Preparation of McFarland Turbidity Standards

By Acharya Tankeshwar •  Updated: 05/03/22 •  2 min read

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/ml). 0.5 McFarland standard is commercially available.

For performing antimicrobial susceptibility testing using Kirby Bauer disc diffusion method, a cell suspension of organisms equivalent to a 0.5 McFarland standard is used.

Procedure

  1. Prepare a 1% solution of anhydrous barium chloride (BaCl2).
  2. Prepare a 1% solution of sulfuric acid (H2SO4)
  3. 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.
  4. Place the resulting mixture in a foil-covered screw-cap tube.
  5. Store the McFarland standard 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.
  6. Prepare a fresh standard solution every 6 months.

Table 1: McFarland turbidity standards

McFarland turbidity standard no.0.51234
1% barium chloride (ml)0.050.10.20.30.4
1% sulfuric acid (ml)9.959.99.89.79.6
Approx. cell density (1×1^8 CFU/ml)1.536912

Matching with turbidity standards

Density of the suspension of bacterial cells is compared to the McFarland turbidity standard (0.5  McFarland turbidity standard for antimicrobial susceptibility testing purposes) by holding the suspension and McFarland turbidity standard in front of light against a white background with contrasting black lines.

If 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.

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

Uses

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please email at microbeonline@gmail.com

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6 responses to “Inducible Clindamycin Resistance (D Test)”

  1. very useful information, thanks a lot 🙂

  2. K says:

    Thank you sir. This is most understandable explanation of the concept I have come across so far.

    • Isha says:

      Hello sir, I am preparing for PG entrance ,Can u pls right for various types of resistance in staph it’s highly confusing….

      • Jerry says:

        Thanks for your good work. Please can you write on the pharmacokinetics and pharmacodynamics of antibiotics.

  3. Jerry says:

    Thanks for making me understand this topic.

  4. Donna Eliason says:

    Have you seen this used to demonstrate ICR in S agalactiae?

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