why always we use 0.5 McFarland standard specifically in determine susceptibility testing of anti microbial agent?
I also use 0.5 Mcfarland to determine number of bacteria cell after incubation but in most articles they use Uv/visible spectrophotometer at 600 nm to determine cell concentration. Which way is more reliable?
Hello Abbas,
We use 0.5 McFarland standard specifically because it presents the number of bacteria within a given range to standardize microbial testing.
Dear Alla, CLSI recommend to use bacteria with McFarland 0.5 turbidity for antimicrobial testing. The number of cells in McFarland 0.5 vary on the basis of cell size. In the attached file, you can find a McFarland densitometer manual, the number of bacteria and principles was explained.
Best regards.
By international convention we adjust the turbidity of our culture against the standard 0.5 MacFarland to be able to compare between microorganisms, antibiotics according to the specifications of the NCCL.
In microbiology, McFarland standards are used as a reference to adjust the turbidity of bacterial suspensions so that the number of bacteria will be within a given range to standardize microbial testing
I agree with Dr. Vielma. A 0.5 McFarland standard, by convention, is used in many studies and tests on antimicrobial susceptibility. We use it since majority of previous literatures have used it; to save effort and standardize our reporting initiatives.
If we didn't follow the standard we may produce false positive results
I just want to ask, is this also applicable for antifungal assay?thank you
https://www.researchgate.net/profile/Mert_Sudagidan/post/A_05_McFarland_standard/attachment/59d64b8a79197b80779a592f/AS%3A480510678966273%401491574081980/download/DEN-1B.pdf
I also use 0.5 Mcfarland to determine number of bacteria cell after incubation but in most articles they use Uv/visible spectrophotometer at 600 nm to determine cell concentration. Which way is more reliable?
Dear Kadriye, antimicrobial tests especially CLSI standards recommend to use bacterial concentration as McFarland 0.5. It is a turbidity value. You can measure bacterial cell concentration at 600 nm also. McFarland standart is usefull to repeat experiments in the same cell concentration.
Best regards.
Dear Dr We must follow the International turbidity standard. 0.5 McFarland. If the density less or more you will be reporting false positive results.
Mert Sudagidan does that mean I can use McFarland standard at 600nm to measure bacterial cell concentrations? and which value does 0.5 McFarland could be equivalent to in terms of cell concentration?
Thank you
Dear Umar, in the attachment there is the manual of a densitometer. In page 6, it was shown the McFarland numbers and equivalent cell number/ml. However, it depends on cell size.
Best regards.
Calibrate your UV-Vis regularly to make sure the absorbance at the frequency you are using is close to the cfu required. Especially as some methods use 620 and higher frequencies. A plate test of the sample, followed by a colony count will give you the cfu.
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