Dear Masoomeh Shafiei,

McFarland Turbidity Standards

McFarland standards can be used to visually approximate the concentration of cells in a suspension. The McFarland Scale represents specific concentrations of CFU/mL and is designed to be used for estimating concentrations of gram negative bacteria such as E. coli. Note that this estimate becomes uncertain with organisms outside the normal usage as different species of bacteria differ in size and mass, as do yeast and mold. Use of this method would require calibration and validation.

McFarland Standards are generally labeled 0.5 through 10 and filled with suspensions of Barium salts. (Note – latex bead suspensions are now also available which extend the shelf life of the material). The standards may be made in the lab by preparing a 1% solution of anhydrous BaCl2 and a 1% solution of H2SO4 – mix them in the proportions listed in the table. They should be stored in the dark, in a tightly sealed container at 20-25oC and should be stable for approximately 6 months (4).

The advantage of the use of these standards is that no incubation time or equipment is needed to estimate bacterial numbers. The disadvantage is that there is some subjectivity involved in interpreting the turbidity, and that the numbers are valid only for those microorganisms similar to E. coli. In addition, the values are not in the appropriate range for the AET inoculum and so further dilutions may be required.

Approximate E. coli concentrations on McFarland Scale

McFarland Scale      CFU (x106/mL)           1% BaCl2/ 1% H2SO4 (mL)

0.5                                     

Hi:

You should compare turbidity of McFarland with your sample in front of light or you can determine OD of McFarland with spectrometer in 600nm and adjust OD of your sample with it.

Thanks for your guidance, it was very useful but More detailed:

I want to know obtaining a 108 concentration  of Yersinia ruckeri in the liquid medium is be used  from a concentration of this solution with OD 0.08 to 0.13?