[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"$fxLN3MUwXCdr5RPjwZYIDpOj8CHyjOmngWTgoKXPtZbg":3,"$faK-20eK-72T3ONf2MpmkdgTCikpeASdtxVFvjBWDtfM":32,"$f3Ft0rKFJHppdzE-vuveecxx1BUcg9iOlMLtyzf_MJDg":244},[4,8,12,16,20,24,28],{"title":5,"slug":6,"path":7},"About Microbeonline.com","about-microbeonline-com","\u002Fabout-microbeonline-com\u002F",{"title":9,"slug":10,"path":11},"About Me","about-me","\u002Fabout-microbeonline-com\u002Fabout-me\u002F",{"title":13,"slug":14,"path":15},"Advertise with Us","advertise-us","\u002Fadvertise-us\u002F",{"title":17,"slug":18,"path":19},"Privacy Policy","privacy-policy","\u002Fprivacy-policy\u002F",{"title":21,"slug":22,"path":23},"Abbreviations","abbreviations","\u002Fabbreviations\u002F",{"title":25,"slug":26,"path":27},"Microbes","microbes","\u002Fmicrobes\u002F",{"title":29,"slug":30,"path":31},"Books","recommended-books","\u002Frecommended-books\u002F",{"type":33,"data":34},"blog",{"slug":35,"title":36,"description":37,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":40,"lastUpdatedDate":40,"draft":41,"category":42,"image":43,"body":44,"faq":45,"tags":61,"related":63},"leucine-aminopeptidase-lap-test-principle-procedure-results","Leucine Aminopeptidase (LAP) Test: Principle, Procedure, Results","LAP (leucine aminopeptidase) test: principle, procedure, and expected results, and how it works alongside PYR to identify catalase-negative Gram-positive cocci.",null,"Acharya Tankeshwar","2026-07-07",false,"biochemical-tests","https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002Flap-test-microbeonline.png","A blood culture flags positive. The Gram stain shows Gram-positive cocci in pairs and short chains, and the catalase test is negative, so you are somewhere in the crowded family of streptococci, enterococci, and their look-alikes. Then the vancomycin disk shows the organism growing right up to the edge of the zone: it is vancomycin resistant. In a hospitalized patient, the first worry is vancomycin-resistant *Enterococcus* (VRE).\n\nLeucine Aminopeptidase (LAP) test is used to identify catalase-negative, Gram-positive cocci. Specifically, it helps differentiate *Aerococcus* and *Leuconostoc* from *Streptococcus*, *Enterococcus*, *Lactococcus*, and *Pediococcus*.\n\nLAP is used for the preliminary characterization of *Streptococcus*-like organisms, and it is only one of a battery of tests for identifying catalase-negative, Gram-positive cocci. It is almost always read next to the [PYR test](https:\u002F\u002Fmicrobeonline.com\u002Fpyrrolidonyl-arylamidase-pyr-test-principle-procedure-results\u002F), and the streptococcal identification panel (API Rapid Strep) includes a LAP reaction as well. Because nearly all of these cocci are LAP positive, the informative result is usually the negative one: a negative LAP points you toward *Aerococcus* or *Leuconostoc* rather than the larger LAP-positive group.\n\n### Principle\n\nThe LAP disk is a rapid test for detecting the enzyme leucine aminopeptidase. A disk impregnated with L-leucine-beta-naphthylamide serves as the substrate. Leucine aminopeptidase cleaves the substrate and releases beta-naphthylamine, which then condenses with the **cinnamaldehyde** reagent to produce a red color. No color means the enzyme is absent.\n\n### Quality control\n\nBefore use, perform QC on each new lot or shipment of LAP disks and color reagent. Because the test is performed infrequently and most Gram-positive cocci are LAP positive, verify potency by testing the positive control each time a negative result is obtained with a test organism.\n\n1. *Enterococcus faecalis* ATCC 29212 — LAP positive\n2. *Aerococcus viridans* ATCC 11563 — LAP negative\n\n### Procedure\n\n1. Before inoculation, slightly dampen the LAP disk with reagent-grade water. Do not supersaturate the disk.\n2. Using a wooden applicator stick, rub a small amount of several colonies from an 18 to 24 hour pure culture onto a small area of the LAP disk.\n3. Let the disk stand at room temperature for 5 minutes. This is the time the enzyme needs to act on the substrate; no reagent is added yet.\n4. After this reaction time, add 1 drop of cinnamaldehyde reagent and read the color.\n\n![](\u002FLAP Test Microbeonline.png)*Image: Disk showing a positive LAP test*\n\n### Expected results\n\n1. **Positive:** development of a deep red to reddish-purple color within 3 minutes after adding the cinnamaldehyde reagent.\n2. **Weak reaction:** pink.\n3. **Negative:** no color change, or development of a slight yellow color.\n\n### LAP Test Results\n\nThe LAP reactions of some catalase-negative, Gram-positive cocci are:\n\n- **LAP negative:** *Leuconostoc*\n- **LAP positive:** *Streptococcus*, *Enterococcus*, *Lactococcus*, and *Pediococcus*\n- **LAP variable:** *Aerococcus* (*A. viridans*, the negative control strain, is typically LAP negative)\n\nRead alongside PYR, LAP places an unknown into a genus quickly:\n\n| PYR | LAP | Points toward |\n| --- | --- | --- |\n| Positive | Positive | *Enterococcus*, *Streptococcus pyogenes*, *Gemella* |\n| Positive | Negative | *Aerococcus* |\n| Negative | Positive | *Streptococcus* (viridans group), *Lactococcus*, *Pediococcus* |\n| Negative | Negative | *Leuconostoc* |\n\n### Limitations\n\n1. LAP is one of several tests for identifying catalase-negative, Gram-positive cocci. It is most helpful when kit tests give an uncommon identification, or when the Gram stain does not show the expected cocci in chains.\n2. An insufficient inoculum can produce a false-negative result. When a negative is obtained, confirm disk potency with the positive control before reporting.\n\n## How to Remember\n\n- **The negative is the news.** Almost every catalase-negative Gram-positive coccus is LAP positive, so a positive tells you little. A negative is the flag that narrows you to *Aerococcus* or *Leuconostoc*. When you read a LAP disk, ask yourself \"is this the boring expected positive, or the useful negative?\"\n- **Two-enzyme spot panel.** Think of LAP and PYR as a pair you read together. PYR splits off *Enterococcus* and *S. pyogenes* (both PYR positive); LAP splits off *Aerococcus* and *Leuconostoc* (LAP negative). The single organism that is PYR positive **and** LAP negative is *Aerococcus*, which is the one clean corner of the grid worth memorizing.\n- **Same red, different reagent.** LAP and PYR both end in a red color from the same released beta-naphthylamine, which is exactly why students mix them up. Anchor the difference on the reagent: **LAP uses cinnamaldehyde; PYR uses dimethylamino-cinnamaldehyde (DMACA).** Different enzyme, different substrate, same colored finish line.\n\n## Key exam facts in one table\n\n| Feature | Detail |\n| --- | --- |\n| Enzyme detected | Leucine aminopeptidase |\n| Substrate | L-leucine-beta-naphthylamide (on the disk) |\n| Reagent added | Cinnamaldehyde |\n| Reaction chemistry | Enzyme releases beta-naphthylamine, which reacts with cinnamaldehyde to give red |\n| Positive result | Deep red to reddish-purple within 3 minutes |\n| Negative result | No change or faint yellow |\n| Positive control | *Enterococcus faecalis* ATCC 29212 |\n| Negative control | *Aerococcus viridans* ATCC 11563 |\n| LAP-positive genera | *Streptococcus*, *Enterococcus*, *Lactococcus*, *Pediococcus* |\n| LAP-negative | *Leuconostoc* (*Aerococcus* variable) |\n| Read together with | PYR test (and gas from glucose, vancomycin susceptibility) |\n| Main use | ID of catalase-negative Gram-positive cocci, including the vancomycin-resistant workup |\n\n## Where Students Get Confused\n\n- **LAP vs PYR.** This is the number-one trap. Both are rapid enzyme spot tests, both use a beta-naphthylamide substrate, and both finish in a red color. They are *different* tests detecting *different* enzymes (leucine aminopeptidase vs pyrrolidonyl arylamidase) with *different* reagents (cinnamaldehyde vs DMACA). Do not treat them as interchangeable.\n- **Thinking a positive is diagnostic.** Because most of these cocci are LAP positive, a positive rarely narrows anything. The discriminating value is in the negative.\n- **The \"5 minutes\" vs \"3 minutes.\"** These are two different clocks. The 5 minute wait is the enzyme acting on the substrate *before* reagent is added. The 3 minute window is how fast the red color should appear *after* cinnamaldehyde is added. They are not contradictory.\n- **The \"variable\" *Aerococcus*.** Students want a clean plus or minus. *Aerococcus* is genuinely variable by species, which is why *A. viridans* serves as the negative QC strain while other *Aerococcus* can read positive. Interpret LAP for *Aerococcus* alongside PYR, not on its own.\n- **Which vancomycin-resistant coccus is which.** *Leuconostoc* and *Pediococcus* are intrinsically vancomycin resistant and can be mistaken for VRE. LAP helps separate them: *Leuconostoc* is LAP negative (and produces gas from glucose), while *Pediococcus* is LAP positive (no gas).\n\n**References**\n\n1. Leber AL, editor. *Clinical Microbiology Procedures Handbook*. 4th edition. Washington, DC: ASM Press; 2016.\n2. Procop GW, Church DL, Hall GS, et al. *Koneman's Color Atlas and Textbook of Diagnostic Microbiology*. 7th edition. Philadelphia: Wolters Kluwer; 2017.\n3. Tille PM. *Bailey & Scott's Diagnostic Microbiology*. 15th edition. St. Louis: Elsevier; 2022.",[46,49,52,55,58],{"question":47,"answer":48},"What does a positive LAP test indicate?","A positive LAP test shows the organism produces leucine aminopeptidase, seen as a deep red to reddish-purple color within 3 minutes of adding cinnamaldehyde. Most catalase-negative Gram-positive cocci are LAP positive, including Streptococcus, Enterococcus, Lactococcus, and Pediococcus, so a positive result mainly confirms you are in this broad group rather than pinning down a single genus.",{"question":50,"answer":51},"What is the difference between the LAP test and the PYR test?","They detect different enzymes. LAP detects leucine aminopeptidase using an L-leucine-beta-naphthylamide substrate and cinnamaldehyde reagent, while PYR detects pyrrolidonyl arylamidase using a pyrrolidonyl-beta-naphthylamide substrate and DMACA reagent. Both release beta-naphthylamine and end in a red color, which is why they are easy to confuse, but they are read together to place an organism into the right genus.",{"question":53,"answer":54},"Which organisms are LAP negative?","Leuconostoc is reliably LAP negative, and Aerococcus is variable (Aerococcus viridans, the negative control strain, is typically LAP negative). Because nearly everything else in this group is LAP positive, a negative LAP is the useful clue that narrows the identification toward these organisms.",{"question":56,"answer":57},"Why is the LAP test useful in a vancomycin-resistant Gram-positive coccus?","Leuconostoc and Pediococcus are intrinsically resistant to vancomycin and can be mistaken for vancomycin-resistant Enterococcus. LAP helps separate them: Leuconostoc is LAP negative and produces gas from glucose, while Pediococcus is LAP positive and produces no gas. Run with PYR and a gas check, LAP helps confirm whether you have a true Enterococcus or a resistant look-alike.",{"question":59,"answer":60},"What causes a false-negative LAP result?","An insufficient inoculum is the common cause. Rubbing too few colonies onto the disk can leave too little enzyme to generate color. Whenever a negative is obtained, confirm disk potency with the positive control (Enterococcus faecalis ATCC 29212) before reporting.",[62],"gram-positive-cocci",[64,80,94,107,120,154,189,215],{"slug":65,"title":66,"description":67,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":68,"lastUpdatedDate":69,"draft":41,"category":70,"image":38,"faq":71,"tags":78},"gram-positive-cocci-of-medical-importance","Gram Positive Cocci of Medical Importance","Gram positive cocci by arrangement, clusters, chains, pairs, and tetrads, covering Staphylococcus, Streptococcus, Enterococcus, and Micrococcus with key identification tests","2022-09-09","2026-07-18","bacteriology",[72,75],{"question":73,"answer":74},"What are the main genera of gram-positive cocci of medical importance?","The most clinically significant genera are Staphylococcus, Streptococcus, and Enterococcus. Micrococcus, Peptococcus, and Peptostreptococcus are also gram-positive cocci but are rare pathogens, mostly normal flora.",{"question":76,"answer":77},"How does cell arrangement (clusters, chains, pairs, tetrads) help identify gram-positive cocci?","Arrangement under the microscope narrows identification before any biochemical test is run: clusters suggest Staphylococcus, chains suggest Streptococcus, pairs (diplococci) suggest S. pneumoniae or Enterococcus, and tetrads suggest Micrococcus. This is typically followed by the catalase test to confirm the genus-level call.",[62,79],"bacterial-classification",{"slug":81,"title":82,"description":83,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":84,"lastUpdatedDate":85,"draft":41,"category":70,"image":38,"faq":86,"tags":93},"staphylococcus-saprophyticus","Staphylococcus saprophyticus: Properties, Pathogenesis, and Lab Diagnosis","Staphylococcus saprophyticus morphology, urease and adhesin virulence factors, and the novobiocin test used to confirm this cause of UTI in young women","2022-06-23","2026-07-04",[87,90],{"question":88,"answer":89},"Why is Staphylococcus saprophyticus often missed if labs use the standard 100,000 CFU\u002FmL cutoff for UTI?","S. saprophyticus UTIs are a recognized exception to the usual significant-bacteriuria threshold. Colony counts below 100,000 CFU\u002FmL can still represent a real infection, especially in young, sexually active women with consistent symptoms across sequential specimens.",{"question":91,"answer":92},"What's the difference between Staphylococcus saprophyticus and Staphylococcus epidermidis?","Both are coagulase-negative staphylococci, but they're separated by the novobiocin susceptibility test: S. saprophyticus is resistant, S. epidermidis is sensitive. Clinically, S. saprophyticus causes UTIs in young women, while S. epidermidis is more associated with catheter and prosthetic device infections.",[62],{"slug":95,"title":96,"description":97,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":98,"lastUpdatedDate":85,"draft":41,"category":70,"image":38,"faq":99,"tags":106},"difference-staphylococcus-micrococcus"," Staphylococcus vs. Micrococcus: Key Differences and Tests","How to tell Staphylococcus and Micrococcus apart: morphology, catalase, bacitracin, furazolidone, and microdase test results compared.","2015-11-24",[100,103],{"question":101,"answer":102},"How can you tell Staphylococcus and Micrococcus apart if both are catalase-positive?","Catalase doesn't separate them since both genera are catalase-positive. Differentiation relies on other tests: bacitracin (Staph resistant, Micrococcus sensitive), furazolidone (Staph sensitive, Micrococcus resistant), lysostaphin (Staph sensitive, Micrococcus resistant), and the microdase test (Staph negative, Micrococcus positive).",{"question":104,"answer":105},"Is Micrococcus ever a real pathogen, or always a contaminant?","Usually a contaminant, since it's normal skin flora, but it can cause genuine opportunistic infection in immunocompromised or catheterized patients. It shouldn't be dismissed automatically just because it's typically harmless.",[62],{"slug":108,"title":109,"description":110,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":111,"lastUpdatedDate":85,"draft":41,"category":70,"image":38,"faq":112,"tags":119},"difference-staphylococcus-streptococcus","Staphylococcus vs. Streptococcus: Differences and Comparison Table","Compare Staphylococcus and Streptococcus by morphology, catalase test, hemolysis pattern, and diseases caused, with a full comparison table.","2015-11-06",[113,116],{"question":114,"answer":115},"Does a Gram stain alone tell you whether an infection is Staphylococcus or Streptococcus?","Largely yes, for a first impression. Clusters point to Staphylococcus, chains point to Streptococcus, and this distinction is often used to guide empiric antibiotic choice before culture results return. Confirmation still relies on the catalase test.",{"question":117,"answer":118},"Why is catalase the key test for separating Staphylococcus from Streptococcus?","Catalase positivity is consistent across Staphylococcus and negative across Streptococcus, making it a fast, reliable, single-step test that pairs directly with the morphology seen on Gram stain (clusters with catalase-positive, chains with catalase-negative).",[62],{"slug":121,"title":122,"description":123,"seoTitle":124,"seoDescription":125,"author":39,"createdDate":126,"lastUpdatedDate":69,"draft":41,"category":42,"image":38,"faq":127,"tags":152},"catalase-test-principle-uses-procedure-results","Catalase Test: The 3-Second Test That Separates Staph from Strep, and Five Ways It Lies","Bubbles in 3 seconds means Staphylococcus. 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.","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",[128,131,134,137,140,143,146,149],{"question":129,"answer":130},"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":132,"answer":133},"Why is the catalase test important in clinical microbiology?","It separates Staphylococcus (catalase-positive) from Streptococcus and Enterococcus (catalase-negative), guiding further identification. It also helps identify Mycobacterium tuberculosis and differentiate Bacillus from Clostridium.",{"question":135,"answer":136},"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":138,"answer":139},"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":141,"answer":142},"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":144,"answer":145},"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":147,"answer":148},"Are all Staphylococcus species catalase positive?","Almost all Staphylococcus species are catalase positive, distinguishing them from Streptococcus and Enterococcus. Rare catalase-negative staphylococcal strains exist, so results should be interpreted with other tests.",{"question":150,"answer":151},"What is pseudocatalase and which bacteria produce it?","Pseudocatalase is a cytochrome-based mechanism in some Enterococcus and Lactobacillus strains that weakly decomposes H₂O₂, producing delayed weak bubbling after 20-30 seconds — unlike the immediate vigorous bubbling of true catalase-positive organisms.",[153,62],"gram-negative-rods",{"slug":155,"title":156,"description":157,"seoTitle":158,"seoDescription":159,"author":39,"createdDate":160,"lastUpdatedDate":161,"draft":41,"category":162,"image":38,"faq":163,"tags":188},"blood-agar-composition-preparation-uses-and-types-of-hemolysis","Blood Agar: Composition, Preparation, and How to Read Hemolysis","Blood agar composition and preparation, how to tell alpha, beta, gamma, and alpha-prime hemolysis apart, and the double-zone target pattern, with a colony-appearance table for 20+ organisms and common modifications (chocolate, CNA, CVBA).","Blood Agar: Preparation, Hemolysis Patterns, and Identification Clues","Learn blood agar composition and preparation, distinguish alpha, beta, and gamma hemolysis, and use colony patterns to support bacterial identification.","2013-08-22","2026-07-22","culture-media",[164,167,170,173,176,179,182,185],{"question":165,"answer":166},"What is the difference between alpha and beta hemolysis?","Alpha: partial lysis, green\u002Fbrown discoloration — S. pneumoniae, viridans streptococci. Beta: complete clear lysis — S. pyogenes, S. agalactiae, S. aureus. Gamma: no hemolysis — Enterococcus, Klebsiella.",{"question":168,"answer":169},"Why is sheep blood used instead of human blood?","Consistent availability, no biohazard risk, reliable hemolysis patterns. Human blood may contain antibiotics or inhibitors and introduces infection risk.",{"question":171,"answer":172},"Why does S. pneumoniae produce alpha not beta hemolysis?","The H₂O₂ produced by S. pneumoniae oxidizes hemoglobin to green products (verdohemoglobin), a partial degradation rather than true lysis. S. pneumoniae lacks the streptolysins O and S that produce the complete, clear lysis of beta hemolysis.",{"question":174,"answer":175},"What does the size of the beta-hemolytic zone tell you?","GAS (S. pyogenes): large zone 2-4× colony diameter. GBS (S. agalactiae): narrow zone barely beyond colony edge. Helps preliminary differentiation at 24 hours with CAMP test and bacitracin.",{"question":177,"answer":178},"What is the umbilicated colony appearance of S. pneumoniae?","Autolysin LytA causes central autolysis at 48-72 hours — raised ring with sunken centre. Umbilicated appearance + alpha hemolysis = strong presumptive S. pneumoniae.",{"question":180,"answer":181},"How does incubation atmosphere affect blood agar hemolysis?","Streptolysin O is oxygen-labile — best seen in stab areas or anaerobically. Streptolysin S is oxygen-stable — visible aerobically on surface. Always stab blood agar.",{"question":183,"answer":184},"Why does C. perfringens produce double-zone hemolysis?","Theta-toxin: outer partial (alpha) zone. Alpha-toxin\u002Flecithinase: inner complete (beta) zone. Double-zone target pattern on anaerobic blood agar = strong presumptive C. perfringens.",{"question":186,"answer":187},"Can blood agar be used for susceptibility testing?","Yes — MH-F (Mueller-Hinton + 5% sheep blood) is CLSI-recommended for fastidious organisms: S. pneumoniae, S. pyogenes, H. influenzae, N. gonorrhoeae.",[62],{"slug":190,"title":191,"description":192,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":193,"lastUpdatedDate":194,"draft":41,"category":42,"image":38,"faq":195,"tags":214},"novobiocin-susceptibility-test-principle-procedure-and-interpretations","Novobiocin Susceptibility Test: How One Disk Tells S. saprophyticus From a Contaminant","A 5 ug novobiocin disk separates S. saprophyticus, a real cause of UTI in young women, from S. epidermidis, usually just skin contamination. Full procedure, the corrected 16mm breakpoint, and the species this test does and doesn't apply to.","2013-07-21","2026-07-13",[196,199,202,205,208,211],{"question":197,"answer":198},"What does the novobiocin susceptibility test actually identify?","It presumptively distinguishes Staphylococcus saprophyticus, which is novobiocin-resistant, from other coagulase-negative staphylococci like S. epidermidis, which are novobiocin-susceptible. It is most reliable when performed on urinary isolates from young, sexually active women.",{"question":200,"answer":201},"What is the breakpoint for a susceptible result?","A zone diameter of 16 mm or greater is reported as susceptible. Anything below 16 mm is reported as resistant.",{"question":203,"answer":204},"Why is this test unreliable outside urinary specimens?","Other novobiocin-resistant coagulase-negative staphylococci species, such as S. cohnii, S. xylosus, and S. kloosii, also exist. Outside urinary isolates from the typical patient population, a resistant result cannot be assumed to mean S. saprophyticus.",{"question":206,"answer":207},"What does novobiocin actually inhibit in the bacterial cell?","Novobiocin blocks DNA gyrase, an enzyme required for DNA replication, which is why susceptible organisms fail to grow within the diffusion zone around the disk.",{"question":209,"answer":210},"Why does it matter clinically whether an isolate is S. saprophyticus or S. epidermidis?","S. saprophyticus is a genuine cause of urinary tract infection, particularly in young sexually active women, while S. epidermidis isolated from urine is usually a skin contaminant. Confusing the two can lead to either missing a real infection or unnecessarily treating a contaminated sample.",{"question":212,"answer":213},"What are the quality control strains for this test?","Staphylococcus saprophyticus ATCC 15305 is used as the resistant positive control, and Staphylococcus epidermidis ATCC 12228 is used as the susceptible negative control.",[62],{"slug":216,"title":217,"description":218,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":219,"lastUpdatedDate":220,"draft":41,"category":42,"image":38,"faq":221,"tags":243},"bacitracin-test-principle-procedure-expected-results-and-quality-control","Bacitracin Test: Principle, Procedure, Results","Bacitracin susceptibility test principle, procedure, and zone interpretation for presumptive identification of Streptococcus pyogenes (Group A Strep)","2013-05-17","2026-07-12",[222,225,228,231,234,237,240],{"question":223,"answer":224},"Does the bacitracin test need a minimum zone size to count as sensitive?","No. Unlike the optochin test, which uses zone-size cutoffs, the bacitracin test is a binary call: any visible zone of inhibition around the disk counts as sensitive, and no zone at all is resistant.",{"question":226,"answer":227},"Can organisms other than Streptococcus pyogenes be bacitracin sensitive?","Yes. Group C and G streptococci occasionally show susceptibility to the 0.04 IU bacitracin disk as well. When the result is ambiguous, a PYR test resolves it, S. pyogenes is the only beta-hemolytic streptococcus that gives a positive PYR reaction.",{"question":229,"answer":230},"Why does it matter which agar the bacitracin disk is placed on in a throat culture?","When bacitracin and optochin are both used in a combined respiratory culture workup, bacitracin should be placed on chocolate agar (where it also suppresses normal flora and improves detection of Haemophilus influenzae), while optochin goes on blood agar. Using the wrong medium for either disk can affect the reading.",{"question":232,"answer":233},"How does the bacitracin test identify Group A streptococci?","Group A beta-hemolytic streptococci (Streptococcus pyogenes) are inhibited by the small amount of bacitracin in a 0.04 unit Taxo A disk, while most other beta-hemolytic streptococci are not. So any zone of inhibition around the disk is a presumptive identification of Group A strep, and no zone is resistant. The test is a binary call: any visible zone counts as sensitive, with no minimum size cutoff, which is different from the optochin test where zone size matters.",{"question":235,"answer":236},"Why is the PYR test used alongside bacitracin?","Because bacitracin susceptibility is not fully specific to Group A strep: Lancefield groups C and G streptococci occasionally show susceptibility too, which can cause a false-positive Group A call. PYR resolves it, since Streptococcus pyogenes is the only beta-hemolytic streptococcus that is PYR positive. A bacitracin-sensitive, PYR-positive beta-hemolytic streptococcus is a confident presumptive Group A strep.",{"question":238,"answer":239},"Why is there no zone-size cutoff in the bacitracin test?","Because the 0.04 unit differential disk gives a binary result: any visible zone of inhibition, however small, is read as sensitive, and no zone is resistant. This differs from the optochin test, which uses a measured cutoff (14 mm or greater around a 6 mm disk). Importing optochin's measurement logic into the bacitracin test is a common error; here the presence or absence of any zone is what matters.",{"question":241,"answer":242},"Why should bacitracin be read from a pure subculture rather than a primary plate?","Because reading straight off a primary culture plate is less reliable. Mixed flora and overcrowded colonies near the disk are the main causes of misreads. If a primary-plate result looks ambiguous, the correct next step is a purified subculture, not a second look at the same plate. A light inoculum can also produce a misleading zone, so confluent growth from a pure culture gives the most reliable reading.",[62],[245,251,258,263,267,271,276,281,285,289],{"slug":246,"name":39,"description":247,"image":248,"body":249,"postCount":250},"acharya-tankeshwar","Editor-in-chief","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Ftankeshwar-acharya-author-microbeonline.jpg","***Tankeshwar Acharya, MSc (Medical Microbiology)***\n\n*Tankeshwar Acharya is an Assistant Professor in the Department of Microbiology at Patan Academy of Health Sciences (PAHS), Nepal, where he has been teaching and practicing clinical microbiology for over 14 years. He is the founder of Microbe Online, one of the leading free microbiology education resources on the web, covering bacteriology, mycology, parasitology, immunology, and clinical laboratory diagnostics written from direct experience in both the classroom and the diagnostic laboratory.*",433,{"slug":252,"name":253,"description":254,"image":255,"body":256,"postCount":257},"ashma-shrestha","Ashma Shrestha","SEO Copywriter and Science Communicator\nKathmandu, Nepal","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Fashma-shrestha.png","Ashma Shrestha holds a Master of Science in Medical Microbiology from the Institute of Science and Technology (IOST), Tribhuvan University, Nepal, where she developed a strong foundation in virology, molecular biology, and diagnostic microbiology.\n\nShe now works as an SEO Copywriter at Resolution Digital, where she combines her scientific training with research-driven content strategy. She is certified in Google Analytics and Google Business Profile (GBP), and brings a data-informed approach to science communication writing content that is not only accurate but structured to reach and serve the students who need it most.\n\nAt microbeonline, Ashma contributes articles primarily in virology and molecular biology, areas she finds most compelling for their mechanistic depth and their growing clinical relevance. Her writing reflects the same standard the site is built on: factual rigor, clear explanation of the *why* behind microbiology concepts, and content that helps students move from memorization to genuine understanding.\n\nShe is passionate about making complex microbiological concepts accessible without sacrificing accuracy; a skill that sits at the intersection of her scientific training and her professional work in content and SEO.",81,{"slug":259,"name":260,"description":261,"image":38,"body":38,"postCount":262},"sushmita-baniya","Sushmita Baniya","Author \u002F Contributor",32,{"slug":264,"name":265,"description":261,"image":38,"body":38,"postCount":266},"samikshya-acharya","Samikshya Acharya",20,{"slug":268,"name":269,"description":261,"image":38,"body":38,"postCount":270},"alisha-tripathi","Alisha Tripathi",6,{"slug":272,"name":273,"description":274,"image":38,"body":38,"postCount":275},"aastha-shrestha","Aastha Shrestha"," Author \u002F Contributor",10,{"slug":277,"name":278,"description":279,"image":38,"body":38,"postCount":280},"guest-author","Guest Author","Guest Author \u002F Contributor",2,{"slug":282,"name":283,"description":261,"image":38,"body":38,"postCount":284},"srijana-khanal","Srijana Khanal",18,{"slug":286,"name":287,"description":279,"image":38,"body":38,"postCount":288},"dr-poonam-acharya","Dr. Poonam Acharya",1,{"slug":290,"name":291,"description":261,"image":38,"body":292,"postCount":293},"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]