[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"$fxLN3MUwXCdr5RPjwZYIDpOj8CHyjOmngWTgoKXPtZbg":3,"$fm7FP3TYUqPh9-1P7VHtUdj2Mhb3qXz1-aG85AgrSfBM":32,"$f3Ft0rKFJHppdzE-vuveecxx1BUcg9iOlMLtyzf_MJDg":176},[4,8,12,16,20,24,28],{"title":5,"slug":6,"path":7},"About Microbeonline.com","about-microbeonline-com","\u002Fabout-microbeonline-com\u002F",{"title":9,"slug":10,"path":11},"About Me","about-me","\u002Fabout-microbeonline-com\u002Fabout-me\u002F",{"title":13,"slug":14,"path":15},"Advertise with Us","advertise-us","\u002Fadvertise-us\u002F",{"title":17,"slug":18,"path":19},"Privacy Policy","privacy-policy","\u002Fprivacy-policy\u002F",{"title":21,"slug":22,"path":23},"Abbreviations","abbreviations","\u002Fabbreviations\u002F",{"title":25,"slug":26,"path":27},"Microbes","microbes","\u002Fmicrobes\u002F",{"title":29,"slug":30,"path":31},"Books","recommended-books","\u002Frecommended-books\u002F",{"type":33,"data":34},"blog",{"slug":35,"title":36,"description":36,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":39,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"body":43,"faq":44,"tags":45,"related":47},"bacterial-culture-media-their-ph-indicators-and-color-of-bacterial-colonies","Culture Media: PH Indicators, Color of Colonies",null,"Acharya Tankeshwar","2013-05-04","2026-07-13",false,"culture-media","Colony color or pigmentation is mostly exploited characteristics of bacteria which aids in the differentiation and identification of the isolates. Except when there is a growth of **pigmented bacteria**. The characteristics pigmentation observed in the culture media is because of the changes in the pH of the medium (i.e. development of the characteristics color based on the use of pH indicator).\n\npH indicators give characteristics color in different pH e.g., phenol red gives yellow color in acidic pH. So whenever fermenters grow in sugar-containing media, they produce acid and give yellow-colored colonies. For e.g. mannitol fermenter [S. aureus](\u002Fstaphylococcus-aureusdisease-properties-pathogenesis-and-laboratory-diagnosis\u002F) in mannitol salt agar, sugar fermenter in XLD agar, etc. In this post, we are grouping some of the bacteriological media on the basis of pH indicator used.\n\n### pH indicators and their range\n\n### pH indicators used in different culture media\n\n\u003Ctable style=\"min-width: 75px;\">\n\u003Ccolgroup>\u003Ccol style=\"min-width: 25px;\">\u003Ccol style=\"min-width: 25px;\">\u003Ccol style=\"min-width: 25px;\">\u003C\u002Fcolgroup>\u003Ctbody>\u003Ctr>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cstrong>Bromothymol Blue\u003C\u002Fstrong> \u003Cstrong>pH range: 6.0 (yellow)- 8.0 (blue)\u003C\u002Fstrong>\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cstrong>Phenol Red\u003C\u002Fstrong>\u003C\u002Fp>\u003Cp>\u003Cstrong>pH range: 6.8 (yellow)- 8.4 (red)\u003C\u002Fstrong>\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>\u003Cstrong>Neutral Red\u003C\u002Fstrong>\u003C\u002Fp>\u003Cp>\u003Cstrong>pH range: 6.8 (red)- 8 (yellow)\u003C\u002Fstrong>\u003C\u002Fp>\u003C\u002Ftd>\u003C\u002Ftr>\u003Ctr>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>Cystine Lactose Electrolyte Deficient Agar (CLED)\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>Mannitol Salt Agar\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>MacConkey Agar\u003C\u002Fp>\u003C\u002Ftd>\u003C\u002Ftr>\u003Ctr>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>Thiosulfate Citrate Bile Salts Sucrose (TCBS) Agar\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>Triple Sugar Iron (TSI) Agar\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>Deoxycholate Citrate Agar (DCA)\u003C\u002Fp>\u003C\u002Ftd>\u003C\u002Ftr>\u003Ctr>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>OF Medium\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>Urease Test Medium\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>Salmonella-Shigella Agar\u003C\u002Fp>\u003C\u002Ftd>\u003C\u002Ftr>\u003Ctr>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>Simmons Citrate Agar\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp>Xylose Lysine Deoxycholate Agar (XLD)\u003C\u002Fp>\u003C\u002Ftd>\u003Ctd colspan=\"1\" rowspan=\"1\">\u003Cp> \u003C\u002Fp>\u003C\u002Ftd>\u003C\u002Ftr>\u003C\u002Ftbody>\n\u003C\u002Ftable>\n\n## pH indicator: Bromothymol blue\n\nIt is a color indicator that turns **yellow** at acidic pH. **At a neutral pH, bromthymol blue is** green. At pH 7.5 or above, bromthymol blue turns **royal blue**.\n\n![Lactose fermenting (Yellow colonies) and Lactose Non fermenting colonies in CLED - Lactose fermenting (Yellow colonies) and Lactose Non-fermenting colonies in CLED](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FLactose_non_lactose_fermenters_on_CLED_agar-300x213.jpg)Figure: Lactose fermenting (Yellow colonies) and Lactose Non-fermenting colonies in CLED\n\n1. [Cystine Lactose Electrolyte Deficient Agar (CLED)](\u002Fcled-agar-composition-uses-typical-colony-characteristics\u002F): In  CLED media, **lactose fermenting colonies** of *Escherichia coli* give **pale yellow colonies** whereas **non-lactose fermenting colonies** of Proteus, Salmonella, etc gives **blue colored colony**.\n2. **Simmons Citrate Agar**: Simmons citrate agar tests the ability of organisms to utilize citrate as a sole carbon source and ammonium dihydrogen phosphate as the sole source of nitrogen. Organisms that **can utilize citrate** as their sole carbon source also utilize ammonium dihydrogen phosphate creating an alkaline environment in the medium, **turning the medium blue**.\n3. **Hugh and Leifsons Medium (OF Medium)**: OF medium (oxidative-fermentative test) is used to determine if gram-negative bacteria metabolize carbohydrates oxidatively (producing weak acid), by fermentation (producing mixed acid), or are nonsaccharolytic and therefore have no ability to use the carbohydrate in the media (no acid production). The **high concentration of acid produced during fermentation** will turn the bromthymol blue indicator from **green to yellow** in the presence or absence of oxygen.\n4. **Thiosulfate Citrate bile salts sucrose (TCBS) agar** is used for the selective isolation of *Vibrio* spp. TCBS has a very high pH (8.5 to 9.5) to suppress other intestinal flora. pH indicator bromothymol blue is used in this media, so if the organism **utilizes the sucrose**, it lowers the pH of the media and **yellow-colored colonies** are seen in TCBS.\n\n### pH indicator: Neutral Red\n\nRed at pH 6.8 and below, yellow at pH 8 and above.\n\n![Mixed growth of mucoid Lactose fermenting colonies and NLF colonies in MacConkey Agar - Mixed growth of mucoid Lactose fermenting colonies and NLF colonies in MacConkey Agar](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FMacConkey-Agar-282x300.jpg)Figure: Mixed growth of mucoid Lactose fermenting colonies and NLF colonies in MacConkey Agar\n\n1. **MacConkey Agar:** It is a selective and differential medium. The pH indicator helps to differentiate between lactose fermenting and lactose non-fermenter. *E coli* and other **lactose fermenting bacteria** give **pink-colored colonies** in MacConkey agar. Whereas **non-lactose fermenter** gram-negative bacilli produce **pale yellow colonies**.\n2. **Deoxycholate Citrate Agar (DCA)**: **Lactose non-fermenter** produces **colorless colonies**. **Coliform bacteria** if present form **pink colonies**.\n3. **Salmonella-Shigella Agar**: It is a selective media used to isolate *Salmonella* and Shigella. **Lactose fermenter** produces **red-pigmented colonies**. Whereas **non-lactose fermenter** (e.g., Salmonella) grows as **translucent colonies** (colorless) **with or without black centers** (neutral red does not have any role in black color formation, it’s because of action of sodium thiosulphate and ferric citrate which are also other constituents of SS Agar).\n\n### pH indicator: Methylene Blue and Eosin dyes\n\n**Eosin Methylene Blue (EMB) Agar**: EMB agar is useful for the isolation and differentiation of lactose fermenting and non-lactose fermenting enteric bacilli.\n\n**Coliform bacteria: Purplish black colonies Non-coliform bacteria: Colorless colonies**\n\n## Malachite green as pH indicator\n\n**Lowenstein Jensen Medium**: Lowenstein Jensen medium is used to isolate *Mycobacterium tuberculosis.*\n\n## pH indicator: Phenol red\n\nIt is **yellow at pH 6.8** and red at pH 8.4 and above.\n\n![Yellow colonies of S. aureus in Mannitol Salt Agar (MSA). Image source: ASM - Yellow colonies ofS. aureusin Mannitol Salt Agar (MSA).Image source: ASM](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002Fthumb_MRSA.MSA_-300x225.jpg)Figure: Yellow colonies of S. aureus in Mannitol Salt Agar (MSA).Image source: ASM\n\n1. **Mannitol Salt Agar (MSA)**: *Staphylococcus aureus* grows in mannitol salt agar, **ferments mannitol**, and produces (acid) **yellow colonies with yellow zones**. Whereas most coagulase-negative staphylococci (CONS) and micrococci **do not ferment mannitol and grow as small red colonies** surrounded by red or purple zones.\n2. [Triple Sugar Iron Agar (TSI) Test](\u002Ftriple-sugar-iron-agar-tsi-principle-procedure-and-interpretation\u002F): TSI contains three sugar (glucose, sucrose, and lactose) along with an iron source. It helps in the identification of members of the family *Enterobacteriaceae* on the basis of their fermentative capability, gas production, and H2S gas production.\n\n**Lactose fermenters: yellow\u002Fyellow Non-lactose fermenter: red\u002Fyellow Non-fermenter: red\u002Fred**\n\n1. **Urea Agar base:** **Urease-producing** organisms give **pink-red color** to the media as they utilize urea with the formation of ammonia the pH of the media changes to an alkaline condition.\n2. **Xylose Lysine Deoxycholate (XLD) Agar**: It is useful for the isolation and differentiation of *Salmonella* and *Shigella* spp which appear as **pink-colored colonies** (as they do not ferment carbohydrate). Whereas the **other nonpathogenic gram-negative enteric bacilli** which **produce yellow colonies** (as they ferment one or more of the sugars present in the media).\n\n**References**\n\n1. Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). [Brock Biology of Microorganisms](https:\u002F\u002Famzn.to\u002F2USOj0v) (15th Edition). Pearson.\n2. [Color Atlas and Textbook of Diagnostic Microbiology](https:\u002F\u002Famzn.to\u002F2vRkUvk), Koneman, 5th edition",[],[46],"bacterial-culture-media",[48,64,90,106,112,146],{"slug":49,"title":50,"description":51,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":52,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":53,"tags":63},"agar-properties-uses","Bacteriological Agar: Properties, Composition, and Uses in Microbiology","Bacteriological agar is the gelling agent used in virtually all solid culture media. Learn its properties, why it's preferred over gelatin, melting and solidification temperatures, and what happens when agar fails.","2022-11-05",[54,57,60],{"question":55,"answer":56},"Why is agar preferred over gelatin as a solidifying agent in culture media?","Agar replaced gelatin in bacteriological culture media for three critical reasons: (1) Temperature stability — agar melts at 96-100°C but does not resolidify until 40-45°C, remaining solid at 37°C incubation temperature. Gelatin melts at 37°C, making it useless for culture at body temperature. (2) Resistance to bacterial degradation — most bacteria cannot break down agar, while many produce gelatinase that liquefies gelatin, destroying the solid medium. (3) Better solidification properties — agar produces a firmer, more transparent gel at lower concentrations than gelatin. The suggestion to use agar came from Angelina Fanny Eilshemius Hesse in 1881, and Robert Koch adopted it immediately, making modern solid culture media possible.",{"question":58,"answer":59},"What is the difference between bacteriological grade and technical grade agar?","Bacteriological grade agar is purified to remove inhibitory substances — heavy metals, sulphated polysaccharides, and other impurities that inhibit microbial growth or interfere with biochemical reactions. Technical grade agar (used in the food industry for gelling) retains these impurities and is inhibitory to many bacteria and fungi. Culture media preparation always requires bacteriological grade agar specifically. Using technical grade agar would produce media that appears normal visually but inhibits or kills the organisms it should be supporting — a subtle quality failure that could generate false-negative culture results.",{"question":61,"answer":62},"What agar concentration is used for different types of culture media?","Agar concentration determines the firmness of the medium: 1.5-2.0% agar produces standard solid media (blood agar, MacConkey agar, Mueller-Hinton agar) suitable for colony isolation and identification. Concentrations below 0.5% produce semi-solid media used for motility testing (SIM medium, motility agar) — firm enough to hold shape but soft enough for motile bacteria to migrate through. Concentrations of 0.1-0.3% produce soft agars used in some transport media. The agar concentration in a medium is a fixed quality parameter — varying it changes the medium's properties and can affect selectivity, differential reactions, and organism growth.",[46],{"slug":65,"title":66,"description":67,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":68,"lastUpdatedDate":69,"draft":41,"category":42,"image":37,"faq":70,"tags":89},"preparation-of-culture-media","Preparation of Culture Media: Step-by-Step Guide, Best Practices, and Troubleshooting","A complete guide to in-house culture media preparation — weighing, dissolving, autoclaving, pH verification, dispensing, drying, and storage — with a troubleshooting table for common problems including clumping, wrong pH, soft agar, and poor growth.","2022-10-30","2026-07-19",[71,74,77,80,83,86],{"question":72,"answer":73},"Why does incorrect Mueller-Hinton agar depth cause false antibiotic susceptibility results?","Mueller-Hinton agar depth affects antibiotic diffusion patterns because the agar acts as a three-dimensional diffusion medium. The standard depth of 4 ± 0.5 mm is calibrated against the interpretive breakpoints published by CLSI and EUCAST — the zone size thresholds for susceptible, intermediate, and resistant were established using plates of exactly this depth. When agar is too thick (e.g., 6 mm), the antibiotic diffuses through more medium before reaching any given radial distance from the disc. This means the antibiotic concentration at any given distance from the disc is lower than it would be on a correctly poured plate — the inhibition zone is therefore smaller than it should be, and an organism that is truly susceptible may produce a zone below the susceptibility breakpoint, generating a false resistant result. Thin agar has the opposite effect: the inhibition zone is larger than it should be, potentially generating false susceptible results for resistant organisms. Pouring to a consistent depth requires either a calibrated dispenser or careful measurement — simply eyeing the plate and estimating is insufficient for this critical measurement.",{"question":75,"answer":76},"Why must certain selective media like TCBS, XLD, and DCA agar never be autoclaved?","TCBS, XLD, DCA, SS agar, and HE agar contain heat-labile selective and differential components that are chemically destroyed by autoclaving at 121°C. In TCBS agar, the alkaline pH (approximately 8.6), the bile salts, and the thiosulfate-citrate combination — all critical for selective inhibition of non-Vibrio organisms and differentiation by sucrose fermentation — are disrupted by autoclaving. In XLD agar, the selective mechanism depends on a specific combination of xylose, lysine, deoxycholate, and sodium thiosulfate operating at precise concentrations; heat causes chemical reactions between these components that destroy the differential capacity. The practical consequence of autoclaving these media is subtle and dangerous: the agar may appear grossly normal (correct colour, correct consistency) but will lack selectivity, allowing organisms that should be inhibited to grow freely. This produces false-negative cultures — the plate appears to show no Salmonella or Vibrio when in fact the organism is present but the selective pressure that would have suppressed competing flora has been eliminated. These media must be prepared by boiling only (one minute with constant stirring), not autoclaving.",{"question":78,"answer":79},"How should a microbiologist investigate when a freshly prepared batch of culture media gives unexpected results during quality control testing?","A systematic approach works through the most common causes in order of likelihood. First, verify the autoclave function: check that the autoclave indicator tape changed colour correctly and review the temperature and pressure log for the sterilization cycle — incomplete sterilization or overheating are both possible. Second, check the water quality: most failures in media preparation in resource-limited settings are due to water with excessive mineral content, incorrect pH, or contaminating substances — test the water conductivity and pH. Third, review the preparation record: were the correct amounts weighed (check against the logbook), was the medium heated to complete dissolution before autoclaving, was the correct incubation temperature and duration used for QC testing. Fourth, test a fresh batch of the same medium prepared in parallel — if the new batch performs correctly, the problem is in the previous preparation process; if both batches fail, the problem may be in the water supply or the dehydrated medium itself (contamination or deterioration). Finally, check the shelf life and storage conditions of the dehydrated medium — improperly stored or expired dehydrated media frequently cause batch failures that appear unexpectedly.",{"question":81,"answer":82},"What is the correct agar depth for Mueller-Hinton agar and why does it matter?","Mueller-Hinton agar must be poured to 4 mm ± 0.5 mm depth (approximately 20-25 mL per 90 mm Petri dish). Agar that is too thick (greater than 4.5 mm) forces antibiotic discs to diffuse through more medium before reaching any given radial distance, producing smaller inhibition zones and false resistance results. Agar that is too thin (less than 3.5 mm) produces larger zones and false susceptibility results. This depth requirement is specified by CLSI and is one of the most important quality parameters in AST plate preparation — a seemingly minor variation in pouring volume can directly affect antibiotic susceptibility reports and clinical treatment decisions.",{"question":84,"answer":85},"What type of water should be used for preparing culture media and why?","Distilled, deionised, or reverse osmosis water should be used for culture media preparation. Tap water contains dissolved minerals (calcium, magnesium, chlorine, fluoride) that can alter the pH of the medium, interfere with selective agents, inhibit organism growth, or affect biochemical reactions. For Mueller-Hinton agar specifically, excess calcium and magnesium ions directly affect aminoglycoside and tetracycline zone sizes. The water quality used in media preparation is therefore a quality control parameter, not merely a procedural preference.",{"question":87,"answer":88},"What should be done if condensation water is seen on the agar surface or inside the lid after preparation?","Condensation on the agar surface or lid should never be shaken off — this spreads moisture across the agar surface, which causes spreading of colonies and compromises selective properties. Instead, dry plates at 35-37°C for 20-30 minutes with plates inverted (agar side up) so condensation drains away from the surface. Do not over-dry — cracking of the agar surface indicates excessive drying and the plates should be discarded. A simple visual check before plating: the surface should appear uniformly matte (not shiny with moisture) and crack-free.",[46],{"slug":91,"title":92,"description":93,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":94,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":95,"tags":105},"quality-control-of-microbiological-culture-media","Quality Control of Culture Media: Why a Plate Can Look Perfect and Still Mislead","A batch of agar that passes every visual check can still distort the exact reaction it's supposed to reveal. The real difference between a visual inspection and genuine quality control, explained.","2019-07-23",[96,99,102],{"question":97,"answer":98},"What are the three components of quality control for culture media?","Quality control of culture media has three components: (1) Physical\u002Fvisual inspection — checking appearance, colour, clarity, pH, and agar depth before use; (2) Sterility testing — incubating 5-10% of each new batch at 35°C for 48-72 hours without inoculation to confirm no contamination occurred during preparation; (3) Performance testing — inoculating with known ATCC reference strains to confirm the medium supports expected growth, selectivity, and differential reactions. All three must pass before a batch is released for clinical use. A batch that fails any component must be quarantined and investigated.",{"question":100,"answer":101},"Which ATCC strains are used for quality control of MacConkey agar?","MacConkey agar QC requires testing with both a target organism and a selectivity control: Escherichia coli ATCC 25922 should produce good growth with pink lactose-fermenting colonies (positive performance); Staphylococcus aureus ATCC 25923 should be inhibited or show no growth (selectivity check — confirming gram-positive organisms are suppressed). Both results must be as expected before the batch is used for clinical specimens. Using only a positive control without a selectivity control can miss medium batches where the selective agents have degraded, allowing gram-positive contamination to go undetected.",{"question":103,"answer":104},"What should happen to clinical results when a batch of culture media fails quality control?","When a batch of culture media fails QC — whether sterility testing, performance testing, or visual inspection — the entire batch must be quarantined and not used for clinical specimens. If clinical specimens were already processed on a failed batch before the failure was detected, all results from those specimens must be flagged for clinical review and the requesting clinicians notified. Repeat testing of available specimens should be offered. The root cause of the failure must be investigated (autoclave records, pH records, preparation logbook) and documented before the next batch is prepared. QC failures must be recorded in the laboratory QC logbook regardless of outcome.",[46],{"slug":107,"title":108,"description":108,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":109,"lastUpdatedDate":40,"draft":41,"category":42,"image":37,"faq":110,"tags":111},"media-used-culture-identification-salmonella","Culture media for Salmonella typhi and paratyphi","2015-01-27",[],[46],{"slug":113,"title":114,"description":115,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":116,"lastUpdatedDate":117,"draft":41,"category":42,"image":37,"faq":118,"tags":143},"commonly-used-anaerobic-media-for-anaerobic-bacteriology","Commonly Used Anaerobic Culture Media in the Diagnostic Bacteriology Laboratory","A complete guide to anaerobic culture media — non-selective, selective, and differential media used in clinical anaerobic bacteriology, including primary plating battery, PRAS media, and key organisms recovered.","2013-05-24","2026-07-18",[119,122,125,128,131,134,137,140],{"question":120,"answer":121},"What is the difference between selective and non-selective anaerobic media?","Non-selective (blood agar, RCM, thioglycollate): support all anaerobes for broad recovery. Selective: use antibiotics to target specific organisms — BBE for B. fragilis, LKV for Prevotella\u002FBacteroides, CCFA for C. difficile, PEA for gram-positive anaerobes.",{"question":123,"answer":124},"Why does Robertson's Cooked Meat Medium support anaerobic growth without a reducing agent?","Sulfhydryl groups in muscle proteins chemically reduce oxygen, lowering Eh naturally. Meat particles physically absorb dissolved oxygen. Supports C. tetani and C. botulinum without added chemical reducing agents.",{"question":126,"answer":127},"What is PRAS media?","Pre-Reduced Anaerobically Sterilized — manufactured under anaerobic conditions, 6-month shelf life. Superior recovery of fastidious anaerobes vs laboratory-prepared plates (use within 2 weeks).",{"question":129,"answer":130},"What does resazurin color indicate in thioglycollate broth?","Colorless = sufficiently anaerobic. Pink = oxygen has penetrated. If more than one-third of tube is pink, the broth is compromised — boil briefly to drive off oxygen, or discard.",{"question":132,"answer":133},"Why is laked blood used in LKV agar?","Hemolyzed blood releases hemin and growth factors that enhance and accelerate brown-black pigment in Prevotella and Porphyromonas — allowing identification at 48-72 hours rather than 5-7 days.",{"question":135,"answer":136},"What is the minimum anaerobic primary plating battery?","Anaerobic blood agar (non-selective) + BBE or LKV (Bacteroides\u002FPrevotella) + PEA (gram-positive anaerobes) + enrichment broth. Add CCFA for suspected C. difficile. Incubate anaerobically at 35-37°C; examine at 48 and 72 hours.",{"question":138,"answer":139},"How does CCFA select for Clostridioides difficile?","Cycloserine inhibits most gram-positives; cefoxitin inhibits gram-negatives. C. difficile: yellow ground-glass colonies with horse-barn odor and yellow-green UV fluorescence. Always combine with toxin immunoassay or PCR.",{"question":141,"answer":142},"Why is Bacteroides fragilis the most clinically important anaerobe?","Most frequently isolated from intra-abdominal infections. Has polysaccharide capsule, fragilysin toxin, intrinsic penicillin resistance, and better oxygen tolerance than other obligate anaerobes.",[144,145,46],"anaerobic-bacteriology","anaerobic-culture-techniques",{"slug":147,"title":148,"description":149,"seoTitle":37,"seoDescription":37,"author":38,"createdDate":150,"lastUpdatedDate":117,"draft":41,"category":42,"image":37,"faq":151,"tags":175},"types-of-bacteriological-culture-medium","Bacterial Culture Media: Classification, Types, Uses","A complete guide to bacteriological culture media: classification by composition, consistency, and functional use, with examples of 35+ media, their selective agents, and clinical applications.","2010-07-24",[152,155,158,161,164,167,170,173],{"question":153,"answer":154},"What is the difference between selective media and enrichment media?","Selective media are solid (agar-based) and allow direct colony isolation. Enrichment media are liquid (broth-based) and are used as a pre-enrichment step before plating, allowing the pathogen to multiply and increase in relative concentration. For example, selenite F broth enriches Salmonella before plating on XLD or SS agar.",{"question":156,"answer":157},"What is the difference between selective media and differential media?","Selective media suppress unwanted organisms while permitting target organisms to grow. Differential media allow multiple organisms to grow but distinguish them by colony color or reaction. Many media are both — MacConkey agar is selective (bile salts inhibit gram-positives) and differential (lactose fermenters produce pink colonies).",{"question":159,"answer":160},"What is the role of agar in culture media and why can most bacteria not digest it?","Agar is a polysaccharide from red seaweed that solidifies culture media. It melts at ~100°C and solidifies at 42-45°C, remaining solid at 37°C incubation temperature. Almost no bacteria produce enzymes capable of digesting agar, so the surface remains stable throughout incubation.",{"question":162,"answer":163},"What makes a bacterium fastidious and which media are used to grow fastidious bacteria?","Fastidious bacteria have complex nutritional requirements that cannot be met by simple media. They require specific growth factors like vitamins, blood factors, or serum. Examples include Neisseria gonorrhoeae, Haemophilus influenzae, Bordetella pertussis, and Legionella pneumophila. Enriched media such as blood agar, chocolate agar, BCYE agar, and Bordet-Gengou agar are used.",{"question":165,"answer":166},"What is the purpose of transport media and what do they contain?","Transport media preserve clinical specimens during transit to the laboratory. They maintain pathogen viability, prevent desiccation, and suppress overgrowth of commensal organisms. They are deliberately low in nutrients with a buffered salt solution and reducing agents. Examples include Stuart's medium, Amies medium, and Cary-Blair medium.",{"question":168,"answer":169},"What is the difference between alpha, beta, and gamma hemolysis on blood agar?","Alpha hemolysis produces a greenish discoloration (partial lysis) — seen with S. pneumoniae. Beta hemolysis produces a clear complete zone of lysis — seen with S. pyogenes and S. aureus. Gamma hemolysis produces no change in the medium — seen with Enterococcus faecalis.",{"question":171,"answer":172},"Why do some bacteria require anaerobic culture media?","Obligate anaerobes lack superoxide dismutase and catalase, making oxygen exposure lethal. Anaerobic media contain reducing agents (sodium thioglycollate, cysteine) to maintain low oxygen tension. Indicators like resazurin or methylene blue turn pink or blue when oxygen is present, alerting lab staff that conditions have been compromised.",{"question":174,"answer":174},"",[46],[177,183,190,195,199,203,208,213,217,221],{"slug":178,"name":38,"description":179,"image":180,"body":181,"postCount":182},"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":184,"name":185,"description":186,"image":187,"body":188,"postCount":189},"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":191,"name":192,"description":193,"image":37,"body":37,"postCount":194},"sushmita-baniya","Sushmita Baniya","Author \u002F Contributor",32,{"slug":196,"name":197,"description":193,"image":37,"body":37,"postCount":198},"samikshya-acharya","Samikshya Acharya",20,{"slug":200,"name":201,"description":193,"image":37,"body":37,"postCount":202},"alisha-tripathi","Alisha Tripathi",6,{"slug":204,"name":205,"description":206,"image":37,"body":37,"postCount":207},"aastha-shrestha","Aastha Shrestha"," Author \u002F Contributor",10,{"slug":209,"name":210,"description":211,"image":37,"body":37,"postCount":212},"guest-author","Guest Author","Guest Author \u002F Contributor",2,{"slug":214,"name":215,"description":193,"image":37,"body":37,"postCount":216},"srijana-khanal","Srijana Khanal",18,{"slug":218,"name":219,"description":211,"image":37,"body":37,"postCount":220},"dr-poonam-acharya","Dr. Poonam Acharya",1,{"slug":222,"name":223,"description":193,"image":37,"body":224,"postCount":225},"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]