[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"$fxLN3MUwXCdr5RPjwZYIDpOj8CHyjOmngWTgoKXPtZbg":3,"$fCVAAT73CYZ1GTOyUldw-9vMlf7KdBD-Y_h-q8sKWCz0":32,"$f3Ft0rKFJHppdzE-vuveecxx1BUcg9iOlMLtyzf_MJDg":47},[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":46},"preparation-mcfarland-turbidity-standards","Preparation of McFarland Turbidity Standards",null,"Acharya Tankeshwar","2016-06-09","2025-12-29",false,"bacteriology","McFarland turbidity standards are prepared by mixing various volumes of 1% sulfuric acid and 1% barium chloride to obtain solutions with specific optical densities. 0.5 McFarland turbidity standard provides an optical density comparable to the density of a bacterial suspension 1.5x 10^8 colony forming units (CFU\u002Fml). 0.5 McFarland standard is commercially available.\n\nFor performing antimicrobial susceptibility testing using **Kirby Bauer disc diffusion method**, a cell suspension of organisms equivalent to a 0.5 McFarland standard is used.\n\n## Procedure\n\n1. Prepare a 1% solution of anhydrous barium chloride (BaCl2).\n2. Prepare a 1% solution of sulfuric acid (H2SO4)\n3. Combine and completely mix the barium chloride and sulfuric acid solutions to form a turbid suspension and BaSO4 in a specific proportion for each McFarland turbidity standard as shown in **Table 1.**\n4. Place the resulting mixture in a foil-covered screw-cap tube.\n5. Store the it at room temperature (25 °C) when not in use. McFarland standard density solution will precipitate and clump over time, and it needs vigorous vortexing before each use. Mark the tube to indicate the level of liquid, and check before use to be sure that evaporation has not occurred.\n6. Prepare a fresh standard solution every 6 months.\n\n| McFarland turbidity standard no. | 0.5 | 1 | 2 | 3 | 4 |\n| --- | --- | --- | --- | --- | --- |\n| 1% barium chloride (ml) | 0.05 | 0.1 | 0.2 | 0.3 | 0.4 |\n| 1% sulfuric acid (ml) | 9.95 | 9.9 | 9.8 | 9.7 | 9.6 |\n| Approx. cell density (1×1^8 CFU\u002Fml) | 1.5 | 3 | 6 | 9 | 12 |\n\n> An alternative way is to use a McFarland densitometer for preparing bacterial suspension as per the required McFarland standard. Here the preparation of BaCl ₂ and H₂SO₄ can be avoided.\n\n**Table 1: McFarland turbidity standards**\n\n## Matching with turbidity standards\n\nDensity of the suspension of bacterial cells is compared to the McFarland standard *(0.5  McFarland turbidity standard for antimicrobial susceptibility testing purposes)* by holding the suspension and McFarland standard in front of light against a white background with contrasting black lines.\n\nIf the density is too heavy, the suspension should be diluted with saline or broth (whichever was used to make the suspension). If the density is not sufficient, additional bacteria should be added to the suspension. The adjusted suspensions should be used as inocula **within 15 minutes**.\n\n![Bacterial suspension prepared to match the turbidity of the 0.5 McFarland turbidity standard - Bacterial suspension prepared to match the turbidity of the 0.5 McFarland turbidity standard](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002Fkirby-bauer-antibiotic-sensitivity-testing-9-728.jpg)Figure: Bacterial suspension prepared to match the turbidity of the 0.5 McFarland turbidity standard\n\n## Uses\n\n- **Standardization of inoculum for Antimicrobial susceptibility testing** If the inoculum does not contain a concentration of bacteria that approximates the 0.5 McFarland turbidity standard, antimicrobial susceptibility test results will be affected. For instance, a resistant organism could appear susceptible if too few bacteria are used in the inoculum. **Various other purposes**(such as DNA extraction, tests, etc. ).\n\n**References**\n\n1. Zapata, A., & Ramirez-Arcos, S. (2015). A comparative study of McFarland turbidity standards and the Densimat photometer to determine bacterial cell density. *Current microbiology*, *70*(6), 907–909. \u003Chttps:\u002F\u002Fdoi.org\u002F10.1007\u002Fs00284-015-0801-2>\n2. Swenson, J. M., & Thornsberry, C. (1984). Preparing inoculum for susceptibility testing of anaerobes. *Journal of clinical microbiology*, *19*(3), 321–325. \u003Chttps:\u002F\u002Fdoi.org\u002F10.1128\u002Fjcm.19.3.321-325.1984>",[],[],[],[48,54,61,66,70,74,79,84,88,92],{"slug":49,"name":38,"description":50,"image":51,"body":52,"postCount":53},"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":55,"name":56,"description":57,"image":58,"body":59,"postCount":60},"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":62,"name":63,"description":64,"image":37,"body":37,"postCount":65},"sushmita-baniya","Sushmita Baniya","Author \u002F Contributor",32,{"slug":67,"name":68,"description":64,"image":37,"body":37,"postCount":69},"samikshya-acharya","Samikshya Acharya",20,{"slug":71,"name":72,"description":64,"image":37,"body":37,"postCount":73},"alisha-tripathi","Alisha Tripathi",6,{"slug":75,"name":76,"description":77,"image":37,"body":37,"postCount":78},"aastha-shrestha","Aastha Shrestha"," Author \u002F Contributor",9,{"slug":80,"name":81,"description":82,"image":37,"body":37,"postCount":83},"guest-author","Guest Author","Guest Author \u002F Contributor",2,{"slug":85,"name":86,"description":64,"image":37,"body":37,"postCount":87},"srijana-khanal","Srijana Khanal",18,{"slug":89,"name":90,"description":82,"image":37,"body":37,"postCount":91},"dr-poonam-acharya","Dr. Poonam Acharya",1,{"slug":93,"name":94,"description":64,"image":37,"body":95,"postCount":96},"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]