[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"$fxLN3MUwXCdr5RPjwZYIDpOj8CHyjOmngWTgoKXPtZbg":3,"$fgj2jyFb3EylN0JRac9fZNkXewZkpzNHZRKnYLSZN-qQ":32,"$f3Ft0rKFJHppdzE-vuveecxx1BUcg9iOlMLtyzf_MJDg":243},[4,8,12,16,20,24,28],{"title":5,"slug":6,"path":7},"About Microbeonline.com","about-microbeonline-com","\u002Fabout-microbeonline-com\u002F",{"title":9,"slug":10,"path":11},"About Me","about-me","\u002Fabout-microbeonline-com\u002Fabout-me\u002F",{"title":13,"slug":14,"path":15},"Advertise with Us","advertise-us","\u002Fadvertise-us\u002F",{"title":17,"slug":18,"path":19},"Privacy Policy","privacy-policy","\u002Fprivacy-policy\u002F",{"title":21,"slug":22,"path":23},"Abbreviations","abbreviations","\u002Fabbreviations\u002F",{"title":25,"slug":26,"path":27},"Microbes","microbes","\u002Fmicrobes\u002F",{"title":29,"slug":30,"path":31},"Books","recommended-books","\u002Frecommended-books\u002F",{"type":33,"data":34},"blog",{"slug":35,"title":36,"description":37,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":40,"lastUpdatedDate":41,"draft":42,"category":43,"image":38,"body":44,"faq":45,"tags":64,"related":66},"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.",null,"Acharya Tankeshwar","2013-07-21","2026-07-13",false,"biochemical-tests","A young, sexually active woman comes in with dysuria and urgency. Urine culture grows gram-positive cocci in clusters, catalase-positive, coagulase-negative. Without the novobiocin test, this could be dismissed as skin contamination, the same picture *S. epidermidis* would give. With it, a resistant result confirms a genuine pathogen, *S. saprophyticus*, the second most common cause of uncomplicated UTI in this population after *E. coli*. One disk is the difference between treating a real infection and writing it off as a contaminated sample.\n\nNovobiocin test is used to differentiate coagulase-negative staphylococci (CONS) and presumptively identify the isolate as *Staphylococcus saprophyticus*  (novobiocin resistant).  It is usually performed on CONS isolated from a urine sample of women of a reproductive age group.\n\n![ - Flow chart for the identification ofStaphylococcus saprophyticus](\u002Fblogs\u002FIdentification-flow-chart-for-Staphylococcus-saprophyticus.png)Figure: Flow chart for the identification of *Staphylococcus saprophyticus*\n\n## Principle of Novobiocin Test\n\n*S. saprophyticus* is second only to *E. coli* as the most frequent causative organism of uncomplicated urinary tract infections (UTIs) in young sexually active women.  So when an organism that looks like staphylococci is isolated from a woman of reproductive age [coagulase test](\u002Fdiagnostic-tests-biochemical-tests-coagulase-test\u002F) is done. If the organism is coagulase-negative i.e. CONS the laboratory must further identify that isolate and find out if this is a true pathogen (*Staphylococcus saprophyticus-novobiocin resistant*) or a contaminant (i.e. *Staphylococcus epidermidis-novobiocin sensitive*).\n\n> Mnemonic: On the office’s STAPH retreat, there was NO StRESs. (NO=novobiocin, S=Saprophyticus, R=Resistant, E=Epidermidis, S=Sensitive\n\nLaboratory identification of *S. saprophyticus* is made on the basis of the absence of hemolysis, coagulase, and **resistance to novobiocin**. Novobiocin susceptibility test results are 100% sensitive and 96% specific. *S. saprophyticus* is innately resistant to the antibiotic novobiocin. Therefore, screening coagulase-negative staphylococci from urine cultures for novobiocin resistance is reliable presumptive identification of *S. saprophyticus.*\n\n**Test organism**\n\n- Testing should be performed on isolated colonies of gram-positive cocci in clusters that are catalase-positive and coagulase-negative isolated from urine specimens, usually from sexually active young women.\n- All colonies should be taken from a [blood agar plate](\u002Fblood-agar-composition-preparation-uses-and-types-of-hemolysis\u002F) and growth must be less than 24  hours old, 15-18 hours being optimal.\n\n## Procedure of Novobiocin test\n\n1. Allow containers to come to room temperature before use.\n2. Using a pure 18-24 hour culture, prepare a suspension of the organism; equivalent to a [McFarland 0.5 opacity standard](\u002Fpreparation-mcfarland-turbidity-standards\u002F); to be identified in tryptic soy broth, sterile water,  or [brain heart infusion (BHI) broth](\u002Fbrain-heart-infusion-bhi-broth-composition-preparation-and-uses\u002F)\n3. Inoculate [Mueller Hinton Agar,](\u002Fmueller-hinton-agar\u002F) 5% blood agar, or [tryptic soy agar](\u002Ftryptic-soy-agar-tsa-composition-preparation-uses\u002F) plate with a sterile swab to obtain confluent growth.\n4. Aseptically apply one 5ug novobiocin disk onto the inoculated agar surface and lightly press down to ensure full contact with the medium.\n5. Incubate plate aerobically for 18 to 24 hours at 35 to 37°C.\n6. Measure (in millimeters) the diameter of the zone of inhibition around the novobiocin disk, and record it as susceptible or resistant.\n\n**Interpretation of Novobiocin test**\n\n1. Sensitive (susceptible) – zone diameter ≥ 16 mm\n2. Resistant – zone diameter < 16 mm (typically S. *saprophyticus* shows little to no zone, often growth up to the disk edge)\n\n![ - Novobiocin sensitivity testing A. Novobiocin-resistant(S. saprophyticus)B. Novobiocin Sensitive(S. epidermidis)](\u002Fblogs\u002Fnovobiocin-sensitivity-testing-in-microbeonline.jpg)Figure: Novobiocin sensitivity testing A. Novobiocin-resistant *(S. saprophyticus)*\\\nB. Novobiocin Sensitive *(S. epidermidis)*\n\n**Expected results:**\n\n1. *Staphylococcus saprophyticus* (resistant) – zone ≤16 mm, typically little to no zone with growth up to the edge of the disk\n2. *Staphylococcus epidermidis* (susceptible) – zone >16 mm\n\nNovobiocin is one of three disk-diffusion tests used to separate look-alike Gram-positive cocci, alongside **optochin** (for *S. pneumoniae* among the alpha-hemolytic streptococci) and **bacitracin** (for Group A strep among the beta-hemolytic streptococci). Novobiocin is the one you reach for on the **catalase-positive, coagulase-negative** side: a coagulase-negative *Staphylococcus* from urine. The catalase and coagulase results route you here. See the Gram-positive coccus disk-test grid on the [optochin page ](https:\u002F\u002Fmicrobeonline.com\u002Foptochin-test-principle-procedure-expected-results-and-quality-control\u002F)for the full routing.\n\n## Quality Control\n\nQuality control of novobiocin test should be performed per lot\u002Fshipment date with known organisms.\n\n1. Positive control (resistant) = *Staphylococcus saprophyticus*  (ATCC® 15305)\n2. Negative control (sensitive) = *Staphylococcus epidermidis* (ATCC® 12228)\n\n## Limitation\n\nThe novobiocin disk is not helpful and can give misleading results if it is performed on isolates other than those from urinary specimens. Occasional human isolates that are not *S. saprophyticus*, *S. cohnii* subsp., or *S. xylosis* may also be resistant to novobiocin.\n\n## Novobiocin-Resistant vs. Novobiocin-Susceptible CoNS\n\nKnowing the test result only matters if you know which species fall on which side of it.\n\n\u003Ctable style=\"min-width: 50px;\">\n\u003Ccolgroup>\u003Ccol style=\"min-width: 25px;\">\u003Ccol style=\"min-width: 25px;\">\u003C\u002Fcolgroup>\u003Ctbody>\u003Ctr>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cstrong>Novobiocin-Resistant (little or no zone; growth up to the disk)\u003C\u002Fstrong>\u003C\u002Fp>\u003C\u002Ftd>\u003Cth colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cstrong>Novobiocin-Susceptible (zone ≥16mm)\u003C\u002Fstrong>\u003C\u002Fp>\u003C\u002Fth>\u003C\u002Ftr>\u003Ctr>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cem>S. saprophyticus\u003C\u002Fem> (the clinically important one)\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cem>S. epidermidis\u003C\u002Fem> (the common skin contaminant)\u003C\u002Fp>\u003C\u002Ftd>\u003C\u002Ftr>\u003Ctr>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cem>S. cohnii\u003C\u002Fem>\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cem>S. capitis\u003C\u002Fem>\u003C\u002Fp>\u003C\u002Ftd>\u003C\u002Ftr>\u003Ctr>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cem>S. xylosus\u003C\u002Fem>\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cem>S. hominis\u003C\u002Fem>\u003C\u002Fp>\u003C\u002Ftd>\u003C\u002Ftr>\u003Ctr>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cem>S. kloosii\u003C\u002Fem>\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cem>S. lugdunensis\u003C\u002Fem>\u003C\u002Fp>\u003C\u002Ftd>\u003C\u002Ftr>\u003C\u002Ftbody>\n\u003C\u002Ftable>\n\nThis is exactly why the test result alone is not a species identification. A novobiocin-resistant CoNS from a urine sample in a young woman is presumptively *S. saprophyticus*, but a handful of other resistant CoNS species exist too, which is the basis for the Limitation noted above: this test is only reliable on urinary isolates, not as a general CoNS identification method.\n\n## Exam Facts\n\n|  | Answer |\n| --- | --- |\n| What does novobiocin inhibit? | DNA gyrase, blocking DNA replication |\n| Disk potency used? | 5 ug |\n| Susceptible breakpoint? | Zone diameter ≥ 16 mm |\n| Reported sensitivity and specificity of the test? | 100% sensitive, 96% specific |\n| Why is this test unreliable outside urinary isolates? | Other novobiocin-resistant CoNS species (*S. cohnii*, *S. xylosus*, *S. kloosii*) exist and would give a false impression of *S. saprophyticus* identification |\n|  |  |\n|  |  |\n\n**References**\n\n1. Afanas’eva, T. I., Shcherbakova, N. A., Givental’, N. I., & Bogdanova, L. F. (1979). Znachenie opredeleniia chuvstvitel’nosti k novobiotsinu v taksonomii stafilokokkov \\[Significance of determining novobiocin sensitivity in the taxonomy of staphylococci\\]. *Antibiotiki*, *24*(11), 824–827.\n2. Leighton, P. M., & White, M. (1983). Rapid determination of novobiocin susceptibility for the identification of Staphylococcus saprophyticus. *Diagnostic microbiology and infectious disease*, *1*(3), 261–264. \u003Chttps:\u002F\u002Fdoi.org\u002F10.1016\u002F0732-8893(83)90026-3>\n3. Kloos WE, Schleifer KH. Simplified scheme for routine identification of human *Staphylococcus* species. *J Clin Microbiol.* 1975;1(1):82-88. doi:10.1128\u002Fjcm.1.1.82-88.1975\n4. Procop GW, Church DL, Hall GS, Janda WM, Koneman EW, Schreckenberger PC, Woods GL. *Koneman's Color Atlas and Textbook of Diagnostic Microbiology.* 7th ed. Philadelphia: Wolters Kluwer; 2017.\n5. Tille PM. *Bailey and Scott's Diagnostic Microbiology.* 15th ed. St. Louis: Elsevier; 2022.",[46,49,52,55,58,61],{"question":47,"answer":48},"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":50,"answer":51},"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":53,"answer":54},"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":56,"answer":57},"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":59,"answer":60},"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":62,"answer":63},"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.",[65],"gram-positive-cocci",[67,83,97,110,123,157,190,219],{"slug":68,"title":69,"description":70,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":71,"lastUpdatedDate":72,"draft":42,"category":73,"image":38,"faq":74,"tags":81},"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",[75,78],{"question":76,"answer":77},"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":79,"answer":80},"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.",[65,82],"bacterial-classification",{"slug":84,"title":85,"description":86,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":87,"lastUpdatedDate":88,"draft":42,"category":73,"image":38,"faq":89,"tags":96},"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",[90,93],{"question":91,"answer":92},"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":94,"answer":95},"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.",[65],{"slug":98,"title":99,"description":100,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":101,"lastUpdatedDate":88,"draft":42,"category":73,"image":38,"faq":102,"tags":109},"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",[103,106],{"question":104,"answer":105},"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":107,"answer":108},"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.",[65],{"slug":111,"title":112,"description":113,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":114,"lastUpdatedDate":88,"draft":42,"category":73,"image":38,"faq":115,"tags":122},"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",[116,119],{"question":117,"answer":118},"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":120,"answer":121},"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).",[65],{"slug":124,"title":125,"description":126,"seoTitle":127,"seoDescription":128,"author":39,"createdDate":129,"lastUpdatedDate":72,"draft":42,"category":43,"image":38,"faq":130,"tags":155},"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",[131,134,137,140,143,146,149,152],{"question":132,"answer":133},"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":135,"answer":136},"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":138,"answer":139},"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":141,"answer":142},"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":144,"answer":145},"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":147,"answer":148},"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":150,"answer":151},"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":153,"answer":154},"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.",[156,65],"gram-negative-rods",{"slug":158,"title":159,"description":160,"seoTitle":159,"seoDescription":161,"author":39,"createdDate":162,"lastUpdatedDate":72,"draft":42,"category":163,"image":38,"faq":164,"tags":189},"blood-agar-composition-preparation-uses-and-types-of-hemolysis","Blood Agar: Preparation, Hemolysis Patterns, and Identification Clues","Blood agar: composition, preparation, types of hemolysis (alpha, beta, gamma, alpha-prime), colony morphology of 20+ organisms, modifications, and clinical uses.","Learn blood agar composition and preparation, distinguish alpha, beta, and gamma hemolysis, and use colony patterns to support bacterial identification.","2013-08-22","culture-media",[165,168,171,174,177,180,183,186],{"question":166,"answer":167},"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":169,"answer":170},"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":172,"answer":173},"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":175,"answer":176},"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":178,"answer":179},"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":181,"answer":182},"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":184,"answer":185},"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":187,"answer":188},"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.",[65],{"slug":191,"title":192,"description":193,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":194,"lastUpdatedDate":195,"draft":42,"category":43,"image":38,"faq":196,"tags":218},"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",[197,200,203,206,209,212,215],{"question":198,"answer":199},"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":201,"answer":202},"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":204,"answer":205},"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":207,"answer":208},"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":210,"answer":211},"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":213,"answer":214},"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":216,"answer":217},"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.",[65],{"slug":220,"title":221,"description":222,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":194,"lastUpdatedDate":195,"draft":42,"category":43,"image":38,"faq":223,"tags":242},"optochin-test-principle-procedure-expected-results-and-quality-control","Optochin Sensitivity Test: Principle, Procedure, Results","Optochin test principle, procedure, and the 14mm zone cutoff used with bile solubility to confirm Streptococcus pneumoniae",[224,227,230,233,236,239],{"question":225,"answer":226},"Why are some Streptococcus pneumoniae strains resistant to optochin?","Optochin's molecular target is the bacterial F0F1 ATP synthase. Point mutations in the genes coding for this enzyme, most often atpC and less commonly atpA, alter its structure enough that optochin no longer binds effectively, producing a resistant phenotype despite the isolate still being a true pneumococcus.",{"question":228,"answer":229},"Should an optochin-resistant alpha-hemolytic coccus from a CSF or blood culture be reported as viridans streptococci?","Not without confirmation. In invasive disease specimens, an optochin-resistant or equivocal result should be confirmed with bile solubility or molecular testing before being called viridans streptococci, since misidentifying a resistant pneumococcus carries real clinical consequences.",{"question":231,"answer":232},"What does it mean if colonies are growing inside an otherwise qualifying optochin zone of inhibition?","This can represent a heterogeneous population, a mix of optochin-susceptible and optochin-resistant cells from the same culture. A zone of 14mm or more with colonies visible inside it shouldn't automatically be dismissed as a measurement artifact; it can still indicate a resistant strain and warrants further confirmation.",{"question":234,"answer":235},"Why is Streptococcus pneumoniae sensitive to optochin when other alpha-hemolytic streptococci are not?","Because optochin targets the F0F1 ATP synthase, the enzyme that generates the cell's energy. The pneumococcal version of this enzyme is exquisitely sensitive to optochin at very low concentrations (5 micrograms per mL or less), while the enzyme in most other alpha-hemolytic (viridans) streptococci is not. Cells near the disk cannot sustain the energy metabolism needed to replicate, producing a zone of inhibition. This is why optochin sensitivity presumptively identifies S. pneumoniae.",{"question":237,"answer":238},"Why must the optochin test be incubated in CO2?","Because S. pneumoniae grows poorly in ambient air, producing smaller and less reliable zones of inhibition. Incubating in 5 to 10% CO2 (or a candle jar) gives proper growth and a reliable zone. Skipping CO2 enrichment can shrink the zone enough to manufacture a false equivocal or false resistant reading on a truly susceptible organism, so ambient air is a genuine source of error, not a shortcut.",{"question":240,"answer":241},"What does a zone of inhibition less than 14 mm mean in the optochin test?","Any zone under 14 mm around a 6 mm, 5 microgram disk is equivocal and cannot presumptively identify pneumococcus on its own. Whether the zone is 10 mm or entirely absent, the next step is the same: perform the bile solubility test (or molecular testing). There is no special lower bound that makes a sub-14 mm result safe to call, so all such results are handled identically with a confirmatory test.",[65],[244,250,257,262,266,270,275,280,284,288],{"slug":245,"name":39,"description":246,"image":247,"body":248,"postCount":249},"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":251,"name":252,"description":253,"image":254,"body":255,"postCount":256},"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":258,"name":259,"description":260,"image":38,"body":38,"postCount":261},"sushmita-baniya","Sushmita Baniya","Author \u002F Contributor",32,{"slug":263,"name":264,"description":260,"image":38,"body":38,"postCount":265},"samikshya-acharya","Samikshya Acharya",20,{"slug":267,"name":268,"description":260,"image":38,"body":38,"postCount":269},"alisha-tripathi","Alisha Tripathi",6,{"slug":271,"name":272,"description":273,"image":38,"body":38,"postCount":274},"aastha-shrestha","Aastha Shrestha"," Author \u002F Contributor",10,{"slug":276,"name":277,"description":278,"image":38,"body":38,"postCount":279},"guest-author","Guest Author","Guest Author \u002F Contributor",2,{"slug":281,"name":282,"description":260,"image":38,"body":38,"postCount":283},"srijana-khanal","Srijana Khanal",18,{"slug":285,"name":286,"description":278,"image":38,"body":38,"postCount":287},"dr-poonam-acharya","Dr. Poonam Acharya",1,{"slug":289,"name":290,"description":260,"image":38,"body":291,"postCount":292},"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]