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0.5 McFarland turbidity Standard Indicates

0.5 McFarland turbidity Standard indicates an optical density compared to the density of a bacterial suspension 1.5X10 ∧8 colony forming units (CFU/ml) as shown above picture.

Principle of McFarland Standard Preparation

Original McFarland turbidity standards are the chemical solution of barium chloride (Bacl₂) and sulfuric acid (H₂SO₄) and reaction between these two chemicals results in the production of a fine precipitate of barium sulfate. (Note: Nowadays McFarland standards are prepared from suspensions of latex particles, which lengthens the shelf life and stability of the suspensions.) First shake well, the turbidity of a McFarland Std. and comparable visually to a bacterial or fungal suspension of known concentration. These McFarland turbidity standards are prepared by mixing various volumes of 0.18 M H₂SO4 (1% w/v) sulfuric acid and 0.048 M barium chloride (1.17% w/v Bacl₂H₂O)to obtain solutions with specific optical densities. By adjusting the volume of these two chemical reagents, McFarland turbidity standards of varying degrees of turbidity can be prepared that represent different microbial densities. For example 0.5 McFarland turbidity standard is equivalent to an optical density comparable to the density of a bacterial suspension with 1.5 x 10^8 colony forming units (CFU/ml).

O.5 McFarland Turbidity Standard Preparation

1. Prepare a 1.17% w/v solution of anhydrous barium chloride and 1% solution of sulfuric acid.
2. Add 0.5 ml of barium chloride solution to 99.5 ml of a solution of sulfuric acid with constant stirring.
3. Mix McFarland standard thoroughly to ensure that it is evenly suspended.
4. Using matched cuvettes with a 1 cm light path and water as a blank standard, measure the absorbance in a spectrophotometer at a wavelength of 625 nm.
5. The acceptable range for the turbidity standard is 0.08-0.13.
6. Distribute the standard into screw-cap tubes of the same size and volume as those use to prepare the test inoculum.
7. Seal the tubes tightly to prevent loss by evaporation. Store it and protect it from light at room temperature.
8. Agitate the turbidity standard vigorously on a vortex mixer before use.
9. Standards may be stored for up to 6 months, after which they should be discarded.

Application of McFarland Standard 

1.  0.5 McFarland standard is used in the antimicrobial susceptibility testing (AST) procedure where the bacterial suspension is compared to this McFarland Standard before swabbing on Muller-Hinton agar (MHA).
2. It is also used for antifungal susceptibility testing (AFST) procedure where the fungal suspension is compared to this McFarland Standard prior to swab on modified Muller-Hinton agar (MHA).
3. It is also supportive for quality control to check and adjust the densities of microbial suspension which can be used for identification and susceptibility procedure.

Limitations of 0.5 McFarland Turbidity Standard

• When using commercially available the latex standard, the tubes for the suspension should be the same diameter as the McFarland Latex Standard tube.
• The tubes should be sealed tightly to prevent loss by evaporation and stored protected from light at room temperature otherwise the turbidity measurement can be affected.
• Bacteria on color media can affect this turbidity measurement.

Bibliography

1. Clinical Microbiology Procedure Handbook, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
2. https://www.biodiamed.gr/wp-content/uploads/2017/0 /Manual_on_Antimicrobial_Susceptibility_Testing.pdf
3. https://en.wikipedia.org/wiki/McFarland_standards
4. http://himedialabs.com/TD/M1825.pdf