattern.\n\n**The Widal test is not proof.** A positive Widal is supportive at best and is frequently wrong in both directions. It does not replace culture. This is a heavily tested point.\n\n***Salmonella* Typhi vs *Shigella* on a pale plate.** Both are pale non-lactose fermenters on MacConkey. The separators are H₂S (*Salmonella* positive, black center on selective media; Shigella negative) and motility (*Salmonella* motile, Shigella non-motile).\n\n**References**\n\n1. Tille, P. M. (2022). *Bailey & Scott's Diagnostic Microbiology* (15th ed.). Elsevier.\n2. Procop, G. W., et al. (2017). *Koneman's Color Atlas and Textbook of Diagnostic Microbiology* (7th ed.). Wolters Kluwer.\n3. Coburn, B., Grassl, G. A., & Finlay, B. B. (2007). Salmonella, the host and disease: a brief review. *Immunology and Cell Biology*, 85(2), 112–118. \u003Chttps:\u002F\u002Fdoi.org\u002F10.1038\u002Fsj.icb.7100007>\n4. Crump, J. A., Sjölund-Karlsson, M., Gordon, M. A., & Parry, C. M. (2015). Epidemiology, clinical presentation, laboratory diagnosis, antimicrobial resistance, and antimicrobial management of invasive Salmonella infections. *Clinical Microbiology Reviews*, 28(4), 901–937. \u003Chttps:\u002F\u002Fdoi.org\u002F10.1128\u002FCMR.00002-15>",[50,53,56,59,62,65,68,71],{"question":51,"answer":52},"\u003Cp>Is Salmonella lactose fermenter or non-fermenter?\u003C\u002Fp>","\u003Cp>Non-fermenter. \u003Cem>Salmonella\u003C\u002Fem> does not ferment lactose, so it forms pale, colorless colonies on MacConkey agar. This separates it from \u003Cem>E. coli\u003C\u002Fem>, which ferments lactose and turns pink.\u003C\u002Fp>",{"question":54,"answer":55},"\u003Cp>Does Salmonella produce H₂S?\u003C\u002Fp>","\u003Cp>Most salmonellae produce H₂S, which shows as a black center on selective media such as SS agar and Hektoen enteric agar. \u003Cem>S.\u003C\u002Fem> Typhi produces only weak H₂S. \u003Cem>Shigella\u003C\u002Fem> does not produce H₂S at all, which helps separate the two.\u003C\u002Fp>",{"question":57,"answer":58},"\u003Cp>What are the TSI results for Salmonella Typhi?\u003C\u002Fp>","\u003Cp>Alkaline slant over acid butt (K\u002FA), with weak H₂S and no gas. It ferments glucose but not lactose or sucrose. The weak H₂S and absence of gas are the classic \u003Cem>S.\u003C\u002Fem> Typhi pattern.\u003C\u002Fp>",{"question":60,"answer":61},"\u003Cp>Why is blood culture used for typhoid instead of stool?\u003C\u002Fp>","\u003Cp>Because enteric fever is a systemic infection. The organism survives inside cells and circulates in the blood rather than staying in the gut, so blood culture detects it reliably while stool culture often does not, especially early. Bone marrow culture is even more sensitive.\u003C\u002Fp>",{"question":63,"answer":64},"\u003Cp>What is the difference between typhoidal and non-typhoidal Salmonella?\u003C\u002Fp>","\u003Cp>Typhoidal \u003Cem>Salmonella\u003C\u002Fem> (\u003Cem>S.\u003C\u002Fem> Typhi and \u003Cem>S.\u003C\u002Fem> Paratyphi) infect only humans and cause enteric fever, a systemic illness. Non-typhoidal \u003Cem>Salmonella\u003C\u002Fem> come from animals and food and usually cause self-limited diarrhea, though they can invade the blood in vulnerable people.\u003C\u002Fp>",{"question":66,"answer":67},"\u003Cp>Is the Widal test reliable for diagnosing typhoid?\u003C\u002Fp>","\u003Cp>Not on its own. It supports a diagnosis but is often falsely negative in real cases and falsely positive in malaria and other conditions. Culture is the definitive test.\u003C\u002Fp>",{"question":69,"answer":70},"\u003Cp>How do you tell Salmonella from Shigella in the laboratory?\u003C\u002Fp>","\u003Cp>Both are pale non-lactose fermenters on MacConkey. \u003Cem>Salmonella\u003C\u002Fem> is motile and produces H₂S (black colonies on selective media); \u003Cem>Shigella\u003C\u002Fem> is non-motile and does not produce H₂S.\u003C\u002Fp>",{"question":72,"answer":73},"\u003Cp>Which Salmonella infection is linked to osteomyelitis in sickle cell disease?\u003C\u002Fp>","\u003Cp>Non-typhoidal \u003Cem>Salmonella\u003C\u002Fem> is a classic cause of osteomyelitis in people with sickle cell disease.\u003C\u002Fp>",[75],"enterobacteriaceae",[77,102,111,119,125,161,195,231],{"slug":75,"title":78,"description":79,"seoTitle":42,"seoDescription":42,"author":43,"createdDate":80,"lastUpdatedDate":81,"draft":46,"category":47,"image":42,"faq":82,"tags":101},"Enterobacteriaceae: How to Identify and Tell Them Apart","A working guide to the Enterobacteriaceae: which genera matter, how lactose fermentation and biochemical tests separate them, and how to reason from a MacConkey plate to a genus.","2013-10-01","2026-08-24",[83,86,89,92,95,98],{"question":84,"answer":85},"\u003Cp>What is the single fastest test to know a Gram-negative rod belongs to the Enterobacteriaceae?\u003C\u002Fp>","\u003Cp>The oxidase test. Members of this family are oxidase-negative. If a Gram-negative rod is oxidase-positive, it is not in this family, and you should think of organisms such as \u003Cem>Pseudomonas\u003C\u002Fem> or \u003Cem>Vibrio\u003C\u002Fem> instead.\u003C\u002Fp>",{"question":87,"answer":88},"\u003Cp>Why are Salmonella and Shigella pale on MacConkey agar?\u003C\u002Fp>","\u003Cp>Because they do not ferment lactose. MacConkey agar turns pink only when an organism ferments lactose and lowers the pH. \u003Cem>Salmonella\u003C\u002Fem> and \u003Cem>Shigella\u003C\u002Fem> are non-lactose fermenters, so their colonies stay colorless. This is why a pale colony in a diarrheal stool is the one worth investigating.\u003C\u002Fp>",{"question":90,"answer":91},"\u003Cp>Is Enterobacteriaceae the same as Enterobacterales?\u003C\u002Fp>","\u003Cp>Not exactly. In 2016 the old family was reorganized into a larger order called Enterobacterales, and some genera were moved into separate families. In everyday clinical and exam use, the term Enterobacteriaceae is still used broadly for this whole group of enteric Gram-negative rods.\u003C\u002Fp>",{"question":93,"answer":94},"\u003Cp>What does IMViC stand for and why is it useful?\u003C\u002Fp>","\u003Cp>IMViC stands for Indole, Methyl red, Voges-Proskauer, and Citrate. These four tests together separate the common genera. The classic contrast is \u003Cem>E. coli\u003C\u002Fem> (+ + − −) versus \u003Cem>Klebsiella\u003C\u002Fem> and \u003Cem>Enterobacter\u003C\u002Fem> (− − + +).\u003C\u002Fp>",{"question":96,"answer":97},"\u003Cp>Which Enterobacteriaceae are the main antibiotic-resistance concerns?\u003C\u002Fp>","\u003Cp>Three groups: organisms with inducible AmpC beta-lactamase (such as \u003Cem>Enterobacter\u003C\u002Fem>, \u003Cem>Serratia\u003C\u002Fem>, \u003Cem>Citrobacter freundii\u003C\u002Fem>, \u003Cem>Klebsiella aerogenes\u003C\u002Fem>), ESBL-producers (common in \u003Cem>E. coli\u003C\u002Fem> and \u003Cem>Klebsiella\u003C\u002Fem>), and carbapenem-resistant Enterobacteriaceae (CRE), where treatment options become very limited.\u003C\u002Fp>",{"question":99,"answer":100},"\u003Cp>What is the Vi antigen?\u003C\u002Fp>","\u003Cp>It is a special capsular antigen of \u003Cem>Salmonella\u003C\u002Fem> Typhi. \"Vi\" stands for virulence. It can cover the O antigen on fresh isolates, which is why an O-antigen agglutination test may read negative until the culture is heated.\u003C\u002Fp>",[75],{"slug":103,"title":104,"description":104,"seoTitle":42,"seoDescription":42,"author":43,"createdDate":105,"lastUpdatedDate":106,"draft":46,"category":107,"image":42,"faq":108,"tags":109},"media-used-culture-identification-salmonella","Culture media for Salmonella typhi and paratyphi","2015-01-27","2026-07-13","culture-media",[],[110],"bacterial-culture-media",{"slug":112,"title":113,"description":114,"seoTitle":42,"seoDescription":42,"author":43,"createdDate":115,"lastUpdatedDate":116,"draft":46,"category":107,"image":42,"faq":117,"tags":118},"salmonella-shigella-ss-agar-composition-principle-procedure-results","Salmonella-Shigella (SS) Agar: Composition, Principle, Colony Characteristics, and Limitations","\u003Cp>SS agar is a highly selective medium for \u003Cem>Salmonella\u003C\u002Fem> isolation from stool but despite its name, it inhibits most \u003Cem>Shigella\u003C\u002Fem> strains. Learn its principle, brilliant green mechanism, colony morphology, and when to use XLD or DCA instead.\u003C\u002Fp>","2016-09-06","2026-08-14",[],[110],{"slug":120,"title":121,"description":122,"seoTitle":42,"seoDescription":42,"author":43,"createdDate":105,"lastUpdatedDate":116,"draft":46,"category":107,"image":42,"faq":123,"tags":124},"hektoen-enteric-agar-composition-principle-uses","Hektoen Enteric (HE) Agar: Composition, Principle, Colony Characteristics, and Uses","\u003Cp>Hektoen Enteric (HE) Agar is a selective and differential medium for \u003Cem>Salmonella\u003C\u002Fem> and \u003Cem>Shigella\u003C\u002Fem> with better \u003Cem>Shigella\u003C\u002Fem> recovery than SS agar. Learn its green-medium principle, three-carbohydrate differentiation, H₂S indicator system, and colony colors.\u003C\u002Fp>",[],[110],{"slug":126,"title":127,"description":128,"seoTitle":129,"seoDescription":130,"author":43,"createdDate":131,"lastUpdatedDate":132,"draft":46,"category":133,"image":42,"faq":134,"tags":159},"widal-test-principle-procedure-results","Widal Test: How to Read Titers Against a Local Baseline (with Nepal Data)","Widal test: principle, slide and tube procedure, result interpretation, diagnostic titers by region, false positives, limitations, and comparison with newer typhoid diagnostic tests. Includes Nepal-specific baseline titer data.","Widal Test: Procedure, Titers, Interpretation, and Limitations","Review Widal test antigen reactions, slide and tube procedures, regional titer interpretation, timing, limitations, and frequent causes of false results.","2015-12-01","2026-07-18","immunology",[135,138,141,144,147,150,153,156],{"question":136,"answer":137},"What is the significant titer for a positive Widal test?","No universal threshold — depends on local endemicity. Nepal baseline (Acharya T et al., JHAS 2013): anti-O >1:80, anti-H >1:160. Fourfold rise between acute and convalescent samples is the most reliable criterion.",{"question":139,"answer":140},"What is the difference between O and H agglutination?","O: somatic antigen, compact granular clumps, day 6-8, declines early — marker for active infection. H: flagellar antigen, large fluffy clumps, day 10-12, persists long — may reflect past infection or vaccination.",{"question":142,"answer":143},"Why can the Widal test be falsely positive?","Previous typhoid vaccination, past subclinical infection in endemic areas, cross-reactions with malaria, liver disease, and other Salmonella serotypes all cause false positives.",{"question":145,"answer":146},"Can the Widal test be negative in proven typhoid?","Yes — antibiotics taken before testing, testing too early (before day 6-8), immunocompromised patient, or Vi antigen interference. A negative Widal does not exclude typhoid.",{"question":148,"answer":149},"Why must baseline titers be established locally?","Endemic populations have raised background titers from subclinical exposures. Nepal research (Acharya T et al., JHAS 2013): >1:80 for anti-O and >1:160 for anti-H diagnostically significant based on 490 healthy donors.",{"question":151,"answer":152},"Is the Widal test still recommended?","Widely used in resource-limited settings. Variable sensitivity (47-77%) and specificity (50-92%). Typhidot and Tubex TF offer better specificity. Blood culture is gold standard. Widal useful with clinical judgment and local baseline titers.",{"question":154,"answer":155},"What is the best time to perform the Widal test?","Second week of illness — O antibodies day 6-8, H antibodies day 10-12. First week testing gives false negatives. Paired acute and convalescent samples with fourfold rise is gold standard.",{"question":157,"answer":158},"What does positive AH or BH mean?","AH = Salmonella Paratyphi A flagellar antigen = paratyphoid fever A. BH = Salmonella Paratyphi B = paratyphoid fever B. Cross-reactivity with H antigen can occur.",[160],"immunoassays",{"slug":162,"title":163,"description":164,"seoTitle":42,"seoDescription":42,"author":43,"createdDate":165,"lastUpdatedDate":166,"draft":46,"category":47,"image":42,"faq":167,"tags":192},"blood-culture-indications-timing-and-volume","Blood Culture: Indications, Timing, and Volume","\u003Cp>When to draw blood cultures and why, why volume matters more than timing, how many sets to collect, how to draw from a peripheral vein versus a central line, and how paired cultures diagnose a line infection.\u003C\u002Fp>","2018-09-25","2026-08-25",[168,171,174,177,180,183,186,189],{"question":169,"answer":170},"\u003Cp>Why does blood volume matter more than the timing of collection?\u003C\u002Fp>","\u003Cp>Most adult bacteremias carry very few organisms per millilitre of blood, so the more blood cultured, the higher the chance of catching one. Studies show total volume drives yield far more than the exact minute of collection.\u003C\u002Fp>",{"question":172,"answer":173},"\u003Cp>How many blood culture sets should be collected, and why not just one?\u003C\u002Fp>","\u003Cp>At least two, preferably three, each from a separate venipuncture. A single set gives too little volume and cannot distinguish a skin contaminant from a true pathogen. A contaminant usually appears in only one set, while a real pathogen appears in several.\u003C\u002Fp>",{"question":175,"answer":176},"\u003Cp>How much blood goes into each bottle for an adult?\u003C\u002Fp>","\u003Cp>About 10 mL per bottle, with two bottles (aerobic and anaerobic) per set, so roughly 20 mL per set. Under-filling is a common reason a true infection is missed.\u003C\u002Fp>",{"question":178,"answer":179},"\u003Cp>Should blood cultures be drawn from a peripheral vein or an existing line?\u003C\u002Fp>","\u003Cp>A fresh peripheral venipuncture is preferred, because drawing through a catheter picks up hub organisms and raises the false-positive rate. Draw from a line only when peripheral access fails, or deliberately as a paired sample when the line itself is the suspected source of infection.\u003C\u002Fp>",{"question":181,"answer":182},"\u003Cp>What is differential time to positivity?\u003C\u002Fp>","\u003Cp>When a catheter set and a peripheral set are drawn at the same time, the catheter bottle turns positive earlier if the line is colonized, because it carries a heavier bacterial load. A catheter set flagging positive 2 hours or more before the peripheral set supports a catheter-related bloodstream infection.\u003C\u002Fp>",{"question":184,"answer":185},"\u003Cp>Which skin antiseptic should be used before drawing blood cultures?\u003C\u002Fp>","\u003Cp>Chlorhexidine-alcohol is preferred because it gives lower contamination rates and does not need to be wiped off. Tincture of iodine is an acceptable alternative. Povidone-iodine works but must be left to dry in contact for at least 2 minutes.\u003C\u002Fp>",{"question":187,"answer":188},"\u003Cp>Can blood culture bottles be refrigerated if there is a delay?\u003C\u002Fp>","\u003Cp>No. Keep them at room temperature and get them to the incubator, for a maximum of about 4 hours. Refrigeration harms the organisms you are trying to grow.\u003C\u002Fp>",{"question":190,"answer":191},"\u003Cp>How much blood is safe to take from an infant?\u003C\u002Fp>","\u003Cp>The volume is based on the child's weight, and no more than about 4 to 4.5% of total blood volume should be taken. Pediatric bottles are designed to keep the correct blood-to-broth ratio at these smaller volumes.\u003C\u002Fp>",[193,194],"blood-culture","specimen-collection-transport",{"slug":196,"title":197,"description":198,"seoTitle":199,"seoDescription":200,"author":43,"createdDate":201,"lastUpdatedDate":202,"draft":46,"category":203,"image":42,"faq":204,"tags":229},"oxidase-test-principle-procedure-and-oxidase-positive-organisms","Oxidase Test: Purple in 10 Seconds, Delayed, or a False Negative?","Purple in 10 seconds is positive. Purple at 90 seconds means something else. No color at all from a TCBS or MacConkey plate often means the test failed, not that the organism is negative. Learn to read the oxidase test the way a bench microbiologist does.","Oxidase Test: 10-Second Procedure, Controls, and False Results","Perform the oxidase test within the correct reading window, use appropriate controls, and recognize media, reagent, and timing causes of false results.","2012-12-29","2026-08-05","biochemical-tests",[205,208,211,214,217,220,223,226],{"question":206,"answer":207},"What does a positive oxidase test indicate?","\u003Cp>Presence of cytochrome c oxidase in the electron transport chain. Does NOT mean the organism is a strict aerobe, \u003Cem>Vibrio cholerae \u003C\u002Fem>and \u003Cem>Campylobacter \u003C\u002Fem>are oxidase-positive but not strict aerobes. A positive result immediately excludes the Enterobacteriaceae family, which are all oxidase-negative.\u003C\u002Fp>",{"question":209,"answer":210},"Why are Enterobacteriaceae oxidase-negative?","\u003Cp>They lack cytochrome c oxidase, using other terminal oxidases (cytochrome bd, cytochrome bo) that do not react with TMPD reagent. Absence of cytochrome c oxidase is a defining characteristic of the entire Enterobacteriaceae family, making the oxidase test one of the most powerful single tests for separating gram-negative rods.\u003C\u002Fp>",{"question":212,"answer":213},"Why must the oxidase test not be performed from MacConkey agar?","A lactose-fermenting colony on MacConkey acidifies its immediate surroundings. Below roughly pH 5.1 the cytochrome c oxidase reaction cannot proceed, so a truly oxidase-positive organism reads negative. The dyes in MacConkey and EMB can also obscure color development. The error is a false negative, not a false positive. Always subculture to nutrient agar, blood agar, or tryptic soy agar first.",{"question":215,"answer":216},"Why does a nichrome wire give a false-positive oxidase result?","Nickel and iron in nichrome oxidize TMPD directly, producing the same purple endpoint that the enzyme would, but with no enzyme involved at all. The result looks identical and means nothing. Always use a platinum loop, wooden applicator stick, or plastic loop.",{"question":218,"answer":219},"What is the difference between the standard and modified oxidase (Microdase) test?","\u003Cp>Standard oxidase test: TMPD reagent applied to gram-negative organisms, differentiates Enterobacteriaceae from non-fermenters. Modified oxidase (Microdase): TMPD in DMSO applied to gram-positive cocci, differentiates \u003Cem>Micrococcus\u003C\u002Fem> (positive) from\u003Cem> Staphylococcus \u003C\u002Fem>(negative). Standard reagent gives unreliable results with gram-positive cocci.\u003C\u002Fp>",{"question":221,"answer":222},"Why is timing critical in the oxidase test?","TMPD undergoes spontaneous auto-oxidation in air, turning purple without any bacterial enzyme activity. Purple within 10 seconds is a true positive. Purple at 60 to 90 seconds is reported as a delayed positive. Color appearing after 2 minutes is auto-oxidation, not a result, and a reagent-only blank paper will develop the same color. Do not interpret anything after 2 minutes.",{"question":224,"answer":225},"Is Acinetobacter oxidase-positive or negative?","\u003Cp>Oxidase-NEGATIVE, an important exception among non-fermenting gram-negative rods. \u003Cem>Pseudomonas aeruginosa\u003C\u002Fem> (the other major non-fermenter) is oxidase-positive. \u003Cem>Acinetobacter\u003C\u002Fem> oxidase-negative vs \u003Cem>Pseudomonas\u003C\u002Fem> oxidase-positive is one of the key distinguishing tests between these two MDR nosocomial pathogens.\u003C\u002Fp>",{"question":227,"answer":228},"What quality control organisms are recommended for the oxidase test?","\u003Cp>\u003Cem>Pseudomonas aeruginosa\u003C\u002Fem> ATCC 27853 (positive control, dark purple within 10 seconds) and \u003Cem>Escherichia coli \u003C\u002Fem>ATCC 25922 (negative control, remains colorless). If either control fails, discard the reagent batch, prepare fresh, and repeat controls before reporting patient results.\u003C\u002Fp>",[230],"enzyme-tests",{"slug":232,"title":233,"description":234,"seoTitle":235,"seoDescription":236,"author":43,"createdDate":237,"lastUpdatedDate":238,"draft":46,"category":203,"image":42,"faq":239,"tags":264},"catalase-test-principle-uses-procedure-results","Catalase Test: The 3-Second Test That Separates Staph from Strep, and Five Ways It Lies","\u003Cp>Bubbles in 3 seconds means \u003Cem>Staphylococcus\u003C\u002Fem>. But red blood cells bubble, nichrome loops bubble, and enterococci grown on blood agar bubble weakly. Learn what the catalase test actually detects, why streptococci cannot make the enzyme, and how to tell a true positive from the four things that imitate one.\u003C\u002Fp>","Catalase Test: Procedure, Controls, False Results, and Interpretation","Run and interpret the catalase test with proper controls, distinguish staphylococci from streptococci, and avoid blood agar and loop-related false results.","2013-10-07","2026-08-16",[240,243,246,249,252,255,258,261],{"question":241,"answer":242},"What is the principle of the catalase test?","The catalase test detects the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. Visible bubbling indicates a positive result. Reaction: 2H₂O₂ → 2H₂O + O₂.",{"question":244,"answer":245},"Why is the catalase test important in clinical microbiology?","\u003Cp>It separates \u003Cem>Staphylococcus\u003C\u002Fem> (catalase-positive) from \u003Cem>Streptococcus\u003C\u002Fem> and \u003Cem>Enterococcus \u003C\u002Fem>(catalase-negative), guiding further identification. It also helps identify \u003Cem>Mycobacterium tuberculosis\u003C\u002Fem> and differentiate \u003Cem>Bacillus\u003C\u002Fem> from \u003Cem>Clostridium.\u003C\u002Fem>\u003C\u002Fp>",{"question":247,"answer":248},"What causes a false positive in the catalase test?","False positives are caused by using metal loops (which non-enzymatically decompose H₂O₂), carrying over red blood cells from blood agar, or testing on Mueller-Hinton agar.",{"question":250,"answer":251},"What causes a false negative in the catalase test?","The most common cause is using colonies older than 24 hours. Catalase production is highest during logarithmic growth; older cultures produce less enzyme, leading to insufficient bubbling.",{"question":253,"answer":254},"What is the difference between the slide and tube catalase test?","The slide test is quicker but risks RBC carryover from blood agar. The tube test is preferred for blood agar cultures as it reduces false positive risk.",{"question":256,"answer":257},"Why should you not use a metal loop in the catalase test?","Metal loops non-enzymatically decompose H₂O₂, producing bubbles that mimic a true positive result. Use a platinum loop, wooden stick, or plastic loop instead.",{"question":259,"answer":260},"\u003Cp>Are all \u003Cem>Staphylococcu\u003C\u002Fem>s species catalase positive?\u003C\u002Fp>","\u003Cp>Almost all \u003Cem>Staphylococcus\u003C\u002Fem> species are catalase positive, distinguishing them from \u003Cem>Streptococcus\u003C\u002Fem> and \u003Cem>Enterococcus.\u003C\u002Fem> Rare catalase-negative staphylococcal strains exist, so results should be interpreted with other tests.\u003C\u002Fp>",{"question":262,"answer":263},"What is pseudocatalase and which bacteria produce it?","\u003Cp>Pseudocatalase is a cytochrome-based mechanism in some \u003Cem>Enterococcus\u003C\u002Fem> and \u003Cem>Lactobacillus\u003C\u002Fem> strains that weakly decomposes H₂O₂, producing delayed weak bubbling after 20-30 seconds unlike the immediate vigorous bubbling of true catalase-positive organisms.\u003C\u002Fp>",[230],{"enabled":266,"threads":267,"total":268},true,[],0,[270,276,283,290,296,301,307,312,318,321,328],{"slug":271,"name":43,"description":272,"image":273,"body":274,"postCount":275},"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.*",502,{"slug":277,"name":278,"description":279,"image":280,"body":281,"postCount":282},"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.",79,{"slug":284,"name":285,"description":286,"image":287,"body":288,"postCount":289},"sushmita-baniya","Sushmita Baniya","Author \u002F Contributor","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Fsushmita-baniya-1.png","Sushmita Baniya holds an M.Sc. in Medical Microbiology from Tribhuvan University (National College), with a research focus in Genetics and Molecular Biology. She is actively involved in teaching and research in the field of microbiology.",26,{"slug":291,"name":292,"description":286,"image":293,"body":294,"postCount":295},"samikshya-acharya","Samikshya Acharya","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Fsamikshya-acharya.jpeg","Samikshya Sharma completed her postgraduate studies in Medical Microbiology at the Central Department of Microbiology, Tribhuvan University, Nepal. She contributes to Microbeonline with the goal of making foundational and clinical microbiology concepts clear and useful for students in medical, laboratory science, and allied health programs.",20,{"slug":297,"name":298,"description":286,"image":42,"body":299,"postCount":300},"alisha-tripathi","Alisha Tripathi","Alisha Tripathi holds an M.Sc. in Medical Microbiology from National College, Tribhuvan University. With over a year of teaching experience, her academic interests span Molecular Biology, Immunology, and Genetics.",6,{"slug":302,"name":303,"description":304,"image":42,"body":305,"postCount":306},"aastha-shrestha","Aastha Shrestha"," Author \u002F Contributor","Aastha Shrestha is a Biotechnology graduate with an M.Sc. from National College, Tribhuvan University. Her academic interests center on Molecular Biology and Immunology; two fields that are increasingly converging in modern diagnostic and clinical microbiology. \n\nShe contributes to Microbeonline with the goal of making complex concepts in these areas approachable and exam-relevant for students across medical, biotechnology, and laboratory science programs.",9,{"slug":308,"name":309,"description":310,"image":42,"body":42,"postCount":311},"guest-author","Guest Author","Guest Author \u002F Contributor",1,{"slug":313,"name":314,"description":286,"image":315,"body":316,"postCount":317},"srijana-khanal","Srijana Khanal","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Fsrijana-khanal-1.png","Srijana Khanal is a microbiology educator with nearly a decade of teaching experience, including her role as faculty in the Microbiology Department at National College, NIST. \n\nHer time in the classroom has given her a clear sense of where students struggle and what explanations actually work, a perspective that directly shapes how she writes.\n\nHer academic interests span Immunology, Genetics, Basic Sciences, and Research Methodology, and she brings the same rigor to her writing that she brought to teaching. Alongside academic writing, she has a passion for creative writing -- an instinct that shows in her ability to make dense scientific material readable without sacrificing accuracy.\n\nShe contributes to Microbeonline to extend her teaching reach beyond the classroom, helping medical and laboratory science students across the region build a stronger foundation in microbiology.",15,{"slug":319,"name":320,"description":310,"image":42,"body":42,"postCount":311},"dr-poonam-acharya","Dr. Poonam Acharya",{"slug":322,"name":323,"description":324,"image":325,"body":326,"postCount":327},"nisha-rijal","Nisha Rijal","Microbiologist and AMR Specialist Kathmandu, Nepal","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Fnisha-rijal-1.png","Nisha Rijal is a microbiologist with nearly 15 years of frontline diagnostic and surveillance experience at the National Public Health Laboratory (NPHL), national reference laboratory under the Department of Health Services, Nepal. She currently works as an AMR Support Officer at the World Health Organization (WHO), Nepal, where her work focuses on strengthening antimicrobial resistance surveillance systems and translating AMR data into actionable public health response.\n\nHer research, published in peer-reviewed journals and cited over 220 times, spans some of the most clinically significant infectious disease challenges in Nepal and South Asia: antimicrobial resistance trends in *Vibrio cholerae* across an 11-year national surveillance dataset, sero-epidemiology of scrub typhus in patients with acute febrile illness, lower respiratory tract infections in HIV-positive patients, and gonococcal resistance surveillance. She was a contributor to Nepal's National Antimicrobial Resistance Containment Action Plan, a foundational policy document for AMR governance in Nepal. You can find list of [Nisha Rijal's article here in Google Scholar.](https:\u002F\u002Fscholar.google.com\u002Fcitations?user=N-Ruq54AAAAJ&hl=en)\n\nThis depth of experience is visible in her writing at Microbeonline. Her 53 published articles cover bacteriology, parasitology, mycology, immunology, and laboratory techniques, and are consistently among the most detailed and clinically grounded content on the site. She brings to every article the same standard that national reference laboratory work demands: methodological precision, awareness of real diagnostic constraints, and an understanding of what results actually mean for patient care in resource-limited settings.\n\nHer areas of particular expertise include antimicrobial susceptibility testing and resistance mechanism detection, quality assurance in clinical microbiology, and laboratory-based infectious disease surveillance.\n\n---\n\n*Nisha Rijal contributes to Microbeonline in a personal capacity. Her views and writing do not represent the positions of the World Health Organization or any other institution.*",55,{"slug":329,"name":330,"description":331,"image":332,"body":333,"postCount":311},"padma-shrestha","Padma Shrestha","Author","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Fpadma-shrestha.png","Padma Shrestha is from Kathmandu, Nepal. She has completed Masters degree in Medical microbiology from Tribhuvan University. She has great interest in Microbiology and Molecular Biology.",[335,342,348,352,357,362,366,370,373,378,382,387,391,396,400,405,409,413,418,423,427,431,435,438,442,446,450,454,459,464,469,473,477,481,485,489,493,497,501,505,509,513,517,520,524,528,532,536,541,545,549,553,557,561,564,569,573,577,581,585,589,593,597,601,605,609,613,617,620,624,627,630,632,635,638,641,644,647,650,653,656,659,662,665,668],{"slug":336,"name":337,"description":338,"image":339,"body":340,"postCount":341},"gram-negative-cocci","Gram-Negative Cocci and Coccobacilli","Neisseria, Moraxella, Haemophilus and related gram-negative coccal organisms","https:\u002F\u002Fassets.microbeonline.com\u002Ftags\u002Fgram-negative-cocci.png","# Gram Negative Cocci\n\nNeisseria gonorrhoeae, Neisseria meningitides, Moraxella catarrhalis, and other Neisseria spp. are clinically relevant gram-negative cocci.\n\nN. gonorrhoeae is the leading cause of sexually transmitted disease whereas N. meningitides is a leading cause of fatal bacterial meningitis.",14,{"slug":343,"name":344,"description":345,"image":42,"body":346,"postCount":347},"microscopy","Microscopy","Microscope types, components, and microscopy techniques","These are list of blog posts related to microscopy. ",12,{"slug":349,"name":350,"description":351,"image":42,"body":42,"postCount":347},"gram-positive-cocci","Gram-Positive Cocci","Staphylococcus, Streptococcus, Enterococcus, Micrococcus — organisms, diseases, and identification tests",{"slug":353,"name":354,"description":355,"image":42,"body":42,"postCount":356},"gram-negative-rods","Gram-Negative Rods (Other than Enterobacteriaceae)","\u003Cp>Gram negative rods other than members of Enterobacteriaceae family such as  Pseudomonas, Acinetobacter and related organisms\u003C\u002Fp>",5,{"slug":358,"name":359,"description":360,"image":42,"body":42,"postCount":361},"gram-positive-rods","Gram-Positive Rods","Bacillus, Clostridium, Listeria, Corynebacterium, Actinomyces and related organisms",8,{"slug":363,"name":364,"description":365,"image":42,"body":42,"postCount":347},"mycobacteria","Mycobacteria","Mycobacterium tuberculosis, leprosy, atypical mycobacteria, and acid-fast organism diagnosis",{"slug":367,"name":368,"description":369,"image":42,"body":42,"postCount":347},"anaerobic-bacteriology","Anaerobic Bacteriology","Anaerobic organisms, anaerobic culture methods, and anaerobic infection diagnosis",{"slug":75,"name":371,"description":372,"image":42,"body":42,"postCount":347},"Enterobacteriaceae","Identification, differentiation, and clinical significance of Enterobacteriaceae family members",{"slug":374,"name":375,"description":376,"image":42,"body":42,"postCount":377},"spirochetes","Spirochetes","Treponema, Leptospira, Borrelia and spirochetal infections",7,{"slug":379,"name":380,"description":381,"image":42,"body":42,"postCount":341},"food-microbiology","Food Microbiology","Food-borne pathogens, food safety, spoilage, and preservation",{"slug":383,"name":384,"description":385,"image":42,"body":42,"postCount":386},"antimicrobial-susceptibility-testing","Antimicrobial Susceptibility Testing","Methods for testing antibiotic susceptibility in clinical microbiology",22,{"slug":388,"name":389,"description":390,"image":42,"body":42,"postCount":341},"antimicrobials-moa-amr","Antimicrobials (MOA & AMR)","Mechanisms, detection, and clinical significance of antimicrobial resistance",{"slug":392,"name":393,"description":394,"image":42,"body":42,"postCount":395},"sterilization-disinfection","Sterilization and Disinfection","Methods of sterilization and disinfection in healthcare and laboratory settings",10,{"slug":194,"name":397,"description":398,"image":42,"body":42,"postCount":399},"Specimen Collection and Transport","Collection, handling, and transport of clinical specimens for microbiological testing",27,{"slug":401,"name":402,"description":403,"image":42,"body":42,"postCount":404},"bacterial-structure-physiology","Bacterial Structure and Physiology","Bacterial cell structure, growth, physiology, and environmental factors affecting growth",21,{"slug":406,"name":407,"description":42,"image":42,"body":408,"postCount":300},"horizontal-gene-transfer","Horizontal Gene Transfer","Articles related to **Horizontal Gene Transfer**",{"slug":410,"name":411,"description":42,"image":42,"body":412,"postCount":395},"chromatography","Chromatography","Information about chromatographic techniques.",{"slug":414,"name":415,"description":416,"image":42,"body":417,"postCount":377},"electrophoresis","Electrophoresis","Information about Electrophoresis Techniques ","Detailed information  about Electrophoresis Techniques ",{"slug":419,"name":420,"description":421,"image":42,"body":422,"postCount":300},"pcr-techniques","PCR Techniques","Information about various types of Polymerase Chain Reaction Techniques ","More detailed information about various types of Polymerase Chain Reaction Techniques ",{"slug":424,"name":425,"description":426,"image":42,"body":42,"postCount":300},"bacteriophage","Bacteriophage","Description about Bacteriophage.",{"slug":428,"name":429,"description":430,"image":42,"body":42,"postCount":300},"malaria","Malaria","It is the collections of articles regarding malarial disease. ",{"slug":432,"name":433,"description":434,"image":42,"body":42,"postCount":300},"anaerobic-culture-techniques","Anaerobic Culture Techniques","Posts related with Anaerobic Culture Techniques.",{"slug":160,"name":436,"description":437,"image":42,"body":42,"postCount":404},"Immunoassays","You will get information about all the diagnostic tests that rely on the specific binding between an antigen and an antibody to detect or quantify a substance.",{"slug":439,"name":440,"description":441,"image":42,"body":42,"postCount":377},"biosafety-levels","Biosafety levels ","Articles related to Biosafety Levels",{"slug":443,"name":444,"description":445,"image":42,"body":42,"postCount":356},"environmental-factors","Environmental Factors ","In this case we are talking about growth requirements of microorganisms with deep dive in environmental factors that affect the growth. ",{"slug":447,"name":448,"description":449,"image":42,"body":42,"postCount":300},"pipette","Pipette","Posts related with Pipette. ",{"slug":451,"name":452,"description":453,"image":42,"body":42,"postCount":377},"bacteriology-mcqs","Bacteriology MCQs","This sections lists MCQs in Bacteriology.",{"slug":455,"name":456,"description":457,"image":42,"body":42,"postCount":458},"parasitology-mcqs","Parasitology MCQs","This section lists MCQs in Parasitology.",2,{"slug":460,"name":461,"description":462,"image":42,"body":42,"postCount":463},"virology-mcqs","Virology MCQs","This is the collections of Multiple Choice Questions in Virology.",4,{"slug":465,"name":466,"description":467,"image":42,"body":42,"postCount":468},"mcqs-in-microbiology","MCQs in Microbiology","This section lists the collections of Multiple Choice Questions in General Microbiology Topics. ",3,{"slug":470,"name":471,"description":472,"image":42,"body":42,"postCount":377},"immunology-mcqs","Immunology MCQs","In this section; we are posting collections of Multiple Choice Questions about Immunology. ",{"slug":474,"name":475,"description":476,"image":42,"body":42,"postCount":395},"microbial-curiosities","Microbial Curiosities","In this clusters, we are posting interesting and unique information about Microorganisms. ",{"slug":110,"name":478,"description":479,"image":42,"body":42,"postCount":480},"Bacterial Culture Media","Posts related to Bacterial Culture Media. ",23,{"slug":482,"name":483,"description":484,"image":42,"body":42,"postCount":300},"fungal-culture-media","Fungal Culture Media","Posts related to Fungal Culture Media.",{"slug":486,"name":487,"description":488,"image":42,"body":42,"postCount":356},"motility-test","Motility Test","This lists the procedure regarding various tests methods for bacterial motility.",{"slug":490,"name":491,"description":492,"image":42,"body":42,"postCount":395},"bacterial-enumeration","Bacterial enumeration","These posts are related to isolation and enumeration of bacteria. ",{"slug":494,"name":495,"description":496,"image":42,"body":42,"postCount":458},"gram-positive-coccobacillus","Gram-positive coccobacillus","List of Gram Positive Coccobacilli",{"slug":498,"name":499,"description":500,"image":42,"body":42,"postCount":463},"dimorphic-fungi","Dimorphic Fungi","This is about various dimorphic fungi. ",{"slug":502,"name":503,"description":504,"image":42,"body":42,"postCount":377},"bacterial-classification","Bacterial Classification","These posts are related with various approaches used for the classification of Bacteria. ",{"slug":506,"name":507,"description":508,"image":42,"body":42,"postCount":356},"immunofluorescence","Immunofluorescence ","Various Tests related to Immunofluorescence ",{"slug":510,"name":511,"description":512,"image":42,"body":42,"postCount":306},"antibody-mediated-immunity","Antibody-mediated Immunity","This clusters links the articles that are sharing insights about Antibody-mediated immunity. ",{"slug":514,"name":515,"description":516,"image":42,"body":42,"postCount":377},"hypersensitivity","Hypersensitivity","Articles related to Hypersensitivity.",{"slug":518,"name":519,"description":42,"image":42,"body":42,"postCount":468},"haemophilus","Haemophilus",{"slug":521,"name":522,"description":523,"image":42,"body":42,"postCount":300},"sexually-transmitted-infections-stis","Sexually transmitted infections (STIs)","This is the clusters of infections that are transmitted sexually. ",{"slug":525,"name":526,"description":527,"image":42,"body":42,"postCount":347},"adaptive-immunity","Adaptive Immunity","Blog posts related to B Cell Immunity and T Cell Immunity.",{"slug":529,"name":530,"description":531,"image":42,"body":42,"postCount":341},"fungal-diagnostics","Fungal Diagnostics","Various methods used for the Diagnosis of Fungal Infections. ",{"slug":533,"name":534,"description":535,"image":42,"body":42,"postCount":356},"laboratory-storage-and-preservation","Laboratory Storage and Preservation","Articles about Laboratory Storage of Antimicrobial Disk, Test organisms and Equipment used for this process. ",{"slug":537,"name":538,"description":539,"image":42,"body":540,"postCount":300},"laboratory-heating-equipment","Laboratory Heating Equipment","A guide to laboratory heating equipment, including hot plates, water baths, Bunsen burners, incubators, and dry baths, and how to choose the right one for each task.","Laboratory heating equipment covers the instruments that warm, melt, incubate, or sterilize samples and media in a microbiology laboratory. Each one delivers heat differently. \n\nA hot plate gives high, direct, dry heat; a water bath gives gentle, even, wet heat up to about 100°C; a Bunsen burner gives an open flame for rapid, very high heat; an incubator holds cultures at a steady temperature over hours or days; and a dry bath heats small tubes without water. Choosing the right one depends on the temperature you need, how precise it must be, and whether the sample can tolerate direct or open-flame heat.\n\nThe articles below cover each piece of heating equipment in detail, including its parts, working principle, uses, and the mistakes that most often go wrong at the bench.",{"slug":542,"name":543,"description":544,"image":42,"body":42,"postCount":306},"laboratory-glassware","Laboratory Glassware","Posts about Laboratory Glassware. ",{"slug":546,"name":547,"description":548,"image":42,"body":42,"postCount":306},"helminths","Helminths","In this section, we are covering properties, life cycle, pathogenesis and laboratory diagnosis of Helminths\u002FHelminthic infestations. ",{"slug":550,"name":551,"description":552,"image":42,"body":42,"postCount":300},"protozoan-parasite","Protozoan Parasite","In this cluster, we are covering protozoan parasites. ",{"slug":554,"name":555,"description":556,"image":42,"body":42,"postCount":311},"tests-for-gram-positive-cocci","Biochemical Tests for Gram Positive Cocci","This is the lists of Biochemical Tests that are used for Gram Positive Cocci. ",{"slug":558,"name":559,"description":560,"image":42,"body":42,"postCount":395},"bacterial-staining-technique","Bacterial Staining Technique","Lists of various staining techniques that are used to stain bacteria. ",{"slug":230,"name":562,"description":563,"image":42,"body":42,"postCount":404},"Enzyme Tests","\u003Cp>Various Biochemical Test that are based on enzymatic activity of the microorganisms. \u003C\u002Fp>",{"slug":565,"name":566,"description":567,"image":42,"body":42,"postCount":568},"carbohydrate-utilization","Carbohydrate Utilization","\u003Cp>Various biochemical tests which are related to Carbohydrate fermentation or Utilization\u003C\u002Fp>",11,{"slug":570,"name":571,"description":572,"image":42,"body":42,"postCount":356},"susceptibility-based-id","Susceptibility-based ID","\u003Cp>These are susceptibility based identification test such as optochin sensitivity, bacitracin sensitivity etc. \u003C\u002Fp>",{"slug":574,"name":575,"description":576,"image":42,"body":42,"postCount":463},"microbial-metabolism","Microbial Metabolism","\u003Cp>Tests about Microbial Metabolism. \u003C\u002Fp>",{"slug":578,"name":579,"description":580,"image":42,"body":42,"postCount":361},"substrate-utilization","Substrate Utilization","\u003Cp>The test in which a non-sugar carbon\u002Fnitrogen source is used or degraded (citrate, malonate, decarboxylases, indole, PAD).\u003C\u002Fp>",{"slug":582,"name":583,"description":584,"image":42,"body":42,"postCount":468},"atypical-pneumonia","Atypical Pneumonia","\u003Cp>Organisms responsible for Atypical Pneumonia. \u003C\u002Fp>",{"slug":586,"name":587,"description":588,"image":42,"body":42,"postCount":356},"antigen","Antigen","\u003Cp>Various articles related to Antigens.\u003C\u002Fp>",{"slug":590,"name":591,"description":592,"image":42,"body":42,"postCount":377},"innate-immunity","Innate Immunity","\u003Cp>Articles related to Innate Immunity. \u003C\u002Fp>",{"slug":594,"name":595,"description":596,"image":42,"body":42,"postCount":463},"respiratory-tract-infection","Respiratory Tract Infection","\u003Cp>In this cluster, you can see various etiological agents that causes respiratory tract infection. \u003C\u002Fp>",{"slug":598,"name":599,"description":600,"image":42,"body":42,"postCount":356},"torch-infection","TORCH Infection","\u003Cp>In this section; you can find articles related with TOCH infection. \u003C\u002Fp>",{"slug":602,"name":603,"description":604,"image":42,"body":42,"postCount":361},"microbiology-for-beginners","Microbiology for Beginners","\u003Cp>These articles are very basic articles, which will share general concepts in Microbiology. \u003C\u002Fp>",{"slug":606,"name":607,"description":608,"image":42,"body":42,"postCount":300},"dna-replication","DNA Replication","\u003Cp>Articles related to DNA and Replication of DNA. \u003C\u002Fp>",{"slug":610,"name":611,"description":612,"image":42,"body":42,"postCount":377},"genetic-code","Genetic Code","\u003Cp>Articles related to Genetic Code.\u003C\u002Fp>",{"slug":614,"name":615,"description":616,"image":42,"body":42,"postCount":377},"molecular-technique","Molecular Technique","\u003Cp>Posts related to Molecular Techniques. \u003C\u002Fp>",{"slug":618,"name":619,"description":42,"image":42,"body":42,"postCount":311},"colorimetric-assay","Colorimetric Assay ",{"slug":621,"name":622,"description":623,"image":42,"body":42,"postCount":356},"pharmaceutical-microbiology","Pharmaceutical Microbiology","\u003Cp>Various articles related to Pharmaceutical Microbiology\u003C\u002Fp>",{"slug":625,"name":626,"description":42,"image":42,"body":42,"postCount":468},"blood-and-immune-cells","Blood and Immune Cells",{"slug":628,"name":629,"description":42,"image":42,"body":42,"postCount":356},"host-pathogen-interaction","Host Pathogen Interaction",{"slug":193,"name":631,"description":42,"image":42,"body":42,"postCount":463},"Blood Culture",{"slug":633,"name":634,"description":42,"image":42,"body":42,"postCount":463},"environmental-microbiology","Environmental microbiology ",{"slug":636,"name":637,"description":42,"image":42,"body":42,"postCount":377},"copromicroscopic-technique","Copromicroscopic Technique",{"slug":639,"name":640,"description":42,"image":42,"body":42,"postCount":468},"quality-control","Quality Control",{"slug":642,"name":643,"description":42,"image":42,"body":42,"postCount":377},"dermatophytes","Dermatophytes",{"slug":645,"name":646,"description":42,"image":42,"body":42,"postCount":468},"viral-hemorrhagic-fevers","Viral Hemorrhagic Fevers",{"slug":648,"name":649,"description":42,"image":42,"body":42,"postCount":463},"h2s-production","H2S Production",{"slug":651,"name":652,"description":42,"image":42,"body":42,"postCount":458},"water-quality-testing","Water Quality Testing",{"slug":654,"name":655,"description":42,"image":42,"body":42,"postCount":356},"virology-basics","Virology basics",{"slug":657,"name":658,"description":42,"image":42,"body":42,"postCount":463},"typing-methods","Typing Methods",{"slug":660,"name":661,"description":42,"image":42,"body":42,"postCount":468},"blotting-technique","Blotting Technique",{"slug":663,"name":664,"description":42,"image":42,"body":42,"postCount":463},"history-microbiology","History of Microbiology",{"slug":666,"name":667,"description":42,"image":42,"body":42,"postCount":300},"trematodes","Trematodes",{"slug":669,"name":670,"description":42,"image":42,"body":42,"postCount":463},"coccidian-parasites","Coccidian Parasites"]